Showing papers in "Journal of Petrology in 2008"

A Geochemical Classification for Feldspathic Igneous Rocks

Abstract: In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO þMgO) ratio or Fe-index, the modified alkali^lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index.The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not. The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %. We introduce the feldspathoid silica-saturation index, which, coupled with the alkalinity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe^Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan granites to nepheline syenites that commonly occur in intracratonic environments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline syenites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which originate from mantle that has been enriched in K2O.

Sediment Melts at Sub-arc Depths: an Experimental Study

Abstract: The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T¼600^10508C, P¼25^45 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnet-phengite-clinopyroxene-coesite-kyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na2O (up to 05 wt%), P2O5 (up to 056 wt%), and TiO2 (up to 09 wt%) in all experiments. Phengite is stable up to 10008C at 45 GPa and is characterized by high TiO2 contents of up to 2 wt%. The solidus has been determined at 7008C, 25 GPa and is situated between 700 and 7508C at 35GPa. At 8008C, 45 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 7008C, 35 and 45 GPa, a solute-rich, non-quenchable aqueous fluid was present.This indicates that the solidus is steeply sloping in P^Tspace. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid = melt + garnet. The H2O content of the produced melt decreases with increasing temperature. The K2O content of the melt is buffered by phengite and increases with increasing temperature from 25 to 10 wt%, whereas Na2O decreases from 7 to 23 wt%. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K2O/H2O increases strongly as a function of temperature and nature of the fluid phase. It is 00004-0002 in the aqueous fluid, and then increases gradually from about 1 at 750-8008C to about 1 at 1000C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K2O/H2O of 01^04, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700-9008C.

Rhyolites and their Source Mushes across Tectonic Settings

Abstract: Evolved magmas, including highly explosive rhyolites, are mainly generated by extraction of viscous melts from solid residues either in (1) partial melting zones within the crust (dominantly up-temperature evolution with newly formed silicic melt), or in (2) long-lived crystallizing mush zones fed by mafic to intermediate magmas (dominantly down-temperature evolution with residual silicic melt). Although both processes undoubtedly occur and are generally coupled, allowing for mixing between mantle and crustal components, we argue that combined field, thermal, geochemical, and geophysical observations favor residual melt extraction from crystalline mushes as the likely scenario in all tectonic settings. Depending on the main melting process in the mantle, two end-member differentiation trends occur: (1) a dry lineage leading to hot-reduced rhyolites and granites in magmatic provinces fueled by decompression melting of the mantle; (2) a wet lineage leading to cold-oxidized rhyolites and granites in subduction zones dominated by flux melting of the mantle.

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Abstract: Experimental phase equilibrium and trace element partitioning data are reported for H 2 O-saturated mid-ocean ridge basalt at 2·5 GPa, 750-900°C and oxygen fugacities at the nickel-nickel oxide buffer. Garnet, omphacite and rutile are present at all temperatures. Amphibole and epidote disappear as residual phases above 800°C; allanite appears above 750°C. The Na-Al-rich silicate glass present in all run products is likely to have quenched from a supercritical liquid. Trace element analyses of glasses demonstrate the important control exerted by residual minerals on liquid chemistry. In addition to garnet, which controls heavy rare earth elements (HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanite buffers the light REE (LREE; La-Sm) contents of liquids to relatively low levels and preferentially holds back Th relative to U. In agreement with previous experimental and metamorphic studies we propose that residual allanite plays a key role in selectively retaining trace elements in the slab during subduction. Experimental data and analyses of allanite-bearing volcanic rocks are used to derive a model for allanite solubility in liquids as a function of pressure, temperature, anhydrous liquid composition and LREE content. The large temperature dependence of allanite solubility is very similar to that previously determined for monazite. Our model, fitted to 48 datapoints, retrieves LREE solubility (in ppm) to within a factor of 1· 40 over a pressure range of 0-4 GPa, temperature range of 700-1200°C and for liquids with anhydrous SiO 2 contents of 50-84 wt %. This uncertainty in LREE content is equivalent to a temperature uncertainty of only ± 27°C at 1000 K, indicating the potential of allanite as a geothermometer. Silicic liquids from either basaltic or sedimentary protoliths will be saturated in allanite except for Ca-poor protoliths or at very high temperatures. For conventional subduction geotherms the low solubility of LREE (+ Th) in liquids raises questions about the mechanism of LREE + Th transport from slab to wedge. It is suggested either that, locally, temperatures experienced by the slab are high enough to eliminate allanite in the residue or that substantial volumes of H 2 O-rich fluids must pass through the mantle wedge prior to melting. The solubility of accessory phases in fluids derived from subducted rocks can provide important constraints on subduction zone thermal structure.

On the Pseudomorphing of Melt-filled Pores During the Crystallization of Migmatites

Abstract: Pseudomorphs of melt-filled pores, recognized by their generally cuspate shape, are used as diagnostic for the former presence of partial melt. They are commonly observed in migmatites from the mid- to deep crust although they occur in the smaller pores in migmatites from shallower levels (1^2 kbar). The pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one. We examine three migmatites in detail: a contact metamorphosed cherty band from an iron formation; an Archaean regional granulite from an accretionary prism; and an amphibolite-facies sample from the roots of an Archaean mountain chain. The greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs. A study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt^solid textural equilibrium was probably attained only in the contact aureole.The regional granulite preserves an almost unmodified reaction-controlled melt distribution, with little evidence for either melt^solid textural equilibration or solid^solid re-equilibration, whereas the reactioncontrolled melt distribution in the regional amphibolite-facies example has been modified by a partial approach to solid^solid textural equilibrium. It is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of H2O on grain boundaries.

