Showing papers in "Journal of Petrology in 2014"

Variations in Fe3+/∑Fe of Mariana Arc Basalts and Mantle Wedge fO2

Abstract: Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe^(3+)/∑Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana Trough. For single eruptions, Fe^(3+)/∑Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation, CO_2 ± H_2O degassing) or reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO_2 + H_2O + S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H_2O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe^(3+)/∑Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean ridge basalts (∼QFM, where QFM is quartz–fayalite–magnetite buffer), ranging from QFM + 1·0 to QFM + 1·6 for Mariana arc basalts, whereas back-arc related samples record primary oxygen fugacities that range from QFM + 0·1 to QFM + 0·5. This Mariana arc sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H_2O-rich component in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from the Mariana Trough through the Mariana arc, increasing by 1·5 orders of magnitude as Ba/La increases by a factor of 10 relative to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized.

Experimental Study of the Influence of Water on Melting and Phase Assemblages in the Upper Mantle

Abstract: The role of water in the uppermost mantle has been explored to 6 GPa (~200 km) by a novel experimental approach in which the silicate melting solidus, the stability of hydrous phases and the H2O-contents in nominally anhydrous minerals (NAMs) were determined. The composition studied is a fertile lherzolite modelled as a source for mid-ocean ridge basalts (MORB). The use of crushed olivine as traps for melt or fluid inclusions enables a distinction to be made between quenched hydrous silicate melt and quench material from water-rich vapour phase. The vapor-saturated solidus (water-rich vapor) of fertile lherzolite increases in temperature (T) from a minimum of 970°C at 1.5 GPa (~50 km) to 1375°C at 6 GPa. The Carich amphibole, pargasite, is stable to the vapour-saturated solidus to 3 GPa (~100 km). Based on normative components, at 2.5 GPa the near-solidus melt (1-2 %) in mantle with very low H2O-content is transitional between sodic-dolomitic carbonatite and olivine melilitite. With higher melt fraction (~5 %) at higher T or higher H2O-content it is olivinerich basanite. Both immediately below and above the solidus, the H2O-content in residual lherzolite is ~200 ppm retained in NAMs at 2.5 and 4 GPa. The experimentally determined vapour-saturated solidus corrects recent numerical models of melting of lherzolite+H2O based on inferred high solubilities of H2O in NAMs and an erroneous experimental determination of the vapour-saturated solidus in which very high water/rock ratios were used. At 2.5 ± 0.1 GPa, the water content of experimental charges was varied from 0.05 to 14.5 wt.%. Below the solidus and with increasing water content from 0.05 to 2.9 wt.%, pargasite decreases in K2O and Na2O content and pargasite is absent in experiments with 7.25 and 14.5 wt.% H2O. Also with increasing water content from 0.05 to 14.5 wt.% H2O, the Na2O content of clinopyroxene decreases from 1.6 wt.% to below the limit of detection (0.2 wt.%). The destabilisation of pargasite and change of clinopyroxene composition at 2.5 GPa, 1000°C are attributed to the leaching role (Na2O and K2O particularly) of the water-rich vapour at high water/rock ratios. The hydrous mineral pargasite is the major site of H2O-storage in the fertile uppermost mantle lherzolite but pargasite is unstable at pressure (P) >3 GPa (~100 km depth) causing a sharp drop in water storage capacity of the upper mantle from >2000 to ~200 ppm. For small H2O-contents (<2000 ppm approximately), the temperature of the vapourundersaturated solidus of fertile upper mantle lherzolite decreases sharply with increasing P at ~90 km depth. The negative dT/dP for the vapour-undersaturated solidus has important rheological and geodynamic consequences. In oceanic intraplate settings, the geotherm passes from subsolidus pargasite-bearing lherzolite to garnet lherzolite with incipient melting, creating the rheological boundary at approximately 90 km depth, between Lithosphere and Asthenosphere. The Asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (>500 ppm H2O) overlying the ‘depleted’ MORB source (~200 ppm H2O) in the deeper Asthenosphere. Water also plays a significant role at convergent margins where hydrous silicate melting in the mantle wedge is initiated at the vapour-saturated solidus. Melting of lherzolite at or near the vapour-saturated solidus does not fully dehydrate residual lherzolite or harzburgite. Residual lithosphere returned to the upper mantle may carry ~100-200 ppm H2O. At 6 GPa the low K/Na model mantle composition (MORB_source mantle) with >200 ppm H2O has normal rather than supercritical melting behaviour with the solidus at 1375°C which is ~350°C below the C+H-free solidus.

The Global Systematics of Ocean Ridge Basalts and their Origin

Abstract: Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments.Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(MgþFe) in equilibrium with Fo90 olivine. Both sets of corrected compositions show large variations in Na2O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al90 and Fe90.These new data are not in agreement with the presentation of Niu & O’Hara (Journal of Petrology 49, 633^664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for highand low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 2008C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slowspreading ridges display a characteristic chemical signature of elevated Na90 and Al90 and lowered Si90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle^melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K90. Such variations do not correlate with the other major elements, showing that major element and trace

New Perspectives on the Bishop Tuff from Zircon Textures, Ages and Trace Elements

Abstract: We present zircon textural, trace element and U-Pb age data obtained by secondaryion mass spectrometry (SIMS) (SHRIMPRG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)-(c) combined have a weighted mean age of 777.9±2.2 ka (95% confidence) and a tail back to ~845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766.6±3.1ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (IDTIMS) (767.1±0.9 ka, 2σ) and Ar/Ar (767.4±2.2 ka, 2σ) techniques, the latter using the 28.172Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862±23 ka (95% confidence), significantly older than the currently accepted 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was active for only ~80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3× concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc+Y+REE)/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of theTi-in-zircon thermometer, in addition to variations inherent to the 0.15-0.67 range in values proposed for aTiO.The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and Ar/Ar techniques.

