Anorthite Content Of Plagioclase Research Articles

Lateral uniformity of the Pyroxenite Marker Transition in the western Bushveld Complex, South Africa

Abstract Mineralogically distinctive layers in the Bushveld Complex, South Africa, can usually be traced for hundreds of km in both the eastern and western limbs. They are remarkably uniform laterally in mineral chemical composition. There is one notable exception, namely the Pyroxenite Marker in the middle of the Main Zone in the eastern limb. It defines the boundary between the Lower Main Zone and Upper Main Zone. Toward the south in the eastern limb mineral compositions become more evolved, and ultimately the Pyroxenite Marker layer itself disappears and is replaced by magnetite-bearing rocks. In all previously published profiles through the eastern Bushveld, through a 400 m interval with the Pyroxenite Marker in the middle, there is a regular prolonged reversal in the anorthite content of plagioclase and mg# of pyroxene of about ten units, attributable to magma addition. In contrast, in the western limb there is no actual outcrop of this layer, but it has been located in the BK borehole from the centre of the limb. A similar magnitude of reversals in mineral compositions as in the east was reported in a previous study. In this study, a second profile was taken close to the eastern limit of the western limb to test if there was lateral variation comparable to that observed in the eastern limb. More mineralogical data are also reported from the borehole intersection. These two sections from the western limb show extremely similar changes in mineral compositions. In the eastern limb the location of the Pyroxenite Marker also shows an upward, abrupt change from primary pigeonite (below) to primary orthopyroxene (above). The same change occurs in the west, and at the same mineral compositions as in the east, and so this boundary can be defined by the pigeonite to orthopyroxene transition even in the absence of an actual pyroxenite layer. Hence, the term Pyroxenite Marker Transition is more applicable. Both in the east and west more primitive plagioclase compositions occur well above this boundary, and so magma addition and/or mixing continued well into the Upper Main Zone.

Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau

The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than pre-collapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.

A comparative study of two-phase equilibria modeling tools: MORB equilibrium states at variable pressure and H2O concentrations

AbstractPhase equilibria modeling is a powerful petrological tool to address both forward and inverse geological problems over a broad range of crustal and upper mantle conditions of pressure (P), temperature (T), composition (X), and redox (fO2). The development of thermodynamic databases, relatively realistic activity−composition (a−X) relations for solids, melts and fluids, pressure-volume-temperature (PVT) equations of state (EOS), and efficient numerical algorithms represent an inflection point in our ability to understand the nexus between tectonics and petrogenesis. While developed—and typically applied in isolation—by either metamorphic or igneous petrologists, some of the published thermodynamic models have overlapping P-T-X calibration ranges, which enables comparisons of model outcomes for similar conditions within the range of applicability. In this paper, we systematically compare the results of two such models that are routinely used for calculating phase equilibria in melt-bearing systems: rhyolite-MELTS (Gualda et al. 2012; Ghiorso and Gualda 2015) and the metabasite set of Green et al. (2016) using the thermodynamic database ds62 (Holland and Powell 2011) (hereafter denoted as “HPx-mb16”). We selected a N-MORB composition and modeled closed system equilibrium phase relations as a function of temperature at 0.25 and 1 GPa for N-MORB with 0.5 and 4 wt% H2O. Our results show that phase relations exhibit some key differences that, in some instances, impact geological inferences. For example, clinopyroxene and plagioclase stabilities are expanded to higher temperatures in HPx-mb16 compared to predictions from rhyolite-MELTS. Orthopyroxene and olivine are stable in greater proportions and at wider temperature ranges in rhyolite-MELTS compared to HPx-mb16. Importantly, HPx-mb16 predicts amphibole in all runs presented here, whereas amphibole is only predicted at high-P–high-H2O (1 GPa and 4 wt% H2O) in rhyolite-MELTS, and in lesser amounts. Garnet stability is systematically expanded at higher temperatures, and the proportion is greater in rhyolite-MELTS. In addition to phase assemblage differences, phase compositions may differ. For example, plagioclase anorthite content is systematically higher in HPx-mb16 (for the same set of conditions), whereas garnet Mg# is higher in rhyolite-MELTS. Calculated amphibole compositions are substantially different between the two models as well. Liquid compositions also show important differences. High-T liquids are generally similar in SiO2 contents but diverge at lower temperatures; in these cases, HPx-mb16 liquids are SiO2-depleted compared to those produced by rhyolite-MELTS. Liquids are also systematically and substantially more mafic in HPx-mb16, and alumina and the alkali concentrations are relatively different and show different trends as a function of temperature at constant pressure. Overall, liquid compositions show the greatest differences near the solidus. Differences in modal abundances of phases and liquid compositions influence liquid trace-element signatures, and these differences can affect geological interpretations. Finally, a comparison between melting experiments of basaltic bulk composition and both thermodynamic models shows that rhyolite-MELTS better reproduces the higher temperature experiments, whereas HPx-mb16 better reproduces the lower temperature experiments. We discuss these and other similarities and differences to highlight the strengths and limitations of each model and to recognize that modeling results have important implications for interpretations of geologic processes. We recognize that our results are informed by a small subset of calculations over a limited range of conditions—but encourage further comparisons over a wider range of conditions and compositions.

