Showing papers in "Journal of Petrology in 2010"

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

Abstract: We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

The Magnetite Crisis in the Evolution of Arc-related Magmas and the Initial Concentration of Au, Ag and Cu

Abstract: The association of large Au–Cu–Ag ore deposits with convergent margins is commonly attributed to the higher content of chalcophile elements in the parental magmas generated at subduction zones compared with mid-ocean ridges. We present new geochemical data for arc-like, relatively oxidized mantle-derived basalt to rhyolite magmas from the Pual Ridge and vicinity, Eastern Manus Basin, which show that the initial abundances of base and precious metals in the parental basalts are similar to those of mid-ocean ridge basalt (MORB). The contents of Au, Cu, and Ag are built up in the evolving Pual Ridge liquids to levels considerably in excess of those in MORB because, unlike MORB, they are not saturated in a sulfide phase, which is a consequence of their being more oxidized than MORB. The behaviour of S during the evolution of the Pual Ridge magmas is obscured by late-stage SO2 loss during eruption, but we show that it may be inferred by using Se as a proxy, because this element follows S closely during magmatic evolution except it is not lost during low-pressure (near sea-floor) degassing. The onset of magnetite fractionation at ~60 wt % SiO2 and an Mg-number of ~40 is accompanied by an abrupt decrease in the contents of Au, Cu and Ag, previously attributed to separation of Cu–Au-rich fluid, which is also shown by Se, implying that magnetite fractionation triggers sulfide saturation. Petrological modelling reveals that the amount of magnetite fractionation involved is sufficient to convert most of the S originally dissolved in the magma as sulfate (SO42–) to sulfide (S2–), triggering saturation in a Cu-rich sulfide phase, tentatively identified as bornite (Cu5FeS4). This sulfide phase sequesters Au and Ag, elements with the same valence as Cu in sulfides, but not other potentially chalcophile elements such as Ni, Re, and Pt, which suggests that the phase is crystalline rather than an immiscible sulfide melt. The relatively high contents of Cu and Au characteristic of evolved convergent margin magmas requires no enrichment from subducted material. Instead, the association of major Cu–Au deposits with convergent-margin magmatism results specifically from the process of magmatic evolution under oxidizing conditions. This same property also leads to early magnetite fractionation, triggering the abrupt saturation in the Cu-rich sulfide. Hence the easily recognizable trend of magmatic evolution under oxidizing conditions (i.e. the sharp drop in chalcophile element concentrations) may be an exploration guide to economic Au–Ag–Cu provinces, or a crucial pre-enrichment step in the formation of such deposits. The decrease in P2O5 and Sr at the same stage in the fractionation sequence (~60 wt % SiO2) indicates that saturation in apatite is concomitant with magnetite–sulfide saturation in the Eastern Manus Basin

Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

Abstract: Chemically zoned garnet growth and coeval modification of this zoning through diffusion are calculated during prograde metamorphic heating to temperatures of up to 8508C.This permits quantification of how the preservation or elimination of zoning profiles in garnet crystals of a given size is sensitive to the specific burial and heating (P^T) path followed, and the integrated duration spent at high temperature (dT/dt). Slow major element diffusion in garnet at T56008C means that centimetre-scale zoning profiles may remain for 430 Myr at amphibolite-grade conditions, but small-scale (tens of micrometres) zoning features will be lost early in the prograde stage unless this is ‘rapid’ ( 5 Myr for rocks reaching c. 6008C). Calculations indicate that preservation of unmodified growth compositions in even relatively large (up to 3 mm diameter) pelitic garnet crystals requires prograde and exhumational events to be510 Myr for rocks reaching c. 6008C.This timescale can be 5 Myr for garnet in rocks reaching 6508C or hotter. It is likely, therefore, that most natural prograde-zoned crystals record compositions already partially re-equilibrated between the time of crystal growth and of reaching maximum temperature. However, a large T^t window exists within which crystals begin to lose their growth compositions but retain evidence of crystal-scale zoning trends that may still be useful for thermobarometry purposes.The upper limit of this window for 500 mm diameter crystals can be as much as several tens of millions of years of heating to c. 7008C. Absolute re-equilibration timescales can be significantly different for garnet growing in different rock compositions, with examples of a granodiorite and a pelite given.

The Role of Water in Generating the Calc-alkaline Trend: New Volatile Data for Aleutian Magmas and a New Tholeiitic Index

Abstract: The origin of tholeiitic (TH) versus calc-alkaline (CA) magmatic trends has long been debated. Part of the problem stems from the lack of a quantitative measure for the way in which a magma evolves. Recognizing that the salient feature in manyTH^CA discrimination diagrams is enrichment in Fe during magma evolution, we have developed a quantitative index of Fe enrichment, the Tholeiitic Index (THI):THI1⁄4Fe4·0/Fe8·0, where Fe4·0 is the average FeO* concentration of samples with 4 1wt % MgO, and Fe8·0 is the average FeO* at 8 1wt % MgO. Magmas withTHI41 have enriched in FeO* during differentiation from basalts to andesites and are tholeiitic; magmas with THI51 are calc-alkaline. Most subduction zone volcanism is CA, but to varying extents; the THI expresses the continuum of Fe enrichment observed in magmatic suites in all tectonic settings.To test various controls on the development of CA trends, we present new magmatic water measurements in melt inclusions from eight volcanoes from the Aleutian volcanic arc (Augustine, Emmons, Shishaldin, Akutan, Unalaska, Okmok, Seguam, and Korovin). Least degassed H2O contents vary from 2 wt % (Shishaldin) to47 wt % (Augustine), spanning the global range in arc mafic magmas.Within the Aleutian data, H2O correlates negatively with THI, from strongly calc-alkaline (Augustine, THI1⁄4 0·65) to moderately tholeiitic (Shishaldin, THI1⁄41·16). The relationship between THI and magmatic water is maintained when data are included from additional arc volcanoes, back-arc basins, ocean islands, and mid-ocean ridge basalts (MORBs), supporting a dominant role of magmatic water in generating CA trends. An effective break between TH and CA trends occurs at 2 wt % H2O. Both pMELTs calculations and laboratory experiments demonstrate that the observed co-variation of H2O and THI in arcs can be generated by the effect of H2O on the