Zoned Monazite and Zircon as Monitors for the Thermal History of Granulite Terranes: an Example from the Central Indian Tectonic Zone

Abstract: The growth and dissolution behaviour of detrital, metamorphic and magmatic monazite and zircon during granulite-facies anatexis in pelitic and psammo-pelitic granulites and in garnetiferous granite from the southern margin of the Central Indian Tectonic Zone (CITZ) have been investigated using reconstructed metamorphic reaction history, monazite electron microprobe dating and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon geochronology. Whereas the pelitic granulites record medium-pressure granulite-facies metamorphism (BM1 stage), the psammo-pelitic granulite reached ultrahigh temperatures (TMax > 880°C at 8·7 kbar). The meta-psammite additionally records two stages of granulite-facies recrystallization (BM2 and BM3). Irrespective of variations in the bulk-rock compositions and peak metamorphic conditions, monazite is highly reactive during the BM1 event, producing complex, chemically zoned crystals. Textural, compositional and chemical ages of these grains indicate the stability of six compositional domains (CD1 to CD6 in the paragenetic sequence), of which CD1 represents pre-metamorphic detrital cores of Paleoproterozoic age. CD2 and CD3 (combined mean age of 1612 ± 14 Ma) mark two stages of recrystallization of detrital monazite cores during prograde events. Rims of CD4 monazite (ages between 1615 ± 14 and 1586 ± 14 Ma) on partially to completely equilibrated cores indicate melt crystallization at, or immediately following, peak BM1P metamorphism. CD5 monazite (age of 1574 ± 7 Ma) is restricted to the psammo-pelitic granulites, and marks final melt crystallization at the solidus during post-peak cooling (BM1R stage, where R represents retrograde metamorphism). The metamorphic rim of CD6 monazite (age of 1539 ± 24 Ma) around partially resorbed CD5 domains is linked to the decomposition of BM1 garnet during the terminal hydration event as part of a granulite-facies recrystallization event. Compositionally homogeneous monazite and rims of chemically zoned monazite grains in granite together record a magmatic crystallization age of 1604 ± 9 Ma. SHRIMP U–Pb zircon dating of the psammo-pelitic granulite and garnetiferous granite indicates detrital or inherited cores of Paleo- to Neoarchean age (3584 ± 3 to 2530 ± 3 Ma), which have been variously recrystallized and overgrown by new zircon: (1) at 1658 ± 12 Ma; (2) between 1595 ± 5 and 1590 ± 6 Ma; (3) at 1574 ± 9 Ma. These zircon dates are correlated with the timing of the following: (1) the protoliths of precursor sediments of the metasedimentary granulites, deposited between 2530 and 1658 Ma; (2) a short-lived high-grade event ∼65–70 Myr before the culmination of the BM1 granulite-facies event; (3) a high-T anatectic event, corresponding to the peak BM1P metamorphism at TMax > 900°C; (4) final crystallization of anatectic melt at the solidus (cf. BM1R metamorphic stage). These chronological constraints from monazite and zircon, when integrated with the metamorphic reaction history and published geochronological data, allow recognition of three episodes of granulite-facies metamorphism in the CITZ at 1658 Ma (pre-BM1 event), between 1612 and 1574 Ma (BM1 event), and between 1572 and 1539 Ma (combined BM2 and BM3 events), as part of a latest Paleoproterozoic to Early Mesoproterozoic orogenic event. This orogeny is linked to the growth of the Proto-Greater Indian Landmass.

The Shallow Plumbing System of Piton de la Fournaise Volcano (La Réunion Island, Indian Ocean) Revealed by the Major 2007 Caldera-Forming Eruption

Abstract: The 2007 caldera-forming eruption of Piton de la Fournaise (PdF) erupted the largest volume of magma (210 Mm3) recorded at this volcano in at least three centuries. Major and trace element and Sr–Nd isotope data for bulk-rocks, groundmasses and olivine phenocrysts have been combined with melt inclusion data (major, trace and volatile elements) to track magma evolution over the whole eruptive sequence. We show that each eruptive phase had a distinctive geochemical and petrological signature and that caldera collapse on 5 April was preceded by a marked shift in bulk magma composition and crystal content and size. Aphyric basalt erupted at the beginning of the sequence (February 2007) had relatively high Sr isotope ratio (87Sr/86Sr = 0·70420–0·704180) and low Nd isotopic ratio (143Nd/144Nd = 0·51285–0·51286). Olivine-basalts extruded on 2–5 April just before caldera collapse are less enriched in radiogenic Sr (87Sr/86Sr = 0·70412–0·70416), but characterized by the same Nd isotopic composition. This magma is interpreted as a new deep input, which pressurized the shallow PdF plumbing system and triggered the 2007 activity. Post-collapse oceanite lavas represent the main volume of magma extruded in 2007. Their bulk-rocks and groundmasses have 87Sr/86Sr (∼0·70418) intermediate between those of February and 5 April, and similar to those of the March 2007 and 2001–2006 lavas. We show that the Steady State Basalts (SSB) commonly erupted at PdF are hybrid melts, which result from multistep mixing between ‘alkaline’ and ‘transitional’ end-members. Our results lead us to propose a new model of the PdF plumbing system to reconcile the petrological, geochemical and geophysical observations: (1) the shallow portion (above sea level) of the PdF plumbing system hosts several small sills, in which magma experiences variable degrees of degassing, cooling and crystallization; (2) oceanite lavas result from the withdrawal of shallow harrisitic mushes stored at low pressures (<48 MPa; <1800–2400 m depth) below both the volcano summit and its eastern flank; (3) water degassing plays a major role in fast magma crystallization at shallow depths. Multistep ascent and periodic extrusion of the shallow magmas is promoted by injections of deeper and hotter basaltic magma, containing up to 1·3 wt % H2O and 1630 ppm S. In 2007, the new deep input was the ultimate source of the large excess in sulfur degassing detected by satellites. Lateral draining and intrusion of magma below the eastern flank of the volcano are the cause of major volcano deformation, flank sliding and summit caldera collapse.

Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

Abstract: At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.

Is the ‘Azores Hotspot’ a Wetspot? Insights from the Geochemistry of Fluid and Melt Inclusions in Olivine of Pico Basalts

Abstract: The concept of an 'Azores mantle plume' has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10*2 and 11*1 ± 0*1 Ra. In contrast, melt inclusions hosted in olivine (Fo76-83*5) span a large range of compositions (K2O = 0*7-1*7 wt %; Ce = 32-65 ppm; Nb = 21-94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1*8-2*0 wt % of H2O and 0*4 wt % of CO2. The total fluid pressures, retrieved from the dissolved CO2 and H2O concentrations, and the PCO2 from fluid inclusions, indicate magma ponding and crystallization at the crust-mantle boundary (ca. 18 km deep). The H2O/Cl and H2O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0*036 ± 0*006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H2O = 680-570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge.

Petrogenesis of Mantle Polymict Breccias: Insights into Mantle Processes Coeval with Kimberlite Magmatism

Abstract: Mantle polymict breccias sampled by kimberlite magmas are complex mixtures of mantle minerals and rock clasts, cemented together by olivine, phlogopite, orthopyroxene, ilmenite, rutile and sulphides. Because of the kimberlite-like texture (i.e. mineral clasts of diverse origin and composition set in a magmatic matrix) and the large geochemical heterogeneity preserved in polymict breccias, these rocks are believed to derive from primitive or precursor kimberlite magmas. Therefore, the study of such xenoliths can provide constraints on the processes occurring in the mantle during the early stages of kimberlite ascent, and possibly on the composition of kimberlite melts. To constrain the petrogenesis of these unusual rocks, we have studied two samples of polymict breccia from the Bultfontein kimberlite (Kimberley, South Africa) and compared our results with published data for other polymict breccias. The most abundant phase in the matrix of the studied samples is olivine with a narrow range in Mg# (∼88–89), but variable Ni–Mn–Ca contents. Similar compositions are characteristic of magmatic olivine in the Bultfontein and nearby De Beers kimberlites. Orthopyroxene is the dominant phase in the matrix of polymict breccias surrounding clinopyroxene clasts, which, like the other silicate mineral clasts, are highly resorbed. The matrix orthopyroxene exhibits variable compositions, with significant enrichment in Ca, Na, Cr, Sr, Ba and light rare earth elements in the grains adjacent to clinopyroxene. The other main matrix phases (phlogopite, ilmenite and rutile) also display variable compositions. Matrix olivine hosts primary carbonate-rich inclusions similar to those observed in polymict breccia ilmenite. These inclusions were previously interpreted as an alkali-carbonate melt trapped during ilmenite growth. This alkali-carbonate melt may represent the parental melt to the matrix minerals of the polymict breccias. The variable composition of the matrix minerals is attributed to rapid, small-scale (centimetre to millimetre) variations in the melt composition owing to clast dissolution, possibly coupled with wall-rock assimilation, closely followed by fast cooling. Partial digestion of silicate porphyroclasts increased the Si content of the matrix melt, thus allowing crystallization of orthopyroxene. Further arguments in favour of a genetic relationship between polymict breccias and kimberlite magmas are provided by (1) similar Hf isotope compositions of polymict breccia ilmenite and South African kimberlites, (2) overlapping olivine compositions in polymict breccias and the host Bultfontein kimberlite, and (3) the occurrence of alkali-carbonate inclusions in polymict breccia and kimberlite minerals. Polymict breccias are interpreted as failed kimberlite intrusions, which metasomatized the magmatic conduit through which subsequent pulses of kimberlite magmas ascended. These wall-rock interactions would limit reactions between later pulses of kimberlite melt and mantle wall-rocks, thus enhancing the ability of kimberlite magmas to reach the surface.