Dynamic Magma Systems, Crustal Recycling, and Alteration in the Central Sierra Nevada Batholith: the Oxygen Isotope Record

Abstract: Values of δ 18 O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) δ 18 O provides evidence for modest (0.5‰), but widespread, alteration, which has complicated interpretation in previous whole-rock δ 18 O studies. Four discrete belts of δ 18 O values are recognized in the central Sierra. A small belt of plutons with relatively low δ 18 O(Zrc) values (5·2–6·0‰) intrudes the foothills, with a sharp increase of δ 18 O revealing the concealed Foothills Suture; high δ 18 O(Zrc) values (7·0–8·5‰) dominate the rest of the western SNB. East of the axis of the Sierra, δ 18 O is distinctly lower (6·75–5·75‰), and decreases monotonically to the Sierra Crest. A sharp 1‰ increase of δ 18 O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-δ 18 O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in δ 18 O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, δ 18 O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, δ 18 O reveals diverse magma system behavior at a range of scales in the Sierran arc.

Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

Abstract: Olivine is the principal mineral of kimberlite magmas, and is the main contributor to the ultramafic composition of kimberlite rocks. Olivine is partly or completely altered in common kimberlites, and thus unavailable for studies of origin and evolution of kimberlite magmas. The masking effects of alteration, common in kimberlites worldwide, are overcome in this study of exceptionally fresh diamondiferous kimberlites of the Udachnaya-East pipe from the Daldyn-Alakit province, Yakutia, northern Siberia. The serpentine-free kimberlites contain large amount of olivine (~ 50 vol%) in a chloride-carbonate groundmass. Olivine is represented by two populations (olivine-I and groundmass olivine-II) differing in morphology, colour and grain size, and trapped mineral and melt inclusions. The large fragmental olivine-I is compositionally variable in terms of major (Fo85-94) and trace element concentrations, including H2O content (10-136 ppm). Multiple sources of olivine-I, such as convecting and lithospheric mantle, are suggested. The groundmass olivine-II is recognised by smaller grain sizes and perfect crystallographic shapes that indicate crystallisation during magma ascent and emplacement. However, a simple crystallisation history for olivine-II is complicated by complex zoning in terms of Fo values and trace element contents. The cores of olivine-II are compositionally similar to olivine-I, which suggests a genetic link between these two types of olivine. Olivine-I and olivine–II have oxygen isotope values (+5.6 ± 0.1 ‰ VSMOW, 1 std. dev.) that are indistinguishable from one another, but higher than values (+5.18 ± 0.28 ‰) in “typical” mantle olivine. These elevated values most likely reflect equilibrium with the Udachnaya carbonate melt at low temperatures and 18O - enriched mantle source. The volumetrically significant rims of olivine-II have constant Fo values (89.0 ± 0.2 mol%), but variable trace element compositions. Uniform Fo compositions of the rims imply absence of fractionation of the melt’s Fe2+/Mg, which can be possible in the carbonatite melt – olivine system. The kimberlite melt is argued to have originated in the mantle as a chloride-carbonate liquid, devoid of “ultramafic” or “basaltic” aluminosilicate components, but became olivine-laden and olivine-saturated by scavenging olivine crystals from the pathway rocks and dissolving them en route to the surface. During emplacement the kimberlite magma changed progressively towards an original alkali-rich chloride-carbonate melt by extensively crystallising groundmass olivine and gravitational separation of solids in the pipe.

Origin of Pyroxenite–Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

Abstract: The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase^spinel websterites. The ‘’ part of the Ronda peridotite contains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitionstowardstheirhost peridotites. Previousstudieshave suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modellingstudy ofa layered outcrop ofgroup C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt^ rock reactions. Mg-number [¼ Mg/(Mg þ Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (� 0� 65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (40� 1and41, respectively) to balance Mg^Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convexupward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)N values, two transient, enriched domains between the light REE (LREE)depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite^peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions.The latterare probably volatile-rich, small-volume melt fractions residual after the refertilization reactionsthatgeneratedgroup C pyroxenites, which migrated throughout the massifcincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’subdomain and ‘plagioclase-tectonite’domain) are on average morefertilethan the residual,‘coarsegranular’subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.

Global Correlations of Ocean Ridge Basalt Chemistry with Axial Depth: a New Perspective

Abstract: Niu, Y., O'Hara, M. J. (2008). Global correlations of ocean ridge basalt chemistry with axial depth: A new perspective. Journal of Petrology, 49 (4), 633-664.

Control of the Products of Serpentinization by the Fe2+Mg –1 Exchange Potential of Olivine and Orthopyroxene

Abstract: It is argued that the high-Mg content (mg-number1⁄4 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental FeMg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite.This causes hydrogen to be evolved in an oxygen-conserved reaction.The low-variance mineral assemblage Olþ SrpþBrcþMag sets the chemical potentials of H2O, SiO2 and O2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe3þ content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe/SFe ratios40 4, with an average value (0 69) similar to that of magnetite (0 67). This feature may be attributed to the presence of high-Fe lizardite, as has been found in Mo« ssbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs SFe a.p.f.u.) for olivinepseudomorph lizardite and, with some exceptions, for bastite lizardite, that show a substitution of the cronstedtite component (Fe charge-balanced onTand M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe for Fe. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than lizardite, facts that may be attributed to the greater Fe content of lizardite. Chrysotile veins provide the best record available to us of the environmental FeMg 1 exchange potential in the pore fluid attending serpentinization.This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe can lead to inappropriate petrological conclusions in the case of serpentinites.