Testing solution models for phase equilibrium (forward) modeling of partial melting experiments

This study compares four sets of solution (or activity-composition) models using two internally consistent thermodynamic datasets for calculating isochemical phase diagram sections of six partial melting experiments covering a wide range of metasedimentary bulk compositions. Compared parameters are: (1) biotite-breakdown temperatures; (2) parageneses of Fe–Mg phases and Ti-oxides; (3) proportion of biotite, garnet, liquid and plagioclase; (4) Mg# of biotite, garnet and liquid and the anorthite content of plagioclase. Results reveal significant differences between sets that are mainly related to the construction of the biotite solution, with the model of Tajcmanova et al. (2009) yielding better results for the majority of investigated parameters. Owing to the success of the investigated set of solution models at reproducing the proxy that constitutes partial melting experiments, but also at reproducing natural observations as published elsewhere, it is suggested that this biotite model should be used in the Ti–Mn–Na–Ca–K–Fe–Mg–Al–Si–H–O system for phase equilibrium (forward) modeling of metasediments.

Strontium isotope variations in the Flatreef on Macalacaskop, northern limb, Bushveld Complex: implications for the source of platinum-group elements in the Merensky Reef

The Platreef of the Bushveld Complex is one of three key stratigraphic units, along with the Merensky and UG-2 reefs, in which platinum-group elements are concentrated. The correlation between the Platreef and the stratigraphy from the western and eastern limbs of the BC has been challenging due to the heterogeneous and variable nature of the Platreef along strike. However, the discovery of the Flatreef, interpreted as the down dip, sub-horizontal extension of the Platreef, on the farms Turfspruit and Macalacaskop, opened new avenues for enquiry that allow the study of a magmatic stratigraphy less affected by footwall interaction. In this study, we report on the Sr isotope composition of plagioclase through an ~ 280-m-thick intersection of the Flatreef and its footwalls and hanging walls as intersected by borehole UMT-393 drilled on the farm Macalacaskop. Comparison of the Sr-isotopic composition and the anorthite content of plagioclase across this intersection with data from the Merensky and Bastard cyclic units elsewhere in the Bushveld Complex supports the contention that the Flatreef is a correlative of the Upper Critical Zone–Main Zone transition, including the Merensky and Bastard cyclic units. The available data suggest that the sulfides of the Flatreef and its correlatives in the remainder of the Bushveld Complex originally formed in a deeper staging chamber. During continued magma ascent some of the sulfides were entrained and deposited in the Flatreef.