Dating Polygenetic Metamorphic Assemblages along a Transect across the Western Alps

Abstract: Multichronometric analyses were performed on samples from a transect in the French-Italian Western Alps crossing nappes derived from the Brianconnais terrane and the Piemonte-Liguria Ocean, in an endeavour to date both high-pressure (HP) metamorphism and retrogression history. Twelve samples of white mica were analysed by 39Ar-40Ar stepwise heating, complemented by two samples from the Monte Rosa nappe 100 km to the NE and also attributed to the Brianconnais terrane. One Sm-Nd and three Lu-Hf garnet ages from eclogites were also obtained. White mica ages decrease from c. 300Ma in the westernmost samples (Zone Houille're), reaching c. 3008C during Alpine metamorphism, to 548Ma in the internal units to the east, which reached c. 5008C during the Alpine orogeny. The spatial pattern of Eocene K-Ar ages demonstrates that Si-rich HP white mica records the age of crystallization at 47-48Ma and retains Ar at temperatures of around 5008C. Paleocene-early Eocene Lu-Hf and Sm-Nd ages, recording prograde garnet growth before the HP peak, confirm eclogitization in Eocene times. Petrological and microstructural features reveal important mineralogical differences along the transect. All samples contain mixtures of detrital, syn-D1 and syn-D2 mica, and retrogression phases (D3) in greatly varying proportions according to local variations in the evolution of pressure-temperature-fluid activity-deformation (P-T-a-D) conditions. Samples from the Zone Houille're mostly contain detrital mica. The abundance of white mica with Si46·45 atoms per formula unit increases eastward. Across the whole traverse, phengitic mica grown during HP metamorphism defines the D1 foliation. Syn-D2 mica is more Si-poor and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist-facies conditions Syn-D1 phengite is very often corroded, overgrown by, or intergrown with, syn-D2 muscovite. Most importantly, syn-D2 recrystallization is not limited to S2 schistosity domains; micrometrescale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals, which could be mistaken for syn-D1 mica based on microstructural arguments alone. The Cl/K ratio in white mica is a useful discriminator, as D2 retrogression was associated with a less saline fluid than eclogitization. As petrology exerts the main control on the isotope record, constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data, described in both the present study and the literature. Our approach, which ties geochronology to detailed geochemical, petrological and microstructural investigations, identifies 47-48Ma as the age of HP formation of syn-D1 mica along the studied transect and in the Monte Rosa area. Cretaceous apparent mica ages, which were proposed to date eclogitization by earlier studies based on conventional ‘thermochronology’, are due to Ar inheritance in incompletely recrystallized detrital mica grains. The inferred age of the probably locally diachronous, greenschist-facies, low-Si, syn-D2 mica ranges from 39 to 43Ma. Coexistence of D1 and D2 ages, and the constancy of non-reset D1 ages along the entire transect, provides strong evidence that the D1 white mica ages closely approximate formation ages. Volume diffusion of Ar in white mica (activation energy E=250 kJ mol 1; pressure-adjusted diffusion coefficient D’050·03 cm2 s 1) has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples.

Accessory mineral chemistry of high Ba-Sr granites from northern Scotland: constraints on petrogenesis and records of whole-rock signature

Abstract: The Rogart and Strontian high Ba–Sr plutons (Northern Highlands, Scotland) comprise a range of lithologies from felsic to ultramafic rocks. The latter are mantle-derived and their differentiation to produce the felsic components of the plutons is the result of fractional crystallization and variable assimilation of the surrounding Moine metasediments. New results presented here demonstrate that accessory mineral chemistry can provide further insight into their petrogenesis and highlight the petrological potential of apatite and titanite. The main accessory minerals titanite, apatite and zircon contain most of the rare earth elements (REE) in the high Ba–Sr plutons. Results for apatite and titanite show that careful imaging and in situ trace element analysis provide constraints on the petrogenetic history of the host-rock. In both plutons, apatite and titanite record in situ crystallization and fractionation. In Strontian, both apatite and titanite from the granitoids record a mixing event with mafic magma in their rim compositions. Apatite and titanite chemistries are sensitive to the nature of their host-rocks (felsic versus ultramafic) and some elements (e.g. Sr, V) closely reflect whole-rock chemistry and the degree of fractionation. In some cases, whole-rock trace element concentrations can be calculated based on accessory mineral chemistry. Thus, trace elements in accessory minerals can give direct access to the nature and crystallization history of plutonic rocks. This petrological tool may be helpful in provenance studies using accessory minerals, and because high Ba–Sr plutons have recently been equated with Archaean sanukitoids, this might also be important in constraining the temporal distribution of this important magma type.

The Fate of Sulfur During Fluid-Present Melting of Subducting Basaltic Crust at Variable Oxygen Fugacity

Abstract: To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H2O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity (Graphic). Double-capsule experiments were conducted at 2*0 and 3*0 GPa and 950-1050°C, using Co-CoO, Ni-NiO, NixPd1-x-NiO, and Fe2O3-Fe3O4 external Graphic buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing Graphic. At pyrrhotite saturation, melt S contents range from ∼30 ppm S at Graphic < FMQ - 1 to ∼500 ppm S at FMQ < Graphic ≤ FMQ + 1*1, whereas at anhydrite saturation (Graphic ≥ FMQ + 2*5) melt S concentrations range from ∼700 ppm S to 0*3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ∼15 wt % S at 1050°C at pyrrhotite saturation (Graphic ≤ FMQ + 1*1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that Graphic decreases markedly with increasing Graphic at pyrrhotite saturation, from several thousand at Graphic < FMQ - 1 to ∼ 200-400 at FMQ < Graphic ≤ FMQ + 1*1, owing to the increase of melt S content. At anhydrite saturation, Graphic is very low (<100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, Graphic might be in the range 200 ± 100, irrespective of Graphic. The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all Graphic. Therefore, S transfer from ocean crust to wedge mantle is not Graphic dependent and could take place over a range of Graphic conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle.