Phase Relations and Liquid Lines of Descent in Hydrous Ferrobasalt—Implications for the Skaergaard Intrusion and Columbia River Flood Basalts

Abstract: Crystallization experiments using a hydrous ferrobasalt as starting material, conducted at 200MPa, 940^12008C, at a wide range of water activities (0 1^1) and redox conditions (QFM 3 to QFMþ 4, where QFM is the quartz^fayalite^magnetite oxygen buffer), show that H2O influences significantly the differentiation history of ferrobasaltic magmas. A combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical Fe-rich tholeiitic system under both dry and hydrous conditions.The addition of H2O decreases liquidus temperatures and changes significantly the proportions, temperature range and sequence of crystallizing mineral phases. The dry liquidus is at about 11708C whereas the liquidus for H2O-saturated melts is at 10608C.The main phases crystallizing from H2O-bearing ferrobasalt at the investigated conditions are olivine (OL), clinopyroxene (CPX), plagioclase (PL), magnetite (MT), hematite (HM), ilmenite (ILM) and amphibole (AM). The phase assemblage is similar to that of the dry system except for the presence of HM at extremely oxidizing conditions and AM at low temperatures (59508C) and H2O-saturated conditions. The important observation made in this study is that the stability of Fe^Ti-oxides, and in particular MT, as well as the simultaneous coprecipitation of MTand ILM, are almost independent of the activity of H2O (aH2O) in the system, whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing aH2O.The stabilities of oxides are controlled mainly by the redox conditions prevailing in the system.The most pronounced effect of aH2O on the liquidus temperatures of silicates is observed for PL, which shows a considerable delay in crystallization with progressive magma differentiation. Early crystallization of Fe^Ti-oxides in H2O-bearing ferrobasaltic compositions precludes any significant Fe enrichment during differentiation. As Fe enrichment is a characteristic feature of the Skaergaard intrusion, it implies that the Skaergaard parental magma did not contain considerable amounts of water. On the other hand, our experiments indicate that the differentiation of some ferrobasaltic series from the Columbia River flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles ( 0 5^3 wt%H2O).

Origin and Evolution of Silicic Magmatism at Yellowstone Based on Ion Microprobe Analysis of Isotopically Zoned Zircons

Abstract: The origin of large-volume Yellowstone ignimbrites and smallervolume intra-caldera lavas requires shallow remelting of enormous volumes of variably O-depleted volcanic and sub-volcanic rocks altered by hydrothermal activity. Zircons provide probes of these processes as they preserve older ages and inherited dO values.This study presents a high-resolution, oxygen isotope examination of volcanism at Yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size.We report 357 analyses of cores and rims of zircons, and isotope profiles of 142 single zircons in 11 units that represent majorYellowstone ignimbrites, and post-caldera lavas. Many zircons from these samples were previously dated in the same spots by sensitive high-resolution ion microprobe (SHRIMP), and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination. We additionally report oxygen isotope analyses of quartz crystals in three units.The results of this work provide the following new observations. (1) Most zircons from post-caldera low-dO lavas are zoned, with higher dO values and highly variable U^Pb ages in the cores that suggest inheritance from pre-caldera rocks exposed on the surface. (2) Many of the higher-dO zircon cores in these lavas have U^Pb zircon crystallization ages that postdate caldera formation, but pre-date the eruption age by 10^20 kyr, and represent inheritance of unexposed post-caldera sub-volcanic units that have dO similar to the Lava Creek Tuff. (3) Young and voluminous 0 25^0 1Ma intra-caldera lavas, which represent the latest volcanic activity atYellowstone, contain zircons with both high-dO and lowdO cores surrounded by an intermediate-dO rim. This implies inheritance of a variety of rocks from high-dO pre-caldera and low-dO post-caldera units, followed by residence in a common intermediate-dO melt prior to eruption. (4) Major ignimbrites of Huckleberry Ridge, and to a lesser extent the Lava Creek and Mesa Falls Tuffs, contain zoned zircons with lower-dO zircon cores, suggesting that melting and zircon inheritance from the lowdO hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse. (5) The dO zoning in the majority of zircon core^rim interfaces is step-like rather than smoothly inflected, suggesting that processes of solution^reprecipitation were more important than intracrystalline oxygen diffusion. Concave-downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals.This study suggests that silicic magmatism atYellowstone proceeded via rapid, shallow-level remelting of earlier erupted and hydrothermally alteredYellowstone source rocks and that pulses of basaltic magma provided the heat for melting. Each postcaldera Yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and dO values.The commonly held model of a single, large-volume, super-solidus, mushy-state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data. Rather, the source rocks for theYellowstone volcanism were cooled below the solidus, hydrothermally altered by heated meteoric waters that caused low dO values, and then remelted in distinct pockets by intrusion of basic magmas. Each packet of new melt inherited zircons that retained older age and dO values.This interpretation may have significance for interpreting seismic data for crustal low-velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side. New basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas. We also suggest that the Lava Creek Tuff

Magma Dynamics with the Enthalpy Method: Benchmark Solutions and Magmatic Focusing at Mid-ocean Ridges

Abstract: Magma genesis and transport link mantle convection with surface volcanism and hence with the long-term chemical and morphological evolution of the Earth's; crust. Modeling the dynamics of magma-mantle interaction in tectonic settings remains a challenge, however, because of the complexity of multi-component thermodynamics and melt segregation in a permeable, compactible, and actively deforming mantle matrix. Here I describe a flexible approach to formulating the thermochemistry of such models based on the Enthalpy Method, a technique commonly used in simulations of alloy solidification. This approach allows for melting and freezing based on a familiar binary phase diagram, consistent with conservation of energy and two-phase compaction and flow. I present an extension of the Enthalpy Method to more than two thermodynamic components. Simulation of a one-dimensional upwelling and melting column provides a benchmark for the method. Two-dimensional simulations of the melting region that feeds magma to a rapidly spreading mid-ocean ridge demonstrate the utility of the Enthalpy Method. These calculations provide a new estimate of the efficiency of magmatic focusing along the base of the oceanic lithosphere. Modeled focusing efficiency varies with mantle permeability and resistance to compaction. To yield 5-7 km of oceanic crust with ∼20% melting of a homogeneous, sub-ridge mantle, a focusing efficiency of greater than 70% is required. This, in turn, suggests that matrix permeability and bulk viscosity are at the high end of previously estimated values.