Island-Arc Augite–Bytownite–Labradorite Dacites of the Kara-Dag Massif, Crimea

Island-arc calc-alkaline dacites (66.7% SiO2, 3.4% Na2O, 1.9% K2O) form a subvolcanic body hosted within andesitic and trachyandesitic tuffs on the east of the Kara-Dag Volcanic Massif of the Crimean Mountains. These dacites characteristically contain many phenocrysts of plagioclase (bytownite Ca75–72Na24–27K0.5–1 confined to the cores and labradorite Ca67–52Na32–47K1 to the inner and outer rims) and low-Ti augite (augite Ca44Mg45Fe11 with 4.5% Al2O3 confined to the cores and augite Ca43–41Mg41–38Fe16–21 with 1–2% Al2O3 to the inner and outer rims). Titanomagnetite, ilmenite, and apatite are intergrown with augite. Low-Mg titanomagnetite is enriched in manganese (up to 4.5 wt % MnO) and zinc (up to 1.6% ZnO) and contains the ulvospinel endmember ranging between 39 and 28%. Low-Mn ilmenite contains 10 to 25 mol % of the hematite endmember, suggesting crystallization at elevated $${{f}_{{{{{\text{O}}}_{2}}}}}$$, that is, under water-saturated melt conditions. The Sr, Ce, and S contents in the apatite are low. The fluorine trend shows an increase in the F content from chlorine-hydroxyl-fluorapatite to fluorapatite. The groundmass of rhyolitic dacites (77.3% SiO2, 3.3% Na2O, and 2.5% K2O) consists of labradorite microlites Ca52–50Na46–48K2–3 with quartz, minor andesine Ca49–46Na49–52K2–3, oligoclase Ca27Na68K5, and anorthoclase in the interstitial spaces. An extremely high anorthite content of plagioclase that is typical of island-arc volcanic rocks is characteristic of these dacites. The crystallization temperature for augite is ~1050–950°C. Early crystallization of the coexisting titanomagnetite and ilmenite occurs at ~900°С with $${{f}_{{{{{\text{O}}}_{2}}}}}$$ 1 log unit higher than the QFM buffer. Their late crystallization occurs at ~880°С with $${{f}_{{{{{\text{O}}}_{2}}}}}$$ 2 log units higher than the QFM buffer.

Ultrahigh-temperature mafic granulite in the Huai’an Complex, North China Craton: Evidence from phase equilibria modelling and amphibole thermometers

The Huai’an Complex in the north-western Trans-North China Orogen, North China Craton is recognized to have undergone high-pressure metamorphism with controversial ages. Two mafic granulite samples (17HT13 and 16HT22) from Huangtuyao of the Huai’an Complex are documented for metamorphism based on petrography, mineral chemistry, phase equilibria modelling and zircon dating. The rocks comprise garnet, clinopyroxene, orthopyroxene, amphibole, plagioclase and ilmenite. They show clockwise P-T paths involving the pre-Tmax decompressional heating, the Tmax and the post-Tmax cooling stages. The pre-Tmax decompressional heating is revealed by the core-rim/mantle anorthite-ascending zoning in coarse-grained plagioclase. The Tmax stage is constrained to 8.5–10.5 kbar/980–1010 °C (17HT13) and 7.5–9.5 kbar/1000–1030 °C (16HT22) by the high Ti content of amphibole, and the anorthite content of coarse-grained plagioclase rims/mantles and fine-grained plagioclase cores. These temperatures are slightly higher than the results of amphibole thermometers (∼940–980 °C). The post-Tmax cooling is defined by the later growth of amphibole and some conventional geothermobarometers. Because it is dominated by temperature with ilmenite present, the Ti content of amphibole is proposed as a reliable thermometer, and the Ti-rich amphibole with Ti > 0.28 p.f.u. or TiO2 > 2.4–2.6 wt% is indicative of ultrahigh-temperature (>900 °C, UHT) conditions. The anorthite content of plagioclase is also significant in confining peak temperatures for UHT granulite but requires defining proper water content. Zircon dating for 17HT13 yields an upper intercept age of 1910 ± 35 Ma, representing the cooling of UHT granulite. We conclude that the Huai’an Complex may have at least locally undergone UHT metamorphism at ∼1.92–1.91 Ga after high-pressure granulite metamorphism at ∼1.95 Ga, similar to the Jining Complex. Therefore, the Huai’an and Jining Complexes share the same tectonic evolution involving ∼1.95 Ga crustal thickening, ∼1.92–1.91 Ga extension with regional or local UHT metamorphism, and subsequent cooling and uplifting.