Conditions of Magma Storage, Degassing and Ascent at Stromboli: New Insights into the Volcano Plumbing System with Inferences on the Eruptive Dynamics

Abstract: Stromboli is known for its persistent degassing and rhythmic strombolian activity occasionally punctuated by paroxysmal eruptions. The basaltic pumice and scoria emitted during paroxysms and strombolian activity, respectively, differ in their textures, crystal contents and glass matrix compositions, which testify to distinct conditions of crystallization, degassing and magma ascent. We present here an extensive dataset on major elements and volatiles (CO2, H2O, S and Cl) in olivine-hosted melt inclusions and embayments from pyroclasts emplaced during explosive eruptions of variable magnitude. Magma saturation pressures were assessed from the dissolved amounts of H2 Oa nd CO2 taking into account the melt composition evolution. Both pressures and melt inclusion compositions indicate that (1) Ca-basaltic melts entrapped in high-Mg olivines (Fo89^90) generate Stromboli basalts through crystal fractionation, and (2) the Stromboli plumbing system can be imaged as a succession of magma ponding zones connected by dikes. The 7^10 km interval, where magmas are stored and differentiate, is periodically recharged by new magma batches, possibly ranging from Ca-basalts to basalts, with a CO2-rich gas phase.These deep recharges promote the formation of bubbly basalt blobs, which are able to intrude the shallow plumbing system (2^4 km), where CO2 gas fluxing enhances H2O loss, crystallization and generation of crystal-rich, dense, degassed magma. Chlorine partitioning into the H2O^CO2-bearing gas phase accounts for its efficient degassing (� 69%) under the open-system conditions of strombolian activity. Paroxysms, however, are generated through predominantly closed-system ascent of basaltic magma batches from the deep storage zone. In this situation crystallization is negligible and sulfur exsolution starts at � 170 MPa. Chlorine remains dissolved in the melt until lower pressures, only 16% being lost upon eruption. Finally, we propose a continuum in explosive eruption energy, from strombolian activity to large paroxysmal events, ultimately controlled by variable pressurization of the deep feeding system associated with magma and gas recharges.

Mantle Melting as a Function of Water Content beneath the Mariana Arc

Abstract: Subduction zone magmas are characterized by high concentrations of pre-eruptive H_2O, presumably as a result of an H_2Oflux originating from the dehydrating, subducting slab. The extent of mantle melting increases as a function of increasing water content beneath back-arc basins and is predicted to increase in a similar manner beneath arc volcanoes. Here, we present new data for olivine-hosted, basaltic melt inclusions from the Mariana arc that reveal pre-eruptive H_2O contents of ~1•5-6•0 wt %, which are up to three times higher than concentrations reported for the Mariana Trough back-arc basin. Major element systematics of arc and back-arc basin basalts indicate that the back-arc basin melting regime does not simply mix with wet, arc-derived melts to produce the observed range of back-arc magmatic H_2O concentrations. Simple melting models reveal that the trend of increasing extents of melting with increasing H_2O concentrations of the mantle source identified in the Mariana Trough generally extends beneath the Mariana volcanic front to higher mantle water contents and higher extents of melting. In detail, however, each Mariana volcano may define a distinct relationship between extent of melting and the H_2O content of the mantle source. We develop a revised parameterization of hydrous melting, incorporating terms for variable pressure and mantle fertility, to describe the distinct relationships shown by each arc volcano. This model is used in combination with thermobarometry constraints to show that hydrous melts equilibrate at greater depths (34-87 km) and temperatures (>1300°C) beneath the Mariana arc than beneath the back-arc basin (21-37 km), although both magma types can form from a mantle of similar potential temperature (~1350°C).The difference lies in where the melts form and equilibrate. Arc melts are dominated by those that equilibrate within the hot core of the mantle wedge, whereas back-arc melts are dominated by those that equilibrate within the shallow zone of decompression melting beneath the spreading center. Despite higher absolute melting temperatures (>1300°C), Mariana arc melts reflect lower melt productivity as a result of wet melting conditions and a more refractory mantle source.

High-pressure Hydrous Phase Relations of Radiolarian Clay and Implications for the Involvement of Subducted Sediment in Arc Magmatism

Abstract: Melting of subducted oceanic sediment is considered to play a key role in the generation of the arc magmatic signature.We have carried out an experimental study on hydrous melting of trace element-doped radiolarian clay at 3 GPa and temperatures from 700 to 12508C; 7^15 wt % H2O was added to the sediment to simulate the effects of flushing by fluids derived from underlying dehydrating lithologies, such as serpentinites. Melting begins at 7508C owing to the breakdown of phengiteþ clinopyroxeneþ coesite and a hydrous melt coexists with mostly garnetþ kyanite quartz up to around 12508C. Rutile and Fe^Ti oxides are present to 10008C.Very high degrees of melting occur at relatively low, supra-solidus temperatures (e.g. with 15 wt % added H2O, the clay is 54% molten at 8008C), in marked contrast to fluid-absent melting of similar rock compositions, which yields negligible melt fractions (5 10%) for similar temperatures. A particular focus of this study is residual monazite, which preferentially incorporates light rare earth elements (LREE) and Th, thereby exerting a powerful control on the Th/La ratio of sediment-derived fluids and melts. In contrast to previous studies, we find that DTh/La varies widely and can be significantly above or below unity. Our dataset suggests that this pattern arises because the various members of the monazite solid solution series are influenced independently by different parameters. We also demonstrate that monazite^melt partition coefficients based on doped experiments cannot be used uncritically to predict fractionation processes in nature because of monazite^huttonite solid solution. However, extrapolation of our results to natural concentration levels suggests fractionation of Th from La in the presence of monazite in most cases. We propose that a solid residue with little or no residual monazite is needed to explain a wide range of geochemical features of arc magmas, including Th/La ratios. A monazite-free residue can be achieved at relatively low sub-arc temperatures provided that enough water is made available (e.g. through antigorite breakdown) to promote sufficient melting to dissolve the entire LREEþTh budget of the sediment.