The Titanomagnetite–Ilmenite Equilibrium: New Experimental Data and Thermo-oxybarometric Application to the Crystallization of Basic to Intermediate Rocks

Abstract: Although the titanomagnetite^ilmenite thermo-oxybarometer has been widely used to provide information on temperature and oxygen fugacity during magmatic and metamorphic processes, the available formulations yield unsatisfactory results; for example, at high temperature and low to moderate fO2 (i.e. in conditions relevant to crystallization in basic and intermediate rocks).We present a new version of this thermo-oxybarometer based on numerical fits of a large experimental dataset comprising new results in the Fe^Ti^Al^Mg^O system and those of literature studies. Our new subsolidus experimental results at temperatures in the range 1100^13008C under low to moderate fO2 conditions show that the addition of Mg and/or Al in the concentration ranges that are usual in Fe^Ti oxides from basic magmatic rocks can be accommodated by simple projections. We have taken advantage of this fact and performed numerical fits to generate empirical formulations.With the resulting expressions we can retrieve temperature values from Xusp and Xilm (projected mole fractions) of titanomagnetite^ilmenitess pairs and fO2 values from Xusp and T. The present thermooxybarometer model is designed for assemblages of titanomagnetite and hemoilmenite (with the R 3 space group), with the usual low Al2O3, Cr2O3, MgO and MnO contents (less than about 6 wt %), which equilibrated at high temperatures (T 8008C) and low to moderate oxygen fugacities (^45 NNO5þ2, where NNO is the nickel^nickel oxide buffer).Tests of our model by using the compositions of titanomagnetite^ilmenitess pairs in products of liquidus experiments conducted at knownT^fO2 conditions (literature data and new results) show that the calculated values reproduce the experimental ones within 708C, and in most cases within 508C. The estimates of the oxygen fugacity are mostly within 0 4 log units. This is a significant improvement compared with the previous models.

Experiments and Models Bearing on the Role of Chromite as a Collector of Platinum Group Minerals by Local Reduction

Abstract: Chromite is widely recognized to act as a collector for platinum group elements (PGE), which tend to be observed as discrete grains of platinum group minerals included within magmatic chromite grains. In the course of experiments involving the re-equilibration or growth of chromite and Cr-spinel in molten silicate, we observe that platinum group minerals (PGM; including metal alloys and laurite) form at the mineral^melt interface. The formation of PGM to the extent observed requires a mechanism involving sustained transport of PGE from a source within the experiment to the site of deposition. We propose that the driving force for this process is a redox gradient developed in response to mineral growth or re-equilibration with the surrounding melt. The mechanism is local reduction within the mineral^melt interfacial region as a consequence of the selective uptake of trivalent Cr and Fe from the melt by spinel relative to the divalent species.We have modeled the transient perturbation of fO2 in a compositional boundary layer melt around spinel for both crystal growth and diffusive re-equilibration of mineral and melt.We find that metal solubilities decrease by several per cent in the silicate melt at the melt^crystal interface during crystal growth, providing the driving force for PGM formation. In magmas that are saturated with PGM, as a result of falling temperature and oxygen fugacity during spinel crystallization, nucleation of PGM will be impeded by interfacial tension everywhere except in the reduced boundary layer around spinel crystals. The resulting concentration and trapping of alloy particles in the growing chromite crystals can produce apparent bulk chromite/melt partition coefficients exceeding 20 even if there is no solid solution of PGE in the chromite.The introduction of spinel grains, initially equilibrated with a mafic magma, into a more primitive magma, with higher Cr/Al, would lead to disequilibrium between chromite and melt. The perturbation of fO2 in the compositional boundary layer surrounding a chromite xenocryst would exceed 0 1 log unit, leading to local reduction of alloy solubility of the order of 13^18%. A small number of spinel xenocrysts could serve as collection sites for all of the excess PGE in the magma, leading to the eventual observation that a few chromite crystals contain many PGM inclusions, whereas the rest of the chromite population may be relatively free of PGM.

Concurrent Mixing and Cooling of Melts under Iceland

Abstract: The compositions of 75 melt inclusions, their host olivines and 49 whole-rock samples of their carrier lavas have been determined. These compositions were added to a compilation of the trace element composition of 243 melt inclusions from 10 eruptions in the neovolcanic zones of Iceland and used to investigate melt mixing processes. Whereas the compositional variability of whole-rock samples from single eruptions is limited, there are significant compositional differences between eruptions. The compositions of inclusions are more variable than those of whole-rock samples of their carrier lava. On a flow-by-flow basis, the average composition of trace element ratios such as La/Yb in the inclusions is similar to that of their carrier lava. These observations indicate that, for each lava flow, melts with compositions similar to those of the inclusions crystallized and mixed to produce the magma that transported the host olivines to the surface. Although many of the olivines are not in Mg–Fe equilibrium with their host melt, they are not accidental xenocrysts because they crystallized from melts similar to those that mixed to form the carrier magma. The trace element variability of melt inclusions drops with decreasing forsterite content of the host olivine. This relationship is observed both within single flows and in the compilation of data from 10 flows. Concurrent mixing and crystallization dominate the compositional evolution of basaltic melts in lower crustal magma chambers. This coupled mixing and cooling is likely to result from convective motions in magma chambers. The rate of change of the mixing parameter, M, with temperature of the melt is dM/dT = 0·0094 ± 0·0036 per °C. This relative rate may be used to constrain the fluid dynamics of basaltic magma chambers.