Kinetic and equilibrium Ca isotope effects in high-T rocks and minerals

Calcium isotope data (δ44Ca, μ42/44Ca, and μ48/44Ca) are reported for high-temperature metamorphic rocks and minerals, and compared with density-functional theory (DFT) estimates of equilibrium Ca isotope fractionation factors for plagioclase, garnet, clinopyroxene, orthopyroxene, olivine, and apatite. The data and calculations are used to evaluate equilibrium and kinetic fractionation effects that apply to high-temperature metamorphism, where extended residence at high temperature should promote equilibration, but where centimeter-to-meter scale Ca transport could produce diffusive kinetic effects. At upper-granulite facies conditions (T ≥ ∼900°C), DFT-predicted equilibrium fractionations between minerals are ≤0.8‰, decreasing to ca. 0.6‰ at 1100°C. We find much larger δ44Ca variations in both whole-rock samples (range of ∼4‰) and individual minerals (range of ∼8‰), and large variations in the Ca isotope fractionation between mineral pairs (e.g. Δ44Cagrt-plag from −1.5 to +1.5‰). Deviations from equilibrium tend to be larger in concert with indications of higher temperature, such as increasing whole-rock Mg#, plagioclase anorthite content, orthopyroxene Ca/Mg, and garnet Mg#. These large variations are inferred to be due to intragranular or grain-boundary diffusion during metamorphism, as this is the only mechanism that can produce such large isotopic variations. We can confirm the kinetic origin of the variations using measurements of μ48/44Ca by MC-ICP-MS to distinguish kinetic from equilibrium fractionation processes using a triple-isotope approach. A new variable (Δ48Ca′) quantifies deviations from the Ca-isotope equilibrium slope on a plot of 48Ca/44Ca vs. 42Ca/44Ca. Available geochronological constraints and numerical modeling indicate that observed kinetic isotope fractionations between adjacent high and low Ca rock layers require effective Ca diffusivities of 10−10 to 10−7 m2/yr, and a ratio of Ca isotope diffusivities of D44/D40 ≈ 0.99. The diffusivities are consistent with Ca transport by volume- and grain-boundary diffusion. The apparent contrast in isotopic diffusivities is large and more consistent with silicate liquids than aqueous fluids. This study confirms that kinetic Ca isotope effects are abundant in nature and can overwrite small equilibrium effects, even at high temperatures and even when other techniques (such as Fe-Mg exchange and Ca-in-Opx thermometry) suggest the establishment of chemical equilibrium. Our results imply that kinetic fractionation effects may complicate the use of δ44Ca measurements for geothermometry or as a tracer of carbonate recycling into the mantle.

Geochemistry of granitic aplite-pegmatite dykes and sills and their minerals from the Gravanho-Gouveia area in Central Portugal

Two distinct series of Variscan granitic rocks have been distinguished in the Gravanho-Gouveia area of Portugal, based on field work, variation diagrams for major and trace elements, rare earth patterns and δ18O versus total FeO diagram of rocks, anorthite content of plagioclase, BaO and P2O5 contents of feldspars and AlVI versus Fe2+ diagram for magmatic muscovite. One series consists of a late-orogenic porphyritic biotite > muscovite granite (G1), less evolved beryl-columbite pegmatites and more evolved beryl-columbite pegmatites showing gradational contacts. The other series consists of post-orogenic porphyritic muscovite > biotite granodiorite to granite (G2), slightly porphyritic muscovite > biotite granite (G3) and lepidolite pegmatites. In each series, pegmatites are derived from the parent granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Some metasomatic effects occur like muscovite replacing feldspars, chlorite in pegmatites of the first series and a late muscovite in pegmatites of the second series, probably due to hydrothermal fluids. The lepidolite pegmatites contain cassiterite and two generations of rutile. The first magmatic generation consists of homogeneous crystals and the second generation occurs as heterogeneous zoned crystals derived from hydrothermal fluids. The beryl-columbite pegmatites and lepidolite pegmatites also contain the first magmatic generation and the late hydrothermal generation of zoned columbite-group minerals. More evolved beryl-columbite pegmatites were converted into episyenite by intense hydrothermal alteration and regional circulation of fluids in the granitic rocks.

Strontium isotope systematics for plagioclase of the Skaergaard intrusion (East Greenland): A window to crustal assimilation, differentiation, and magma dynamics

Author(s): Hagen-Peter, G; Tegner, C; Lesher, CE | Abstract: We present in situ laser ablation-multicollector-inductively coupled plasma-mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest 87Sr/86Sri (calculated at 56 Ma) values (~0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of ~0.7044, which is uniform throughout the middle ~2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An65-70 at the base and top to ~An25 approaching the purported Sandwich Horizon. Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic (87Sr/86Sri up to ~0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor in situ contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.