Petrology and Nd–Hf Isotope Geochemistry of the Neoproterozoic Amon Kimberlite Sills, Baffin Island (Canada): Evidence for Deep Mantle Magmatic Activity Linked to Supercontinent Cycles

Abstract: The c. 673 Ma (U^Pb rutile) Amon kimberlites located in northern Baffin Island intruded Late Archean basement rocks of the Rae craton as a subhorizontal sill complex.The Amon sills are part of widespread low-volume, volatile-rich ultramafic magmatism that occurred along the northern and eastern margins of Laurentia, demarcating the temporal and spatial breakout from the Rodinia supercontinent during the Late Neoproterozoic. Numerous other known kimberlite occurrences that are related to these rifting events between c. 680 and 540 Ma are located in mainland Nunavut, Ontario, Quebec, Labrador, andWest Greenland.The magmas that fed the Amon sills are archetypal Group-I kimberlites, based on groundmass mineralogy (e.g. phlogopite, spinel, ilmenite) and bulk-rock compositions, including moderately depleted Sr^Nd^Hf isotope ratios. However, a wide compositional range, together with observed flowage textures, indicates that some magma differentiation occurred during sill emplacement. The Amon samples that are interpreted as parental kimberlite magma compositions overlap published compositions of experimentally derived, near-solidus partial melts of carbonated peridotite between 5 and 10 GPa; that is, equivalent to an origin from 150 to 300 km depth. Furthermore, the Amon kimberlites are characterized by moderately depleted Nd (eNd(i)1⁄4þ1·5 to þ3·5) and Hf (eHf(i)1⁄4þ1·1 to þ8·7) isotope compositions, without pronounced isotope decoupling as known from other kimberlite occurrences worldwide. Among the studied Late Neoproterozoic volatile-rich ultramafic magmatic rocks in Laurentia, the Amon kimberlites have Nd^Hf isotope systematics that are similar to those of a previously identified, carbonate-rich, depleted end-member component. This common component is suggested to represent a widespread near-solidus partial melt of volatile-fluxed fertile peridotite within the uppermost convecting mantle beneath the rifting supercraton. Our preferred model for Late Neoproterozoic kimberlite and related magmatism along the rifted

Thermodynamic Model for Energy-Constrained Open-System Evolution of Crustal Magma Bodies Undergoing Simultaneous Recharge, Assimilation and Crystallization: the Magma Chamber Simulator

Abstract: The Magma Chamber Simulator quantifies the impact of simultaneous recharge, assimilation and crystallization through mass and enthalpy balance in a multicomponent^multiphase (meltþ solids fluid) composite system. As a rigorous thermodynamic model, the Magma Chamber Simulator computes phase equilibria and geochemical evolution self-consistently in resident magma, recharge magma and wallrock, all of which are connected by specified thermodynamic boundaries, to model an evolving open-system magma body. In a simulation, magma cools from its liquidus temperature, and crystals fluid are incrementally fractionated to a separate cumulate reservoir. Enthalpy from cooling, crystallization, and possible magma recharge heats wallrock from its initial subsolidus temperature. Assimilation begins when a critical wallrock melt volume fraction (0·04^0·12) in a range consistent with the rheology of partially molten rock systems is achieved.The mass of melt above this limit is removed from the wallrock and homogenized with the magma body melt. New equilibrium states for magma and wallrock are calculated that reflect conservation of total mass, mass of each element and enthalpy. Magma cooling and crystallization, addition of recharge magma and anatectic melt to the magma body (where appropriate), and heating and partial melting of wallrock continue until magma and wallrock reach thermal equilibrium. For each simulation step, mass and energy balance and thermodynamic assessment of phase relations provide major and trace element concentrations, isotopic characteristics, masses, and thermal constraints for all phases (meltþ solids fluid) in the composite system. Model input includes initial compositional, thermal and mass information relevant to each subsystem, as well as solid^melt and solid^fluid partition coefficients for all phases. Magma Chamber Simulator results of an assimilation^fractional crystallization (AFC) scenario in which dioritic wallrock at 0·1GPa contaminates high-alumina basalt are compared with results in which no assimilation occurs [fractional crystallization only (FC-only)]. Key comparisons underscore the need for multicomponent^multiphase energyconstrained thermodynamic modeling of open systems, as follows. (1) Partial melting of dioritic wallrock yields cooler silicic melt that contaminates hotter magma. Magma responds by cooling, but a pulse of crystallization, possibly expected based on thermal

Sr, Nd, Pb and Os Isotope Systematics of CAMP Tholeiites from Eastern North America (ENA): Evidence of a Subduction-enriched Mantle Source