Volatiles in High-K Magmas from the Western Trans-Mexican Volcanic Belt: Evidence for Fluid Fluxing and Extreme Enrichment of the Mantle Wedge by Subduction Processes

Abstract: Primitive, high-K minettes and basanites erupted during the Pleistocene from cinder cones on the flanks of the Colima Volcanic Complex in the western Trans-Mexican Volcanic Belt Melt inclusions in olivine (Fo89^92) from tephra at these cones reveal that both magma types are oxidized and volatile rich, with high H2 O( � 6� 2 wt%), CO2 (� 5300 ppm), S (� 6700 ppm), Cl (� 2300 ppm), and F (� 8100 ppm) contents A nearby calc-alkaline basaltic andesite cinder cone with more evolved composition (Fo78^80 olivine) has melt inclusions with similarly high H2O (� 5� 5 wt%) but much lower CO2, S, and Cl compared with the potassic magmas Melt inclusions from each cone have highly variable H2O and CO2, corresponding to crystallization pressures of5100 bars to � 7 kbar This indicates that olivine crystallized from variably degassed melts over a wide range of depths extending from the lower crust (4 25 km depth) to very shallow levels The H2O and CO2 variations cannot be explained by simple degassing models but instead requiring more complex, open-system processes or possibly reflect disequilibrium degassingTrace element variations in the melt inclusions suggest that phlogopite and garnet were residual minerals during melting in the mantle source, and the presence of garnet suggests an origin in asthenospheric rather than lithospheric mantle Decompression melting of phlogopite^garnet peridotite cannot produce the high H2O contents of the potassic magmas, and thus the presence of fluids during melting is required Trace element modeling of a mantle source (intermediate in composition between enriched mid-ocean ridge basalt and ocean island basalt sources) that is fluxed with an H2O-rich fluid or hydrous melt from the subducting slab can reproduce most of the trace element characteristics of the potassic melts, demonstrating that they are clearly linked with subduction processes Formation ofthe potassic magmasprobably involved slab rollback, trenchward migration of the arc into the region above metasomatically enriched forearc mantle, and heating of this veined and fluid-fluxed mantle as a result of upwelling of hot mantle through a tear between the subducted Cocos and Rivera plates

Juxtaposition of Melt Impregnation and High-Temperature Shear Zones in the Upper Mantle; Field and Petrological Constraints from the Lanzo Peridotite (Northern Italy)

Abstract: Results of a field and microstructural study between the northern and the central bodies of the Lanzo plagioclase peridotite massif (NW Italy) indicate that the spatial distribution of deformation is asymmetric across kilometre-scale mantle shear zones. The southwestern part of the shear zone (footwall) shows a gradually increasing degree of deformation from porphyroclastic peridotites to mylonite, whereas the northeastern part (hanging wall) quickly grades into weakly deformed peridotites. Discordant gabbroic and basaltic dykes are asymmetrically distributed and far more abundant in the footwall of the shear zone. The porphyroclastic peridotite displays porphyroclastic zones and domains of igneous crystallization whereas mylonites are characterized by elongated porphyroclasts, embedded between fine-grained, polycrystalline bands of olivine, plagioclase, clinopyroxene, orthopyroxene, spinel, rare titanian pargasite, and domains of recrystallized olivine. Two types of melt impregnation textures have been found: (1) clinopyroxene porphyroclasts incongruently reacted with migrating melt to form orthopyroxene plagioclase; (2) olivine porphyroclasts are partially replaced by interstitial orthopyroxene. The meltrock reaction textures tend to disappear in the mylonites, indicating that deformation in the mylonite continued under subsolidus conditions. The pyroxene chemistry is correlated with grain size. High-Al pyroxene cores indicate high temperatures (11001030C), whereas low-Al neoblasts display lower final equilibration temperatures (860C). The spinel Cr-number [molar Cr/(Cr Al)] and TiO2 concentrations show extreme variability covering almost the entire range known from abyssal peridotites. The spinel compositions of porphyroclastic peridotites from the central body are more variable than spinel from mylonite, mylonite with ultra-mylonite bands, and porphyroclastic rocks of the northern body. The spinel compositions probably indicate disequilibrium and would favour rapid cooling, and a faster exhumation of the central peridotite body, relative to the northern one. Our results indicate that melt migration and high-temperature deformation are juxtaposed both in time and space. Meltrock reaction may have caused grain-size reduction, which in turn led to localization of deformation. It is likely that melt-lubricated, actively deforming peridotites acted as melt focusing zones, with permeabilities higher than the surrounding, less deformed peridotites. Later, under subsolidus conditions, pinning in polycrystalline bands in the mylonites inhibited substantial grain growth and led to permanent weak zones in the upper mantle peridotite, with a permeability that is lower than in the weakly deformed peridotites. Such an inversion in permeability might explain why actively deforming, fine-grained peridotite mylonite acted as a permeability barrier and why ascending mafic melts might terminate and crystallize as gabbros along actively deforming shear zones. Melt-lubricated mantle shear zones provide a mechanism for explaining the discontinuous distribution of gabbros in oceancontinent transition zones, oceanic core complexes and ultraslow-spreading ridges.