A triple S-shaped compositional profile in a Karoo dolerite sill—Evidence of concurrent multiple fractionation processes

Thick dolerite sills show a range of vertical geochemical variation trends attributable to various processes during slow crystallization. We have identified chemical parameters in a 169-m-thick sill from the Karoo igneous province in South Africa that define three different lower crossover levels (maximum or minimum concentrations) creating S-shaped variation trends. The crossover level for wholerock MgO is at 20 m height (due to mechanical sorting of olivine); the anorthite content of plagioclase is at 52 m (due to addition of primitive magma); and that of the incompatible trace elements is at 75 m (due to different proportions of early formed grains to trapped liquid). Each process can operate independently and concurrently, leading to their maximum effects occurring at different levels in the intrusion. The independence of these processes and the triple S-shaped geochemical profiles have not been recognized before in any mafic-ultramafic sills.

Quantitative analysis of mass transfer during polymetamorphism in pelites of the Transangarian Yenisei Ridge

The study provides geological, structural, mineralogical, petrological, and geochronological evidence for polymetamorphic evolution of gneisses from the Garevka complex of the Yenisei Ridge. The results of the study provide significant insight into the geochemical behavior of major and trace elements in zoned garnet crystals and mineral inclusions formed during prograde and retrograde metamorphism of pelitic rocks. It was shown that the concentrations of Y and HREE in garnet decrease with increasing P and T and increase with decreasing pressure and temperature. The combined study of multicomponent chemical zoning patterns of coexisting minerals and metamorphic mineral reactions in metapelites was conducted. The results show that the main reason for a drastic increase in CaO content in garnets during collisional metamorphism is a mass exchange between garnet and plagioclase. The deviation from this trend, as indicated by the concurrent increase inthe grossular content of garnet and anorthite content of plagioclase, arises from the breakdown of epidote. The calculated metamorphic reactions, mass balance analysis, and changes in mineral chemistry during metamorphism reinforce the evidence for the isochemical character of processes with respect to most components of the system. The minimum volume of the system in which chemical exchange between reacting phases is balanced for all major and trace elements did not exceed ~1 mm3. The total HREE balance requires a greater reaction volume (up to ~8 mm ) involved in the redistribution of these elements, which provide evidence for their relatively higher mobility during metamorphism relative to other rare earth elements. The specific distribution and quite substantial mass transport of HREE are controlled by heterovalent isomorphic substitution between these elements and CaO in garnet.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

Open-system processes in the differentiation of mafic magma in the Teide–Pico Viejo succession, Tenerife

Oceanic island basalts are commonly thought to differentiate by fractional crystallization, yet closed-system fractionation models have so far failed to reproduce major and trace element variations observed in mafic lavas from the Teide–Pico Viejo stratovolcano complex on Tenerife. Here, new high-precision plagioclase trace element data are fed into such a fractionation model. The results confirm that fractionation of phenocrysts found in the lavas does not reproduce trace element variations, in particular enrichment of Sr and Zr observed in the Teide–Pico Viejo mafic suite. This enrichment of Sr and Zr is tested by an energy-constrained recharge, assimilation and fractional crystallization (EC-RAFC) model at high T and low Δ T intervals, consistent with previously determined magma storage beneath Tenerife at sub-Moho depths. Published mineral–melt equilibrium relations using the plagioclase anorthite content (0.4 X An Supplementary material: Detailed average mineral compositions of low and high Sr–Zr lavas, composition of the bulk extracts and full graphs of the recalculated closed-system model are available at www.geolsoc.org.uk/SUP18604.