Abstract: The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 10e7km2. Here we document the geochemical characteristics of CAMP basalts from Triassic-Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr-Nd-Pb initial (t=201Ma) isotopic compositions (206Pb/204Pbini. = 18·155-18·691, 207Pb/204Pbini. = 15·616-15·668, 208Pb/204Pbini. = 38·160-38·616, 143Nd/144Ndini. = 0·512169-0·512499). On the basis of stratigraphy, rare earth element (REE) chemistry and Sr-Nd-Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206Pb/204Pbini. >18·5 and 143Nd/144Ndini.>0·51238, comprises all the latest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206Pb/204Pbini.>18·5 and 0·51233>143Nd/144Ndini.>0·51225, comprises the intermediate units. The Orange Mountain group has the highest La/Yb ratios and 143Nd/144Ndini.<0·51235 and involves all the earliest and stratigraphically lowest units, including the entire North Mountain basalts from Nova Scotia. In this last group, three sub-groups may be distinguished: the Rapidan sill, which has 206Pb/204Pbini. higher than 18·5, the Shelburne sub-group, which has 143Nd/144Ndini.<0·51225, and the remaining Orange Mt samples. With the exception of one sample, the Eastern North America (ENA) CAMP basalts display initial 187Os/188Os ratios in the range of mantle-derived magmas (<0·15). Simple modelling shows that the composition of the ENA CAMP basalts cannot plausibly be explained solely by crustal contamination of oceanic island basalt (OIB), mid-ocean ridge basalt (MORB) or oceanic plateau basalt (OPB) magmas. Mixing of such magma compositions with sub- continental lithospheric mantle (SCLM)-derived melts followed by crustal contamination, by either assimilation-fractional crystallization (AFC) or assimilation through turbulent ascent (ATA) pro- cesses is somewhat more successful. However, this latter scenario does not reproduce the REE and isotopic composition of the ENA CAMP in a fully satisfactory manner. Alternatively, we propose a model in which asthenospheric mantle overlying a subducted slab (i.e. mantle wedge) was enriched during Cambrian to Devonian subduction by sedimentary material, isotopically equivalent to Proterozoic-Lower Paleozoic crustal rocks. Subsequently, after subduction ceased, the isotopic composition of this mantle evolved by radioactive decay for another 170 Myr until the CAMP magmatic event. Varying amounts and compositions of the incorporated sedimentary component coupled with radiogenic ingrowth over time can account for the main geochemical characteristics of the ENA CAMP (enriched incompatible element patterns, negative Nb anomalies, enriched Sr-Nd-Pb isotopic composition) and the differences between the three chemical groups.

Multiple metamorphic stages within an eclogite-facies terrane (Sesia Zone, Western Alps) revealed by Th-U-Pb petrochronology

Abstract: Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels, but the details of such dynamics are controversial.To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internalWestern Alps.This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples.Two independent tectono-metamorphic ‘slices’ showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex).The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ~85 and 60 Ma, with evidence of intermittent decompression (∆P~0.5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ~85 and 75Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel.

Magma Storage Conditions of Large Plinian Eruptions of Santorini Volcano (Greece)

Abstract: The intensive variables of dacitic-rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850-900°C, fluid (H2O + CO2)-saturation, XH2O [= molar H2O/(H2O + CO2)] between 0*6 and 1 (melt H2O contents of 2-10 wt %), and redox conditions of FMQ (fayalite-magnetite-quartz buffer) or NNO + 1 (where NNO is Ni-NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850-900°C and P ≥ 2 kbar, under moderately reduced conditions (ΔNNO = −0*9 to −0*1), and were poor in fluorine (∼500-800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5-6 wt % and 2500-3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H2O, but most probably saturated with respect to H2O + Cl ± CO2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.

Zonation of H2O and F Concentrations around Melt Inclusions in Olivines

Abstract: Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H_(2)O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H2O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0·03 ± 0·01 ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0·03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0·00002 ± 0·00002. Olivines from the two localities display contrasting H_(2)O and F compositions: Sommata olivines contain 27 ± 11 ppm H_(2)O and 0·28 ± 0·07 ppm F, whereas Jorullo olivines have lower and proportionately more variable H_(2)O and F (11 ± 12 ppm and 0·12 ± 0·09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H_(2)O and F contents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions. This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H_(2)O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-axis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H_(2)O loss that is rate-limited by the ‘proton–polaron’ mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0·0007 ± 0·0003 for H_(2)O and 0·0005 ± 0·0003 for F. The H_(2)O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt. Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H_(2)O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H2O decrease in the external melt and post-entrapment enrichment of H_(2)O in the inclusions. The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton–polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H_(2)O content of the Sommata melt inclusions before diffusive loss of 6 wt % H_(2)O. The findings provide new insights on rapid H_(2)O loss during magma ascent and improve our ability to assess the fidelity of the H_(2)O record from melt inclusions.

Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand

Abstract: TheTaupoVolcanic Zone (TVZ) is well known for its extraordinary rate of rhyolitic magma generation and caldera-forming eruptions. Less is known about how large volumes of rhyolitic magma are extracted and stored prior to eruption, and the role tectonics might play in the process of melt extraction and control of caldera eruption(s). Here we present a new model for the extraction, storage and simultaneous eruption of the4245 km paired Mamaku and Ohakuri magmas sourced from calderas centred 30 km apart (the Rotorua and Ohakuri calderas, respectively) in the central TVZ. The Mamaku and Ohakuri ignimbrites share a similar bulk pumice composition and the same phenocryst assemblage; however, bulk-rock compositions suggest several poorly mixed magma types in each erupted volume, which are randomly distributed throughout the eruptive deposits. To refine models of the pre-eruptive geometry of the magmatic system and discuss a possible origin for triggering of each eruption, we present an expanded database of matrix glass and quartz-hosted melt inclusion compositions along with the existing bulk-rock and mineral compositions. Major and trace element compositions show that the region produced five different magma batches, extracted from the same source region, and a continuous intermediate mush zone beneath the Mamaku^Ohakuri region is suggested here. These magma batches were most probably juxtaposed but isolated from each other in the upper crust, and evolved separately until eruption. The observed geochemical differences between the batches are likely to be generated by different extraction conditions of the rhyolitic melt from a slightly heterogeneous mush. The lack of evidence for more mafic recharge prior to eruption (for example, there are no bright cathodoluminescence rims on quartz crystals) suggests that a magmatic input is unlikely to be an eruption trigger. However, tectonic activity could be an efficient way to trigger the eruption of isolated magma batches, with the evacuation of one magma batch causing a disturbance to the local stress field and activating regionally linked faults, which then lead to the eruption of additional magma batches and associated caldera subsidence. In addition, the