Plagioclase Peridotites in Ocean–Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere

Abstract: complex history of regional-scale melt infiltration and melt^rock reaction, which has erased most of the ancient history. Simple calculations suggest that up to 12% of mid-ocean ridge basalt-type melt can be stored in plagioclase peridotite, relative to a depleted residue. Such a ‘lithospheric sponge’ provides an explanation for the fertile compositions of the peridotites and the rare occurrence of volcanic rocks in magma-poor rifted margins. We suggest that magma-poor vs magma-rich margins are largely determined by the efficiency of melt extraction and not so much by melt generation processes, given a similar initial composition of the upwelling mantle. It is proposed that refertilization increases textural diversity and chemical heterogeneity related to shallow crystallization in the mantle lithosphere.

Composition of the Lithospheric Mantle in the Siberian Craton: New Constraints from Fresh Peridotites in the Udachnaya-East Kimberlite

Abstract: We present petrographic, major element and trace element data for bulk-rocks and minerals for a suite of 34 remarkably fresh peridotite xenoliths in the Udachnaya kimberlite recovered from deep horizons of the open pit mine The xenoliths are spinel peridotites, granular and deformed garnet peridotites (as well as a megacrystalline dunite) in proportions similar to those reported by previous work on altered xenoliths from less deep levels in the mine Equilibration temperatures (T) range from 760^9658C for the spinel peridotites to 1200^13208C for sheared garnet peridotites; the latter yield pressures (P) of 5·4^6·6 GPaThe majority of the granular garnet peridotites equilibrated at 860^10008C and 2·6^5·5 GPa, but two samples yield much higher T and P values (11768C, 6·1GPa; 13408C, 6·8 GPa) indicating the presence of undeformed rocks near the base of the lithosphere Strong enrichments in silica and opx relative to dry melting residues are not common in our granular peridotites (eg 11 samples out of 18 analyzed plot on Boyd’s ‘oceanic trend’); they have distinctly lower modal opx than low-T peridotites from the Kaapvaal craton and could be formed by 30^45% of melt extraction at 1^5 GPa Minor to moderate enrichments in silica relative to experimental melting trends seen in some xenoliths may indicate that subduction-related settings were involved in their origin Garnets are usually unzoned but some are enriched in Ti at the rims Their contents of CaO (4^7 wt %) and Cr2O3 (2^12 wt %) show inverse power-law correlations with whole-rock CaO, Al2O3 and Al/Cr ratios; the latter can be estimated from garnet analyses Garnets from the dominant cpx-free and cpx-bearing harzburgites and less common low-cpx (5^6%) lherzolites fall into the G9 ‘lherzolite’field; garnets from three cpx-rich (9^15%) lherzolites are in the G5 ‘pyroxenite’ field Rare earth element (REE) patterns in cpx and garnet range from sinusoidal to bell-shaped and light REE (LREE)-depleted; the latter are common in sheared rocks and may be close to equilibrium with metasomatic liquids Garnets from some granular rocks show low heavy REE (HREE) and continuous depletion in LREE, as in melting residues Trace element patterns and ratios in many bulk peridotites are distinct from those in the host kimberlites and may not be much affected by contamination Bulk-rock abundances of moderately incompatible elements are lowest in spinel harzburgites (20^50 times lower than in primitive mantle for HREE) and highest in sheared rocks Some bulk-rocks show depletion from Lu to Er attributed to 30^40% of melt extraction first in the presence of garnet, then in the spinel field Overall, the observations on fresh Udachnaya peridotites show less evidence for silica and opx enrichments and smaller compositional effects of synand post-eruption alteration than earlier work The abundances of cpx and garnet and bulk-rock Al2O3 do not vary systematically with depth except that rare sheared xenoliths anomalously enriched in cpx and garnet occur near the base of the lithosphereThe remarkable preservation of olivine and opx shows in detail the transformation of granular microstructures into different types of porphyroclastic fabrics and their relations to chemical compositions

Continent Formation in the Archean and Chemical Evolution of the Cratonic Lithosphere: Melt-Rock Reaction Experiments at 3-4 GPa and Petrogenesis of Archean Mg-Diorites (Sanukitoids)

Abstract: To better understand the genetic relationship between granitoid rocks of the Archean cratons and the underlying ‘keel’of subcratonic lithospheric mantle (SCLM), we have conducted two types of experiments in a multi-anvil apparatus at 3^4 GPa: (1) peridotite assimilation experiments, in which natural, hydrousTTG (trondhjemite^tonalite^granodiorite) melts are reacted with mantle peridotite at relatively high melt^rock ratios; (2) liquidus saturation experiments on hydrous Mg-rich diorite (sanukitoid) melts. Our results demonstrate that liquids similar to typically lateto post-tectonic sanukitoid intrusions can form by hybridization of initial TTG melts by assimilation of olivine-bearing peridotite, and that these primitive granitoid melts are in equilibrium with reaction residues consisting of olivine-free garnet websterite or garnet pyroxenite.The experimental melts retain the distinctive trace element signature of TTGs, overprinted by a ‘primitive’ mantle signature (i.e. high Mg-number, elevated Cr and Ni abundances), whereas the various phases of the crystalline residues acquire trace element signatures reflecting equilibration with Mg-rich granitoid melts. At low melt:rock ratios, metasomatism by TTG melts may be responsible for the silica enrichment and high modal orthopyroxene content of some cratonic peridotites and cryptic trace element overprints in garnet, clinopyroxene, and orthopyroxene. Our results demonstrate that the lithospheric keel of Archean cratons represents the product of reaction betweenTTG melts and previously depleted mantle peridotite at relatively low melt:rock ratios, as evidenced by the trace element signature in garnet pyroxenite and orthopyroxene-enriched garnet peridotite xenoliths, whereas Late Archean sanukitoids represent the products of these same reactions at relatively high melt:rock ratios.