New Insights into Crustal Contributions to Large-volume Rhyolite Generation in the Mid-Tertiary Sierra Madre Occidental Province, Mexico, Revealed by U–Pb Geochronology

Abstract: Voluminous (≥3·9 × 105 km3), prolonged (∼18 Myr) explosive silicic volcanism makes the mid-Tertiary Sierra Madre Occidental province of Mexico one of the largest intact silicic volcanic provinces known. Previous models have proposed an assimilation–fractional crystallization origin for the rhyolites involving closed-system fractional crystallization from crustally contaminated andesitic parental magmas, with 33 to ∼100% of the dated population; most antecrysts range in age between ∼20 and 32 Ma. A sub-population of the antecrystic zircons is chemically distinct in terms of their high U (>1000 ppm to 1·3 wt %) and heavy REE contents; these are not present in the Oligocene ignimbrites in the northeastern sector of the Sierra Madre Occidental. The combination of antecryst zircon U–Pb ages and chemistry suggests that much of the zircon in the youngest rhyolites was derived by remelting of partially molten to solidified igneous rocks formed during preceding phases of Sierra Madre Occidental volcanism. Strong Zr undersaturation, and estimations for very rapid dissolution rates of entrained zircons, preclude coeval mafic magmas being parental to the rhyolite magmas by a process of lower crustal assimilation followed by closed-system crystal fractionation as interpreted in previous studies of the Sierra Madre Occidental rhyolites. Mafic magmas were more probably important in providing a long-lived heat and material flux into the crust, resulting in the remelting and recycling of older crust and newly formed igneous materials related to Sierra Madre Occidental magmatism.

Crystallization of Groundmass Spinel in Kimberlite

Abstract: Groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared. The majority of the spinel analyses are classified as high-chromium chromite (Chr) and magnesioulvo« spinel^magnetite (Mum) and represent two significant stages of spinel growth. There are also a significant number of spinel grains that are classified as xenocryst spinel (Xen), pleonaste spinel (Ple) and magnetite (Mag). Eight different spinel zoning trends are identified.The majority of the Chr spinel grains are interpreted as a primary phase that crystallized as small octahedra from kimberlite magma on the journey from the upper mantle to the final resting place in the upper crust. Three zoning trends lead directly away from primary chromite. The major zoning trend, Trend 1, is from chromite to magnesio-ulvo« spinel^magnetite. This zoning trend is unique to spinel in kimberlite, carbonatites and lamprophyres. We suggest that this somewhat oxidizing, and more magnesian, trend was influenced by the high carbonate content of Group I kimberlites and the rapid crystallization of the minerals during the evolution of volatiles.The zoningTrend 2 involves increasing titanium and ferric iron as a function of increasing Fe 2þ /(Fe 2þ þ Mg). This trend is similar to the zoning of spinel in basalt and is thought to be due to co-crystallization of magnesium- and aluminum-rich silicate minerals such as olivine and phlogopite in kimberlites, or pyroxene and plagioclase in basalt. Zoning Trend 3 in kimberlite leads away from primary chromite and towards an aluminous pleonaste (Ple) spinel. This trend is characterized by a large decease of Cr/(Cr þAl) parallel to so-called olivine^spinel iso-potential lines. Similar trends of lesser magnitude and cyclic Al^Cr zoning have been identified in basaltic spinel. This trend is thought to be due to very rapid crystallization under conditions of supersaturation where the crystallization of spinel affects the local environment ahead of the growing spinel crystal (i.e. diffusion-controlled crystallization). The tendency for immiscibility between ferrite- or titanate-rich spinel, and aluminate-rich spinel (pleonaste) has a great influence on Trends 1 and 3 zoning and also on atoll-spinel formation. Very local conditions such as nucleation, or lack of nucleation, of other minerals can influence both the textural environment and composition of kimberlitic spinel.

Assimilation of Plutonic Roots, Formation of High-K ‘Exotic’ Melt Inclusions and Genesis of Andesitic Magmas at Volcán De Colima, Mexico

Abstract: Phenocryst-hosted melt inclusions from the 1998^2005 andesite eruptions of Volcan de Colima (Mexico) show broad ranges of major and trace element contents that do not overlap with the bulk- rock compositions and indicate that melt inclusions can be formed by and record a range of processes involved in the genesis of andesites. The melt inclusions that demonstrably record the evolution of the melt feeding the eruption indicate low-pressure (130^10 MPa) crys- tallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcan de Colima. Mingling of dacite melt with gabbroic fragments in the shallow sub-volcanic system is the process responsible for generating the bulk andesitic composition of the magmas. A significant proportion of the melt inclusions have distinctive high large ion lithophile element (LILE) signatures. These 'exotic' high-K melt inclusions in pyrox- enes are thought to result from incongruent melting of interstitial bio- tite during assimilation of gabbroic fragments in the dacitic melt. A second group of exotic high-K melt inclusions found in plagioclase are likely to result from dissolution of higher-pressure (4200 MPa) amphibole, plagioclase, magnetite and biotite cumulates during assimilation in the ascending dacitic melt. Although they are not volumetrically abundant, high-K melts formed during assimilation of plutonic fragments and crystal cumulates made a significant con- tribution to the LILE contents of the magmas and represent a poten- tial source for this group of elements. The range of melt inclusion compositions in Volcan de Colima magmas emphasizes the impor- tance of mixing between ascending evolved melts and crystal popula- tions formed during previous episodes of magmatism over a range of pressures, temperatures and volatile contents. Cannibalization of plutonic roots appears to be a fundamental process in the genesis of andesite magmas and melt inclusions at continental arc volcanoes.