Crystal Storage and Transfer in Basaltic Systems: the Skuggafjöll Eruption, Iceland

Abstract: Magma mixing and crystal mush disaggregation are important processes in basaltic magma reservoirs. We carried out a detailed petrological and geochemical study on a highly plagioclase-phyric eruption within the Eastern Volcanic Zone of Iceland—the Skuggafjoll eruption—to investigate crystal storage and transport processes within a single magmatic system. Crystal content and phase proportions vary between samples: the least phyric samples have phase proportions similar to the low-pressure, three-phase gabbro eutectic (plg:cpx:ol \~ 11:6:3), whereas highly phyric samples are strongly enriched in plagioclase (plg:cpx:ol \~ 8:1:1). Statistically significant geochemical variability in 28 whole-rock samples collected across the eruption can be accounted for by variable accumulation of a troctolitic assemblage containing plagioclase and olivine in an approximately 9:1 ratio. Two macrocryst assemblages are defined using compositional and textural information recorded in QEMSCAN® images: a primitive assemblage of high-anorthite plagioclase (An\textgreater83) and high-forsterite olivine (Fo\textgreater84), and an evolved assemblage of low-anorthite plagioclase (An\textless79), low-forsterite olivine (Fo\textless82) and clinopyroxene (Mg# \~ 82). Plagioclase and olivine have strongly bimodal composition distributions whereas the composition distribution of clinopyroxene is unimodal. The mean trace element composition of melt inclusions hosted within high-forsterite olivine and high-anorthite plagioclase macrocrysts is the same (mean Ce/Y \~ 0·47–0·48), confirming that both primitive macrocryst phases crystallized from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallized from melts with a more incompatible trace element-enriched composition (mean Ce/Y \~ 0·65–71) than the primitive assemblage. Variability in whole-rock, macrocryst and melt inclusion compositions suggests that the Skuggafjoll magma experienced two stages of crystallization. Primitive macrocrysts crystallized first from incompatible trace element-depleted melts within a shallow crustal magma reservoir. These primitive macrocrysts were subsequently stored in crystal mushes that ultimately disaggregated into an evolved and incompatible trace element-enriched magma from which the evolved assemblage crystallized. On average, \~17% of the erupted magma at Skuggafjoll is composed of accumulated macrocrysts entrained from crystal mushes. The timescale between mush disaggregation and eruption, during which crystal accumulation occurred, was short—of the order of years—according to simple diffusion calculations. Striking petrological similarities between Skuggafjoll and other highly phyric eruptions both in Iceland and along mid-ocean ridges indicate that crystal accumulation by mush disaggregation is likely to be an important mechanism for generating highly phyric magmas in basaltic plumbing sy

Polymagmatic Activity at the Monogenetic Mt Gambier Volcanic Complex in the Newer Volcanics Province, SE Australia: New Insights into the Occurrence of Intraplate Volcanic Activity in Australia

Abstract: Monogenetic volcanism can produce eruptive suites showing considerable complexity in compositional features and pre-eruptive magma evolution. The � 5 ka Mount Gambier Volcanic Complex (MGVC), a monogenetic volcanic centre in SE Australia’s Newer Volcanics Province (NVP), is a good example. It displays a complex stratigraphy of interbedded deposits related to different eruption styles from a multi-vent system. Formation of the MGVC proceeded through simultaneous eruption of two alkali basaltic magma batches: a more alkaline and light rare earth element enriched basanite batch (Mg# 58^62) in the west and a trachybasalt batch (Mg# 58^64) enriched in SiO2 and CaO in the east. Trace element modelling suggests an origin of both magma batches from a single parental melt formed by 4^5% partial melting of a metasomatized lherzolite source in the asthenospheric mantle (2· 2G Pa; � 80 km). At the base of the lithosphere, part of this parental melt interacted with a deep-seated pyroxenite contaminant to form the trachybasaltic suite. Further modification of either magma batch at crustal levels appears to have been negligible. Isotope and trace element signatures are consistent with the inferred asthenospheric magma source; Pb isotopes in particular suggest a source with mixed Indian mid-ocean ridge basalt^Enriched Mantle 2 affinities, the latter perhaps related to metasomatic overprinting. It is argued that Cenozoic NVP volcanism in SE Australia is not necessarily related to a mantle plume but can be explained by other models involving asthenospheric upwelling. Fast magma ascent rates in the lithosphere evidenced by the presence of mantle xenoliths may reflect reactivation of lithospheric structures that provide magma pathways to the surface.