Melting Relations of MORB–Sediment Mélanges in Underplated Mantle Wedge Plumes; Implications for the Origin of Cordilleran-type Batholiths

Abstract: This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB melanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source.

Deformation and Fluid–Rock Interaction in the Supra-subduction Mantle: Microstructures and Water Contents in Peridotite Xenoliths from the Avacha Volcano, Kamchatka

Abstract: The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (< 13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1-8 center dot 6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25-506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (< 800-900 degrees C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001].

Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: insights from experimental petrology

Abstract: ‘hyper-calcic’) phases found in the xenoliths and the contaminated zones in Merapi feldspars.The xenoliths also exhibit micro-vesicular textures that can be linked to the CO2 liberation process seen in the experiments.This study, therefore, provides well-constrained petrological insights into the problem of crustal interaction at Merapi and points toward the substantial impact of such interaction on the volatile budget of the volcano.

Composition and Genesis of Depleted Mantle Peridotites from the Wadi Tayin Massif, Oman Ophiolite; Major and Trace Element Geochemistry, and Os Isotope and PGE Systematics

Abstract: The Oman ophiolite consists of several massifs cropping out along a 500 km long band trending NW-SE along the coast of Oman; it is one of the best exposed sections of oceanic crust and mantle in the world. There is a gradient in igneous processes and composition in the ophiolite, with the northern massifs recording a polygenetic igneous history involving an increasingly important subduction component, whereas the southern massifs were formed primarily via a mid-ocean ridge basalt (MORB)-like, single-stage process at a submarine spreading ridge. In this study we use geochemical data from Wadi Tayin, which is one of the southern massifs of the Oman ophiolite, to constrain the composition and genesis of oceanic crust and upper mantle. The Wadi Tayin harzburgites are residues of partial melting that are as depleted as the most depleted mid-ocean ridge peridotites. They have low middle- to heavy rare earth element ratios, most probably reflecting melting close to and beyond the exhaustion of clinopyroxene. Like many abyssal peridotites, the Wadi Tayin samples show enrichment in highly incompatible elements, giving rise to U-shaped MORB-normalized trace element patterns. We favor the idea that this is caused by enrichment of highly incompatible elements along grain boundaries in peridotite, which may be the result of near-equilibrium partitioning between grain boundaries and grain interiors, rather than disequilibrium processes. Equilibrium partitioning between crystals and grain boundaries during melting and melt extraction is also our preferred explanation for ubiquitous high Pb contents relative to Ce and La in our samples as well as in most abyssal peridotites. The Wadi Tayin samples record substantial variability in terms of osmium isotopic composition and platinum group element (PGE) concentrations. The more radiogenic nature of the dunites and impregnated peridotites compared with the residual harzburgites may be due to relatively high Os-187/Os-188 in melt transported through the dunites. The presence of residual peridotites with whole-rock osmium isotopic compositions less radiogenic than MORB may be explained by melting of a veined (upwelling) mantle forming mixed melts in which much of the Os derives from veins with high Re/Os in a matrix of previously depleted peridotite. An important result of this study is that the shallowest samples are refertilized (i.e. anomalously enriched in incompatible elements), and record relatively high metamorphic closure temperatures. These observations suggest that migrating melt underwent crystallization during rapid cooling in the uppermost mantle during and immediately after ridge magmatism. A variety of geothermometers all yield the result that the stratigraphically highest harzburgites equilibrated at higher temperature than the deeper ones. The systematic decrease in closure temperature with increasing depth below the Moho transition zone probably reflects systematic variation in cooling rate as a function of depth in the mantle section. We hypothesize that the refertilization and higher closure temperatures recorded by the uppermost mantle samples are linked. More rapid cooling led to higher closure temperatures, and to partial crystallization of migrating melts in the shallowest part of the mantle section, yielding slightly elevated abundances of elements such as Ca and Na, and incompatible trace elements.

Olivine, and the Origin of Kimberlite

Abstract: relatively rare rock in the mantle and where present its olivine is persistently Fo-rich. The dunitic source of the nodules in kimberlites lacked minerals such as pyroxene and an aluminous phase, which make up about half of most mantle-derived rocks. It appears that these minerals were removed from the mantle peridotite that was to become the source of the nodules, and the Fo content of the retained olivine was modified during interaction with CO2-rich fluids whose arrival at the base of the lithosphere immediately preceded the passage of the kimberlite magmas. Fragments of the resultant dunite were entrained into the kimberlite, where they were retained both as intact nodules and as disaggregated grains in the matrix.

Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite

Abstract: We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001)(liz), corresponding to the pseudofibres observed optically. The plane (001)(liz) is locally parallel to the narrow boundary ol-liz; thus column orientations register the interface of serpentinization. The ol-liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010)(ol). In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010](atg) show domains of very regular modulation with a 43 center dot 5 A wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP-HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100](atg)//[010](ol) and (atg)// (ol); the planes in contact are (001)(atg)//(100)(ol) and (001)(atg)//(010)(ol), respectively. The [010](atg)//[001](ol) and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine-serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.