Slab-Derived Fluids in the Magma Sources of St. Vincent (Lesser Antilles Arc): Volatile and Light Element Imprints

Abstract: It is generally accepted that the parental magmas of the Lesser Antilles arc were generated by partial melting of a mid-ocean ridge basalt (MORB)-type mantle source modified by slab-derived components.To determine the nature of these components, the H2O, S, Cl, F, Li and B contents and Li, B, O and S compositions were systematically determined in olivine-hosted melt inclusions from St.Vincent (southern part of the arc). Both the geochemical and isotopic data define a broad compositional spectrum. On the whole, the melt inclusions have basaltic to CaO-rich (415 0 wt %), SiO2-poor (545 7 wt %) compositions. Most of the entrapped melts result from 10^20% batch partial melting of a MORBtype mantle source modified initially by dehydration fluids with low solute contents and a seawater-like chemical signature. As a result, the melt inclusions are enriched in B, Cl and H2O compared with MORB and have B up toþ15o, S of 2o and O down toþ 3o. In contrast, some others record initial magmatic heterogeneities that require input of fluids derived from (1) the dehydration of altered oceanic crust in agreement with the selective B enrichment (up to 53 ppm) in the melt and negative lithium isotopic compositions, and (2) the dehydration of sediments resulting in distinctive B and S (down to ^20o and ^8o, respectively) and high Li contents in the melts. The CaO-rich melt inclusions cannot be distinguished from the others on the basis of their isotopic signatures.They possibly reflect magma interactions with CaO-rich, amphibole-bearing lithologies. Combination of our results with literature experimental data leads to the conclusion that St. Vincent basaltic melt inclusionscwhose water content varies from 2 2 to 3 6 wt %crepresent magmas derived from a rather limited portion of the mantle wedge, by partial melting at between 13 and 14 5 kbar and a restricted temperature range (1220^11908C).

A Rhyolite Compositional Continuum Governed by Lower Crustal Source Conditions in the Taupo Volcanic Zone, New Zealand

Abstract: Rhyolites generated in the modern Taupo Volcanic Zone (TVZ), New Zealand, have previously been interpreted as having evolved by a combination of extensive fractional crystallization of mantle-derived mafic magmas and limited crustal assimilation (up to 25%). Polytopic vector analysis (PVA), a form of multivariate statistical analysis, of the major-element compositions of over 475 basaltic to rhyolitic bulk-rock samples, representing over 600 kyr of volcanism within the TVZ, has provided a robust platform for rhyolite characterization and new insights into rhyolite petrogenesis. There is a continuum of compositions between two rhyolite end-member magma types (EM1 and EM2), which have been identified on the basis of the PVA and which have distinct petrological and geochemical characteristics, as follows. EM1: crystal-rich (up to 45%), hydrous phases (± hornblende ± biotite ± cummingtonite), high Aluminum Saturation Index [ASI; molar Al 2 O 3 /(CaO + Na 2 O + K 2 O)], low FeO*/MgO (calc-alkaline series), depleted abundances of middle rare earth elements (MREE) and Y, and high Sr; EM2: crystal-poor (<10%), anhydrous phases (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), low ASI, less depleted MREE and Y, and low Sr. The range of ASI values, and relative depletion in MREE and Y in the rhyolites is consistent with the results of experiments to constrain the partial melting behaviour of amphibolite at crustal pressures. The major- and trace-element data are also consistent with 50-60% equilibrium crystallization of a crustally contaminated, hornblende-bearing andesite to produce the TVZ rhyolites. Distinct major- and trace-element variations along the continuum between the two rhyolite end-member types can be effectively modelled by simulating changes in the temperature-fO 2 -fH 2 O conditions in the lower crust where mantle-derived mafic magmas are stored and differentiate. Low T and high fO 2 and fH 2 O in the crustal magma storage zone promote abundant hornblende crystallization and suppress plagioclase crystallization, which produces the EM1 type rhyolite. By increasing the temperature and/or lowering fO 2 and fH 2 O in the magma storage region, plagioclase becomes more dominant and hornblende crystallization is suppressed, producing more EM2-like rhyolitic magma types.

Petrology of a Late Archaean, Highly Potassic, Sanukitoid Pluton from the Baltic Shield: Insights into Late Archaean Mantle Metasomatism

Abstract: at 1^1� 5 GPa. The precise cause of the melting event at 2� 74 Ga is not known, although a model involving upwelling of asthenospheric mantle following slab break-off is consistent with the geochemical evidence for the enrichment of the Karelian subcontinental mantle lithosphere by subduction fluids.

The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1·5 GPa: Implications for the Petrogenesis of MORB

Abstract: We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1·5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at °C; 1270°C. At only ∼5°C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to ≥ 0·8 GPa is strongly dependent on bulk-rock CaO/Na2O and normative plagioclase content in the peridotite. At 1·5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at ∼C; 1350°C. We have determined a near-solidus melt composition at ∼C; 2% melting within 10°C of the solidus. Near-solidus melts at both 1 and 1·5 GPa are nepheline normative, and have low normative diopside contents; also they have the highest TiO2, Al2O3 and Na2O, and the lowest FeO and Cr2O3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1·5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within 'melt traps' (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method. © The Author 2008. Published by Oxford University Press. All rights reserved.