Melt-Rock Reaction in the Mantle: Mantle Troctolites from the Parece Vela Ancient Back-Arc Spreading Center

The Godzilla Megamullion is a giant oceanic core complex located in an extinct slow-spreading segment of the Parece Vela Basin (Philippine Sea). It exposes lower crust and mantle, offering an opportunity to unravel the architecture, composition, and magmatic processes operating in the shallow mantle of a back-arc basin. Here we present data on primitive troctolites and associated olivinegabbros close to the breakaway zone of the megamullion. The troctolites are subdivided into olivine (olivine460 vol. %) and plagioclase (plagioclase460 vol. %) sub-types. Both have textures and mineral compositions suggesting formation through melt^rock reaction.We suggest that the Ol-troctolites formed by dissolution and disaggregation of dunite with crystallization of new plagioclase and diopside by a mid-ocean ridge basalt (MORB)-like melt, producing a high-Mg melt.The Pl-troctolites probably formed by local dissolution of an anorthositic cumulate and crystallization of olivine, diopside and new plagioclase owing to mixing the high-Mg melt residual to the formation of the Ol-troctolites with melt crystallizing an anorthositic cumulate. We show that the Oland Pl-troctolites and associated Ol-gabbros were strongly influenced by melt^rock reaction with the enclosing peridotite, reflecting solidification by reaction and conductive cooling of ascending melts in the mantle. The effects of the melt^rock reaction account for a sharp decrease in plagioclase anorthite content with respect to diopside Mg# and olivine forsterite content. Contrasting olivine nickel contents can constrain the environment of formation of olivine-rich rocks in the lower crust and mantle, the extent and nature of the processes by which they form, and the melt compositions involved in the reaction process. Comparison of olivine nickel contents in troctolite and gabbro occurrences in oceanic and ophiolite settings reveals a set of relationships consistent with this. Olivine gabbros from the 1508 m Hole 735B Atlantis Bank gabbro section have low nickel contents, whereas those from the 1400 m Hole U1309D Atlantis Massif gabbro section have high nickel. This is consistent with less-reacted infiltrating melt at the latter, and suggests that U1309D represents a deeper section of the lower crust near the crust^mantle transition, whereas the former has been shown to have crystallized near the dike^gabbro transition. Godzilla troctolites and gabbros bear clear similarities to East Pacific Rise (EPR) troctolites and gabbroic segregations drilled in dunites crosscutting residual mantle peridotite at Hess Deep Site 895.The latter represent multi-stage crystallization of stagnant melts in conduits beneath the EPR.Thus, it appears that the processes that formed the Godzilla troctolites occur beneath ocean ridges across the spreading rate spectrum.

Continental arc magmatism in a Mesoproterozoic convergent margin: Petrological and geochemical constraints from the magmatic suite of Kondapalle along the eastern margin of the Indian plate

The magmatic suite at Kondapalle represents a Mesoproterozoic (~ ca. 1634 Ma) magmatic arc emplaced in the southern sector of the Eastern Ghats Belt (EGB). Here we present new geological, mineralogical and geochemical data on the various lithological units in this complex including anorthosites, gabbronorites and pyroxenites. The major mineral constituents in these rocks are plagioclase (An 98–57), amphibole (X Mg 0.93–0.52), orthopyroxene (X Mg 0.94–0.51), clinopyroxene (X Mg 0.93–0.63) and chromite (X Mg 0.20–50). The near-absence of plagioclase in the orthopyroxenites, early and abundant crystallization of orthopyroxene, and formation of gabbronorites rather than gabbro or olivine gabbro in the Kondapalle suite are correlated with the features of arc cumulates. The chemistry of chromian spinel and clinopyroxene also displays the trend for arc cumulates. The variations in the anorthite content of plagioclase vs. the Mg# of olivine attest to an arc-related magma source. The rocks display low abundance of incompatible trace elements (Ba, Rb, K and Zr) comparable to the values typically observed in subduction-related magmatic arcs. In trace element N-MORB normalized diagrams, all the rock units show Nb–Ta–Ti–Zr troughs reflecting the features characteristic of arc magmas. We interpret the Kondapalle rocks to represent the root zone of a deeply eroded magmatic arc built during the Mesoproterozoic associated with the subduction of an oceanic lithosphere in a long-lived convergent margin. We identify that the Mesoproterozoic subduction along the eastern margin of the Indian plate generated a wide arc-accretionary complex with an extruded high P–T metamorphic orogen during the final stage of collision. The subduction–accretion process is also supported by recent findings of Mesoproterozoic ophiolite mélanges from this zone, marking the history from the break-up of the Paleoproterozoic Columbia supercontinent to the assembly of the Neoproterozoic Rodinia supercontinent.