Source and evolution of molybdenum in the porphyry Mo(-Nb) deposit at Cave Peak, Texas

Abstract: The 37 � 1 Ma breccia-hosted porphyry Mo(^Nb) deposit at Cave Peak is genetically related to a mafic, alkaline intrusion nearby (the Marble Canyon Stock), thus providing an excellent opportunity to study the influence of mafic magmatism on porphyry Mo formation. Laser ablation inductively coupled plasma mass spectrometry analysis of crystallized melt inclusions allowed reconstruction of pristine concentrations of Mo and other fluid-mobile elements in melts ranging from phonotephrite (� 50 wt % SiO2 )t o rhyolite (� 75 wt % SiO2) in composition. Incompatible trace element abundances in these melt inclusions define smooth, linear trends reaching up to 100 times average upper continental crust, implying that the felsic magmas evolved from a mafic parent by fractional crystallization.The mafic parent has a geochemical signature typical of primitive melts generated in within-plate tectonic settings. Molybdenum increases from 4 ppm in the mafic endmember to 12 ppm in felsic melts immediately before Mo mineralization. After breccia formation, Mo concentrations in the residual melt decreased gradually to 5 ppm, suggesting that Mo was removed from the residual melts by exsolving fluids. This interpretation is supported by fluid^melt partition coefficients (DMo, fluid/melt ¼ 17^20) measured on assemblages of coexisting fluid and melt inclusions. A high sulfur fugacity in the mafic input magma is indicated by early pyrrhotite saturation. To account for the 0·4^1·5 � 10 6 t Mo present in the deposit a volume of � 30^100 km 3 alkali feldspar granite magma is needed, or up to 10 times more of the mafic melt. Significantly less magma is required to provide all the sulfur present in the deposit.The results of this study provide direct evidence for derivation of metals (and sulfur) in porphyry Mo deposits from associated mafic, alkaline magmas through fractional crystallization.

Crystallization Sequence and Magma Chamber Processes in the Ferrobasaltic Sept Iles Layered Intrusion, Canada

Abstract: The Sept Iles layered intrusion (Quebec, Canada; 564 Ma) is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro, and an upper border series with anorthosite, capped by cupolas of A-type granite. Chilled margin compositions suggest a ferrobasaltic parental magma close to that of the Skaergaard intrusion, but much richer in iron and titanium. Samples from drill-cores and surface sampling of the 4·7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 Fe-Ti oxide layers cm-to m-thick and many anorthositic autolithic blocks. The sequence of crystallization in the layered series is: plagioclase (An72-34) and olivine (Fo75-21) followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. An olivine gap is observed between Fo66 and Fo59. The saturation of Fe-Ti oxides before Ca-rich pyroxene is interpreted to be the result of the high FeOt and TiO2 contents and the low CaO content of the parental magma. Contamination by old continental crust has occurred during crystallization of the layered series, as indicated by Sr isotopic compositions (87Sr/86Sr564=0·70360-0·70497). The differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases, Cr content of magnetite and lower Sr isotope ratios. These reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma. © The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org.

Mechanisms of Crustal Anatexis: a Geochemical Study of Partially Melted Metapelitic Enclaves and Host Dacite, SE Spain

Abstract: To shed light on the mechanisms of crustal anatexis, a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at El Hoyazo, SE Spain. Anatectic enclaves, composed of plagioclase þ biotite þ sillimanite þ garnet þ glass K-feldspar cordierite þ graphite, formed during the rapid heating and overstepped melting of a greenschist-facies metapelite, and finally equilibrated at 850 508C and 5^7 kbar. Glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves, and has a major element composition similar to that of peraluminous leucogranites. Melt inclusions and matrix glasses have normative quartz^orthoclase^albite compositions that plot in the vicinity of H2O-undersaturated haplogranite eutectics. Melt inclusions show some compositional variability, with high Li, Cs and B, low Y, first row transition elements (FRTE) and rare earth elements (REE), and zircon and monazite saturation temperatures of 665^7508C.They are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under H2O-undersaturated conditions. Compared with melt inclusions, matrix glasses show less compositional variability, lower large ion lithophile element contents, higher Y, FRTE and REE, and higher zircon and monazite saturation temperatures ( 695^8158C).They are interpreted as former melts recording the onset of biotite dehydration-melting. Matrix glasses in the dacite are compositionally different from glasses in the enclaves, hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest; this opens the possibility for some alternative interpretation. This study shows the following: (1) melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves; (2) melting occurred for the most part under H2O-undersaturated conditions even if, because of the rapid heating, the protolith preserved most of the structurally bound H2O contained at greenschist facies up to the beginning of anatexis, such that the excess H2O maximized the amount of H2O-undersaturated melt generated during anatexis; (3) although a large proportion of accessory minerals are currently shielded within major mineral phases, they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path, as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions; (4) the dacite magma was probably produced by coalescence of melts

Petrology of Mantle Wedge Lithosphere: New Data on Supra-Subduction Zone Peridotite Xenoliths from the Andesitic Avacha Volcano, Kamchatka

Abstract: Peridotite xenoliths in arc volcanoes are very rare, usually small and remain poorly studied. Much of the earlier work focused on peridotites affected by re-crystallization, metasomatism and veining that took place shortly before the eruption of the host magmas; such lithologies may not be widespread in the mantle wedge.This study reports petrographic, major and trace element data for 17 large, fresh peridotite xenoliths from the active Avacha volcano and discusses the origin of supra-subduction zone lithospheric mantle, in particular the role and characteristics of partial melting and metasomatism. The xenoliths are spinel harzburgites containing interstitial cpx (1·5^3%) and amphibole ( 1%). Nearly all are mediumto coarse-grained with protogranular to granoblastic microstructures; some have fine-grained domains and thin cross-cutting veins of secondary opx and olivine. Core^rim zoning and unmixing of cpx and spinel in coarse opx indicate long-term cooling to 900^10008C; Cr#Sp and Al and Cr in opx are correlated with equilibration temperatures.The peridotites are highly refractory, with 44% MgO and very low Al2O3 and CaO (0·4^0·9%), TiO2 ( 0.01%), Na2O ( 0.03%), K2O and P2O5 (below detection) and REE in whole-rocks, 2·1% Al2O3 in opx and 0·1^0·3% Na2O in cpx. Comparisons of Mg, Al and Fe contents with melting experiments indicate 28^35% melt extraction at 1 to 2 GPa, in line with the absence of primary cpx and high Mg#Ol (0·907^0·918) and Cr#Sp (0·53^0·65). Bulk-rock Al2O3 is a more robust melt extraction index than Cr#Sp, Mg#Ol and Mg#WR. Forearc harzburgites and certain xenoliths from the Western Pacific share many of these characteristics with the Avacha suite and may have similar origins. A distinctive feature of the Avacha harzburgites is a combination of variable but commonly high modal opx (18^30%) with very low modal cpx. At a given olivine or MgO content, they have higher opx and SiO2, and lower cpx (as well as Al2O3 and CaO) than typical refractory peridotite xenoliths in continental basalts. These features may indicate fluid fluxing during melting in the mantle wedge. Alternatively, they could have been produced after partial melting by selective metasomatic enrichment in SiO2 by fluids to transform some olivine into opx, although the latter mechanism is hard to reconcile with the very low alkalis and REE contents and the absence of silica correlation with fluid-mobile elements. Bulk-rock enrichments in silica and opx are unrelated to the presence or abundance of late-stage, fine-grained materials and are due to an ancient event rather than recent re-crystallization and veining.