The Roles of Fractional Crystallization, Magma Mixing, Crystal Mush Remobilization and Volatile–Melt Interactions in the Genesis of a Young Basalt–Peralkaline Rhyolite Suite, the Greater Olkaria Volcanic Complex, Kenya Rift Valley

Abstract: The Greater Olkaria Volcanic Complex is a young ( 20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro–syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ± corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt–trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing.

Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs

Abstract: An 40 Ar/ 39 Ar age of 45·1 Ma determined for lavas from northern Saipan confirms that these high-silica rhyolites erupted during the ‘proto-arc’ stage of volcanism in the Izu-Bonin-Mariana system, which is characterized elsewhere by eruption of boninitic lavas. Incompatible trace element concentrations and Sr, Hf, Nd, and Pb isotope ratios for these rhyolites are transitional between those of c. 48 Ma boninitic lavas and post-38 Ma ‘first-arc’ andesites and dacites from Saipan and Rota that have typical subduction-related compositions. These transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust. A second stage of Rayleigh fractionation in the upper crust is required by SiO 2 concentrations that exceed 77 wt % and near-zero compatible element concentrations. First-arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first-arc magmas themselves. The mantle sources of the proto-arc and first-arc lavas from Saipan and Rota are similar to those of Philippine back-arc basin basalts based on Nd and Hf isotopic compositions. The Pb isotope compositions of these lavas are between those of Pacific sea-floor basalts and Jurassic and younger cherty and clay-rich sediments. This contrasts with the boninitic proto-arc volcanic rocks from Guam and Deep Sea Drilling Project Sites 458 and 459 that have Pb isotope compositions similar to Pacific basin basalts and volcaniclastic sediments. The preferred explanation for the difference in the nature of proto-arc volcanism between Saipan and other fore-arc locations is that the crust ceased extending 3-4 Myr earlier beneath Saipan. This was caused by a change from mantle upwelling, fore-arc extension, and shallow melting to an environment dominated by more normal mantle wedge convection, stable crust, and deeper melting.

The Alkaline–Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source

Abstract: The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (ΔFMQ = +2–5 to −1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (ΔFMQ = −2) and intermediate a SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.

Melt Depletion and Enrichment beneath the Western Kaapvaal Craton: Evidence from Finsch Peridotite Xenoliths

Abstract: We present major- and trace-element analyses of mineral phases present in a suite of 16 garnet-peridotite xenoliths from the western terrane of the Kaapvaal craton. The xenoliths were entrained by a Group II Finsch kimberlite at 118 Ma, shortly prior to a major metasomatic event that caused widespread enrichment of the Kaapvaal lithospheric mantle. Compositionally homogeneous grains of olivine, orthopyroxene, garnet and clinopyroxene and coarse–equant textures indicate equilibrium relationships between mineral phases in the majority of xenoliths. Pressure and temperature estimates suggest that clinopyroxene-bearing garnet peridotites last equilibrated at 1130–1270°C and 45–59 kbar whereas clinopyroxene-free xenoliths record temperatures of 1000–1070°C and pressures of 34–42 kbar. The Finsch xenoliths plot on a conductive palaeogeotherm with a surface heat flux of ~46 mW m2. Combined Ca and Cr abundances of Finsch pyrope garnets suggest that both lherzolitic and harzburgitic parageneses are present. Samples bearing sub-calcic (harzburgitic) garnets are from the shallowest depths. The lherzolitic garnets are depleted in light rare earth elements (LREE) relative to the middle and heavy REE (MREE and HREE) and have ‘smooth’ chondrite-normalized patterns. In contrast, the sub-calcic garnets are characterized by sinusoidal chondrite-normalized REE patterns that peak at Nd and Lu and exhibit lows at La and Er. The sub-calcic garnets also have lower Zr, Hf, Ti and HREE, and higher LREE and Sr, than lherzolitic garnets. The variations in REE ratios correlate with temperature and pressure and also Cr/Ca ratio. The high Cr content of harzburgitic and some lherzolitic Finsch garnets may have a significant effect on the crystal framework. Substitution of the larger Cr3+ ion for the smaller Al3+ ion increases with decreasing temperature and pressure and distorts the crystal lattice; this permits a greater substitution of Ca by large cations, such as Sr and the LREE, but also limits the replacement of Al by Ti, Zr and Hf. Positive HREE slopes displayed by harzburgitic garnets on chondrite-normalized plots are believed to result from metasomatic enrichment by a melt that had already undergone significant garnet fractionation during ascent through the lithospheric mantle. The low-temperature Finsch peridotites are characterized by much lower orthopyroxene (< 17%) and higher olivine (up to 96%) modal abundances than have been reported from xenolith suites elsewhere in the Kaapvaal craton. Significantly, they resemble residues generated in partial melting experiments. The Finsch harzburgites have very low Al2O3 (0·18 wt %) and CaO (0·38 wt %) and high MgO contents (49·75 wt %) and appear to be highly refractory. They also have high bulk-rock Mg/(Mg + Fe) and high modal olivine contents, and in this respect resemble some of those recently described from NW Canada and Greenland. We suggest that some of the Finsch low-temperature peridotites represent Kaapvaal lithospheric mantle that formed as a residue of adiabatic decompression melting between 4·5 and 1·5 GPa. The inferred mantle potential temperature of 1550°C would have been similar to that of ambient Archaean mantle. Importantly, it appears that the sub-Finsch lithospheric mantle has remained unmodified by the silica enrichment that has been so prevalent elsewhere in the craton. This may reflect the remoteness from the subduction zone that is believed to have been in existence at 2·9 Ga on the eastern margin of the craton.