Preliminary experimental determination of the partitioning of lithium between plagioclase crystals of different anorthite contents

The role of plagioclase anorthite content in controlling the partitioning of Li has been investigated experimentally. Plagioclase crystals of different anorthite contents were equilibrated with a single plagioclase powder at 1000°C. The results show that the partition coefficient of lithium is a strong function of the anorthite content of the plagioclase, decreasing by a factor of ~3.5 from An60 to An80. This has important implications for the interpretation of the distribution of Li in plagioclase in natural samples. An example of the importance of considering the change in partition coefficient with plagioclase anorthite content is given using published plagioclase Li contents in the products of the Soufriere Hills volcano. It is shown that the distribution of Li in these plagioclase phenocrysts can be reproduced using a model in which Li diffuses into, and out of, the plagioclase crystals if the change in partition coefficient with An content is accounted for. Because Li diffusion in plagioclase is very rapid, the distribution of Li in plagioclase crystal rims can record changing conditions during eruption and pyroclastic flow emplacement (second to minute timescales).

Mineral chemistry, petrogenesis, and tectonic setting of the Wateranga layered intrusion, southeast Queensland, Australia

The Wateranga layered mafic intrusion (28 km2 in area, &gt; 500 m thick) is a tholeiitic, undeformed, unmetamorphosed, Permo-Triassic layered gabbroic pluton intruded into the late Carboniferous Goodnight beds of the Goodnight Block in southeast Queensland. The intrusion mainly consists of gabbro and norite, associated with subordinate amounts of troctolite, anorthosite, and orthopyroxenite, and rare picrite. Olivine gabbro is the dominant rock type of the intrusion. Fractionation followed a tholeiitic trend with iron enrichment in the liquid. Petrographic, mineral chemical, and whole-rock geochemical data have been used to divide the intrusion into Lower, Middle, and Upper zones, which are interpreted as reflecting magma chamber replenishment. The observed changes in the crystallization order between the zones reveal that a single parental magma is inadequate to explain the data. The common differentiation indices, such as An content of plagioclase, Mg#s of olivine, clinopyroxene, orthopyroxene and whole-rocks, and the whole-rock concentrations of various incompatible trace elements (Zr, Y, Nb, La Ba, Rb, Sr, and Nd), all vary widely with stratigraphic depth and display abrupt shifts at the zone boundaries, indicating open system addition of new mafic magma. Temperatures estimated from two-pyroxene geothermometer vary from 1057 to 927 °C. During the course of crystallization, pressure probably was &gt; 2 and &lt; 4 kbar (1 kbar = 100 MPa). The variation trend of anorthite content of plagioclase versus the forsterite content of olivine precludes an arc-related magma source. The composition and geological setting of the intrusion are consistent with emplacement in a post-subduction extensional tectonic environment.

Crustal assimilation in basalt and jotunite: Constraints from layered intrusions

To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r , the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr 0 (initial 87Sr/ 86Sr) and decreases in whole-rock εNd 0 (initial εNd). The r -values of the layered sequences range from ∼0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust/ magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust/ magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions. Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion ( r = 0.27), the initial temperature of the country rocks was ∼450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion ( r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion ( r = 0.12), the country rocks were cooler (∼300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r -values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions. Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r -values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.

Ferric iron partitioning between plagioclase and silicate liquid: thermodynamics and petrological applications

A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO2–Al2O3–Fe2O3–FeO–MgO–CaO–Na2O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe3+,Al3+)Si2O8–Ca(Fe2+,Mg)Si3O8 system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeOt, MgO content and Mg/(Fet+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ±0.34 wt% (relative error =9%) and ±0.08 wt% (14%) and ±0.7 (8%) respectively. Calculated Fe3+–Al exchange chemical potentials of plagioclase, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} $$ agree with those calculated using reported thermodynamic models for multicomponent spinel, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} $$ and clinopyroxene, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} $$ . The FeOt content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe3+/(Fe3++Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase–liquid oxygen barometer are scattered, this study found that plagioclase–spinel–clinopyroxene–oxygen and plagioclase–olivine–oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and fO2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.