Prograde P^T Evolution of a Lawsonite Eclogite from the Monviso Meta-ophiolite (Western Alps): Dehydration and Redox Reactions during Subduction of Oceanic FeTi-oxide Gabbro

Abstract: Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T epsilon 550 degrees C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9 degrees C km(-1) to less than 7 degrees C km(-1), which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O-2. This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

Mantle Melting, Melt Transport, and Delivery Beneath a Slow-Spreading Ridge: The Paleo-MAR from 23°15′N to 23°45′N

Abstract: Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an ∼300–400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large ∼1 km+ thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust–mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt–rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep. The average Kane transform peridotite is a lherzolite with 5·2% Cpx, whereas that from the main core complex is a harzburgite with only 3·5% Cpx. As the area corresponds to a regional bathymetric low, and the crust is apparently thin, it is likely that most residual mantle along the MAR is significantly more depleted. Thus, harzburgitic and lherzolitic ophiolite subtypes cannot be simply interpreted as slow- and fast-spreading ridges respectively. The mantle peridotites are consistent with a transform edge effect caused by juxtaposition of old cold lithosphere against upwelling mantle at the ridge–transform intersection. This effect is far more local, confined to within 10 km of the transform slip zone, and far smaller than previously thought, corresponding to ∼8% as opposed to 12·5% melting of a pyrolitic mantle away from the transform. Excluding the transform, the overall degree of melting over 3 Myr indicated by the peridotites is uniform, ranging from ∼11·3 to 13·8%. Large variations in composition for a single dredge or ROV dive, however, reflect local melt transport through the shallow mantle. This produced variable extents of melt–rock reaction, dunite formation, and melt impregnation. At least three styles of late mantle metasomatism are present. Small amounts of plagioclase with elevated sodium and titanium and alumina-depletion in pyroxene relative to residual spinel peridotites represent impregnation by a MORB-like melt. Highly variable alumina depletion in pyroxene rims in spinel peridotite probably represents cryptic metasomatism by small volumes of late transient silica-rich melts meandering through the shallow mantle. Direct evidence for such melts is seen in orthopyroxenite veins. Finally, a late hydrous fluid may be required to explain anomalous pyroxene sodium enrichment in spinel peridotites. The discontinuous thin lower crust exposed at Kane Megamullion contrasts with the >700 km2 1·5 km+ thick Atlantis Bank gabbro massif at the SW Indian Ridge (SWIR), clearly showing more robust magmatism at the latter. However, the SWIR spreading rate is 54% of the MAR rate, the offset of the Atlantis II Fracture Zone is 46% greater and Na8 of the spatially associated basalts 16% greater—all of which predict precisely the opposite. At the same time, the average compositions of Kane and Atlantis II transform peridotites are nearly identical. This is best explained by a more fertile parent mantle beneath the SWIR and demonstrates that crustal thickness predicted by simply inverting MORB compositions can be significantly in error.

Dacite Petrogenesis on Mid-Ocean Ridges: Evidence for Oceanic Crustal Melting and Assimilation

Abstract: Whereas the majority of eruptions at oceanic spreading centers produce lavas with relatively homogeneous mid-ocean ridge basalt (MORB) compositions, the formation of tholeiitic andesites and dacites at mid-ocean ridges (MORs) is a petrological enigma. Eruptions of MOR high-silica lavas are typically associated with ridge discontinuities and have produced regionally significant volumes of lava. Andesites and dacites have been observed and sampled at several locations along the global MOR system; these include propagating ridge tips at ridge^transform intersections on the Juan de Fuca Ridge and eastern Gala¤ pagos spreading center, and at the 98N overlapping spreading center on the East Pacific Rise. Despite the formation of these lavas at various ridges, MOR dacites show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes are controlling their chemistry. Although most geochemical variability in MOR basalts is consistent with low-pressure fractional crystallization of various mantle-derived parental melts, our geochemical data for MOR dacitic glasses suggest that contamination from a seawater-altered component is important in their petrogenesis. MOR dacites are characterized by elevated U,Th, Zr, and Hf, low Nb andTa concentrations relative to rare earth elements (REE), and Al2O3, K2O, and Cl concentrations that are higher than expected from low-pressure fractional crystallization alone. Petrological modeling of MOR dacites suggests that partial melting and assimilation are both integral to their petrogenesis. Extensive fractional crystallization of a MORB parent combined with partial melting and assimilation of amphibole-bearing altered crust produces a magma with a geochemical signature similar to a MOR dacite.This supports the hypothesis that crustal assimilation is an important process in the formation of highly evolved MOR lavas and may be significant in the generation of evolved MORB in general. Additionally, these processes are likely to be more common in regions of episodic magma supply and enhanced magma^crust interaction such as at the ends of ridge segments.