Textural Evolution of Plagioclase Feldspar across a Shear Zone: Implications for Deformation Mechanism and Rock Strength

Abstract: Caledonian amphibolite-facies shear zones developed in granulitefacies anorthosites and anorthositic gabbros of the Bergen Arcs, western Norway, allow a detailed study of the relationships between fluid-infiltration, mineral reactions, the evolution of microstructure and deformation mechanisms. A sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy, electron microprobe microanalysis (EMPA), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), focusing on the progressive development of microstructure in the plagioclase feldspars, leading up to their deformation in the shear zone. At the outcrop scale, fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac^brown to white. This is associated with the breakdown of the intermediate composition plagioclase (� An50) in the granulite to a complex intergrowth of Na-rich and Ca-rich domains. EBSD analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar, but that the Ca-rich domains contain low-angle grain boundaries. Within the shear zone, this complex intergrowth coarsens by grain boundary migration, annihilating these grain boundaries but retaining the Na-rich and Ca-rich zoning pattern. Analysis of nearestneighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation (CPO) is inherited from the parent granulite grain orientations. Random pair misorientation angle distributions show that there is no CPO in the shear zone as a whole, nor is there significant shape preferred orientation (SPO) in single grains. These observations are interpreted in terms of fluid-induced weakening and deformation by dissolution^ precipitation (pressure solution) creep.

Post-supereruption Magmatic Reconstruction of Taupo Volcano (New Zealand), as Reflected in Zircon Ages and Trace Elements

Abstract: New zircon U^Th model-age and trace element datasets are presented fromTaupo volcano (New Zealand), which are used to investigate the timescales and broad-scale magmatic processes involving zircon crystallization after the caldera-forming 25·4 ka Oruanui supereruption. Detailed C-based chronologies and controls on vent locations allow the timing and location of post-caldera eruptions to be spatially and temporally constrained to an extent not possible for any other supervolcano. After 5 kyr of post-Oruanui quiescence, Taupo erupted three dacitic units, followed by another 5 kyr break, and then a sequence of rhyolitic units in three subgroups (SG1^SG3) from 12 ka onwards. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U^Th zircon model ages inTaupo SG1 rhyolites (erupted from 12 to 10 ka) indicate only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains in some units. U^Pb dating of these older grains shows that both 300^450 ka plutonic-derived and pre100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units that were vented within the outline of the Whakamaru caldera. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolite (Units B and C) have notably high concentrations of U, Th, P, Yþ (REE)3þ and Nb but with only minor variations in Hf andTi. SG2 zircons typically have higher Sc contents, reflecting large-scale changes in melt chemistry and crystallizing mineral phases with time.The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U^Th ages from single post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallization, acting within a crustal protolith chemically independent of that which was dominant in the Oruanui system.These periods of cooling and crystallization alternate with times of rejuvenation and eruption, sometimes

The Beni Bousera Peridotite (Rif Belt, Morocco): an Oblique-slip Low-angle Shear Zone Thinning the Subcontinental Mantle Lithosphere

Abstract: Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW-SE stretching lineation that progressively rotates towards a NNE-SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet-spinel mylonites, Ariegite subfacies fine-grained porphyroclastic spinel peridotites, Ariegite-Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2*0 GPa in garnet-spinel mylonites to 1150°C at 1*8 GPa in the Seiland domain. A pervasive diffuse dunitic-websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for <200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km−1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ∼90 to ∼60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.

Using Multiphase Solid Inclusions to Constrain the Origin of the Baima Fe–Ti–(V) Oxide Deposit, SW China

Abstract: Multiphase solid inclusions within cumulus silicates, particularly olivine, in Fe–Ti oxide ores from the Lower Zone of the Baima intrusion, Emeishan large igneous province, SW China, have been identified for the first time using 2-D scanning electron microscope and 3-D high-resolution X-ray computed tomography. These inclusions are spherical to subspherical and range from 100 to 300 µm in diameter. They are composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. The titanomagnetite in the inclusions has a low Cr content (<700 ppm) similar to the interstitial titanomagnetite, suggesting that these inclusions cannot be early crystallized mineral aggregates. In contrast, the spherical shape of these inclusions provides evidence of early trapped liquids from which these minerals crystallized. Based on the composition and modal proportions of the daughter mineral phases within the inclusions, the trapped liquids are estimated to have 82·1–59·6 wt % FeOT, 11·4–18·5 wt % TiO2, 2·69–6·12 wt % Al2O3, 1·40–4·47 wt % MgO, 0·87–4·93 wt % SiO2 and ∼1 wt % volatiles including F, S, Cl, P and H2O. Such a liquid composition deviates substantially from that of the slightly evolved ferrobasaltic magmas inferred to be parental to the Fe–Ti–(V) oxide-bearing mafic–ultramafic intrusions of the Emeishan large igneous province. It is thus speculated that these trapped liquids are immiscible Fe–Ti-rich melts formed upon cooling of ferrobasaltic magma. The net-textured and disseminated oxide ores have titanomagnetite compositions similar to those in the inclusions, suggesting that the oxide ores of the Baima intrusion also formed from Fe–Ti-rich melts immiscibly separated from ferrobasaltic magmas. We propose that immiscible Fe–Ti-rich liquids with high density percolated down through crystal-bearing silicate magma and crystallized an interconnected Fe–Ti oxide network interstitial to olivine, plagioclase and clinopyroxene. This study highlights that immiscible separation of Fe–Ti-rich liquids from ferrobasaltic magmas is an important mechanism in the formation of magmatic Fe–Ti–(V) oxide deposits hosted in mafic–ultramafic layered intrusions.