The Stability of Plagioclase in the Upper Mantle: Subsolidus Experiments on Fertile and Depleted Lherzolite

Abstract: Plagioclase peridotites are important markers of processes that characterize the petrological and tectonic evolution of the lithospheric mantle in extensional tectonic settings. Studies on equilibrated plagioclase peridotites have documented continuous chemical changes in mantle minerals in response to plagioclase crystallization, potentially tracing the re-equilibration of mantle peridotites up to very low pressure.This experimental study provides new constraints on the stability of plagioclase in mantle peridotites as a function of bulk composition, and the compositional and modal changes in minerals occurring within the plagioclase stability field as a function of P^T^bulk composition. Subsolidus experiments have been performed at pressures ranging from 0·25 to 1·0 GPa, and temperatures ranging from 900 to 12008C on fertile and depleted anhydrous lherzolites modelled in the system TiO2^Cr2O3^Na2O^CaO^FeO^ MgO^Al2O3^SiO2 (Ti,Cr-NCFMAS). In the fertile lherzolite (Na2O/CaO 1⁄4 0·08; XCr 1⁄4 0·07) a plagioclase-bearing assemblage is stable up to 0·7 GPa, 10008C and 0·8 GPa, 11008C, whereas in the depleted lherzolite (Na2O/CaO 1⁄4 0·09; XCr 1⁄4 0·10) the upper limit of plagioclase stability is shifted to lower pressure.The boundary between plagioclase lherzolite and spinel lherzolite has a positive slope in P^T space. In a complex chemical system, the plagioclase-out boundary is multivariant and sensitive to the XCr value [XCr 1⁄4 Cr/(Cr þAl)] of spinel.This latter is controlled by the reaction MgCr2O4 þ CaAl2Si2O8 1⁄4 MgCrAlSiO6 þ CaCrAlSiO6, which is a function of the Cr^Al partitioning between spinel and pyroxenes, and varies with the XCr value and chromite/ anorthite normative ratio of the bulk composition. Within the plagioclase stability field, the Al content of pyroxenes decreases, coupled with an increase in the anorthite content in plagioclase, and Ti and XCr in spinel with decreasing pressure; these chemical variations are combined with systematic changes in modal mineralogy governed by a continuous reaction involving both plagioclase and spinel. As a consequence, the composition of plagioclase varies significantly over a rather narrow pressure range and is similar at the same P^T conditions in the investigated bulk-rocks.This suggests the potential application of plagioclase composition as a geobarometer for plagioclase peridotites.

Crystallization Kinetics in Continuous Decompression Experiments: Implications for Interpreting Natural Magma Ascent Processes

Abstract: Decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma. These experiments differ from previous studies in that they employ a continuous, rather than stepwise, pressure^time trajectory. Four series of experiments were performed at rates of 0·5^10MPa h , corresponding to ascent rates of 0·007^0·14 m s . Experiments quenched along each decompression path allow snapshot-style monitoring of progressive crystallization. As expected, rates of plagioclase nucleation and growth depend strongly on decompression rate. Regardless of decompression rate, approximately equal volumes of feldspar crystallize during a given decompression interval, even when the observed crystallinity is lower than the equilibrium crystallinity. Apparently, a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals. Other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks. Feldspar microlites do not maintain chemical equilibrium with melt. They are more anorthite-rich than equilibrium plagioclase, suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static (phase equilibrium) experiments. Although decompression rate is an important element controlling final textures in volcanic rocks, no single compositional or textural parameter uniquely records this rate. Of the criteria examined, microlite number density and morphology are the best indicators of magma decompression rate. Based on a small number of comparisons between continuous and multi-step decompression style at the same time-integrated decompression rate (1MPa h ), decompression path evidently influences crystal texture, with stepwise decompression yielding textures correlative with faster decompression than indicated by the time-integrated decompression rate. Thus, consideration of magma ascent style, in addition to rate, will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes.

Carbonate-fluxed Melting of MORB-like Pyroxenite at 2·9 GPa and Genesis of HIMU Ocean Island Basalts

Abstract: We present partial melting experiments on a carbonate-added, mid-ocean ridge basalt (MORB)-like pyroxenite composition (G2C; 5 wt % CO2). Experiments were conducted at 2·9 GPa and 1000^15008C and the resulting partial melt compositions were compared with those of alkalic ocean island basalts (OIBs). The solidus is estimated between 1000 and 10508C and the liquidus is between 1450 and 14758C. The subsolidus assemblage is cpx þ garnet þ rutile þ calcio-dolomitic solid solution, and the near-solidus melt is carbonatitic (5 5 wt % SiO2, 51· 3w t % TiO2,50· 5w t % Al2O3 ,3 1wt %5CaO525 wt %). At 1245^ 12758C, with the disappearance of rutile, a carbonated basaltic melt is found to coexist with carbonatitic melt, cpx, and garnet. The silicate melts are alkalic basalts with SiO2 of � 44^47 wt % on a volatile-free basis, and the melt becomes most silica-poor and CO2rich at the temperature of complete mixing of carbonate and silicate melt (i.e. at 1345^13758C). The onset of carbonated silicate melting in our study is � 60^708C cooler than the solidus of the carbonate-free MORB-pyroxenite at a similar pressure. G2C-derived carbonated silicate partial melts are similar to nephelinitic to basanitic ocean island basalts in general, and those derived from the HIMU mantle end-member in particular.The key similarities in major and minor element signatures include low SiO2 and highTiO2 ,F eO*, CaO, and Na2O.The main discrepancies between G2C partial melts and natural alkalic OIBs are higher Al2O3, lower CaO/Al2O3, and lower MgO of the former. We hypothesize that such discrepancies might be resolved if carbonated MORB-like pyroxenite produces partial melts at somewhat higher pressures and if a hybrid peridotite^carbonated pyroxenite source is considered. Geodynamic consideration of carbonated silicate melting of pyroxenite bodies beneath ocean islands suggests that volatile-enriched alkalic OIBs with the HIMU signature are probably generated from subducted, carbonated crust over a depth range, with the onset of melting as deep as 180^200 km, for a potential temperature of � 15008C. However, rather than a direct decompression of carbonated ocean crust, the key processes involved in the generation of a carbonated silicate melt in equilibrium with MORB-pyroxenite may involve melt^rock reaction and melt^melt mixing.

Formation of High-Mg Diorites through Assimilation of Peridotite by Monzodiorite Magma at Crustal Depths

Abstract: The 134^130 Ma Han-Xing diorite^monzodiorite complex intrudes the central part of the North China block (NCB).The high-Mg diorites have intermediate SiO2, high MgO, Cr and Ni contents, elevated Mg-numbers (Mg#), and are characterized by high Sr and Ba, highly fractionated rare earth elements (REE), and strong Nb and Ta depletions. These geochemical signatures are similar to those of high-Mg adakite and sanukitoid magmas that form through complex crust^mantle interactions. We have investigated felsic magma^peridotite interactions in the Han-Xing diorite complex by undertaking a detailed petrological and bulk-rock and mineral trace element study. The high-Mg diorites contain olivine xenocrysts and dunite xenoliths that have been partly resorbed by the quartz-bearing matrix. These textures, coupled with the high Cr and Ni, and low Ca, Al, P, Y, Sc and Zr contents, provide evidence that the olivines were derived from a depleted peridotite. The assimilation of olivine produced diorite with highly variable MgO, Cr, Ni and Mg# values. However, all rocks have similar incompatible element patterns, indicating that they originate from a common pristine magma.The relative timing of olivine assimilation is constrained by reversely zoned clinopyroxene and amphibole crystals. The cores of the reversely zoned minerals have low Mg# values (clinopyroxene, � 75; amphibole, � 60) and REE characteristics indicating an origin by crystallization from a felsic magma compositionally similar to the monzodiorites. Inclusions of andesine, K-feldspar and biotite are present in the low-Mg# cores of reversely zoned clinopyroxenes, and are further proof of the felsic character of the pristine magma.The high-Mg (up to 90) midsections of the reversely zoned minerals have high Cr and Ni contents and lower but subparallel REE, indicating assimilation of peridotite by the monzodiorite magma. An eclogite residue for the pristine magma is precluded by the presence of plagioclase inclusions, and low Na2 Oa nd Al2O3 and unfractionated heavy REE in the low-Mg# cores of the reversely zoned clinopyroxenes. The maximum depth of the peridotite assimilation is estimated to be � 20 km, using the Al-inamphibole geobarometer for the high-Mg# midsection of a reversely zoned amphibole. This provides further evidence that the interaction of felsic magma with peridotite occurred at crustal rather than mantle depths. We suggest that the peridotite was emplaced at middle or upper crustal levels by previous orogenic processes.The original monzodiorite magma was probably produced in an extensional setting by partial melting of lower crust. Our observations indicate that the delamination model cannot be applied to the petrogenesis of the Han-Xing high-Mg diorite. The interaction of felsic melt with ultramafic rocks at crustal pressures has not been considered as a mechanism for the generation of high-Mg adakites and sanukitoids so far. This might have important implications for crustal evolution. Whereas in previous models sanukitoids have been related to either crustal growth by partial melting of enriched mantle peridotite or crust destruction through lower crust delamination, in the case reported here, they document crustal fractionation through anatexis of lower crustal rocks.

Petrology, Geochemistry and Geochronology of Kaua‘i Lavas over 4·5 Myr: Implications for the Origin of Rejuvenated Volcanism and the Evolution of the Hawaiian Plume

Abstract: Kaua‘i lavas provide a unique opportunity to examine over 4·5 Myr of magmatic history at one location along the Hawaiian chain. New field, geochronological, petrological and geochemical results for a large suite of shield, post-shield and rejuvenated lavas are used to examine models for the origin of rejuvenated volcanism, and to evaluate the composition and structure of the Hawaiian plume. Kaua‘i has the most voluminous ( 58 km based on new field and water well interpretations) and longest-lived suite of rejuvenated lavas ( 2·5 Myr) in Hawai‘i. New K^Ar ages and field work reveal an 1 Myr gap (3·6^2·6 Ma) in volcanism between post-shield and rejuvenated volcanism. Isotopic and trace element ratios, and modeling of major elements of Kaua‘i’s rejuvenated lavas require low-degree melting (0·02^2·6%) at 1525 108C and 3·5^4·0 GPa of a heterogeneous, peridotitic plume source. High-precision Pb, Sr, Nd and Hf isotopic, and inductively coupled plasma mass spectrometry trace element data show substantial source variations with a dramatic increase in the depleted component in younger lavas. Some shield, post-shield and rejuvenated lavas (4·3^0·7 Ma) have high Pb*/Pb* (radiogenic Pb produced since the formation of the Earth) values (40·947) indicative of Loa-type compositions, the first reported Loa values in rocks 43 Ma, questioning previous models for the emergence of the Loa component in Hawaiian lavas.The timing, long duration, temporal variation in rock types and voluminous pulse of rejuvenated volcanism (58 km), and the synchronous eruption of compositionally similar rejuvenated lavas, indicating tapping of common components along 350 km of the Hawaiian chain, are inconsistent with current models for this volcanism. Combining the lithospheric flexure and secondary zone of melting models provides a physical mechanism to initiate and focus the melting at shallower levels within the plume (flexural uplift) with a means to extend the duration of Ko loa volcanism at higher degrees of partial melting.

Microstructural and Rheological Evolution of a Mantle Shear Zone

Abstract: We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2^3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domainsca relatively coarse-grained domain (� 550m) that contains only olivine and a finer-grained domain (� 250m) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker.The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution.The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.