Marghaleray Amini

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

A practical guide for the design and implementation of the double-spike technique for precise determination of molybdenum isotope compositions of environmental samples

The isotopic double-spike method allows for the determination of stable isotope ratios by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) with accuracy and precision in the range of ∼0.02 ‰ amu−1, but its adoption has been hindered by the perceived difficulties in double-spike calibration and implementation. To facilitate the implementation of the double-spike approach, an explanation of the calibration and validation of a 97Mo-100Mo double-spike protocol is given in more detail than has been presented elsewhere. The long-term external standard reproducibility is 0.05 ‰ on δ98/95Mo measurements of standards. δ98/95Mo values for seawater and U.S. Geological Survey (USGS) reference materials SDO-1 and BCR-2 measured in this study are 2.13 ± 0.04 ‰ (2 SD, n = 3), 0.79 ± 0.05 ‰ (2 SD, n = 11), and −0.04 ± 0.10 ‰ (2 SD, n = 3) relative to the NIST-SRM-3134. The double-spike method corrects for laboratory and instrumental fractionation which are not accounted for using other mass bias correction methods. Spike/sample molar ratios between 0.4 and 0.8 provide accurate isotope measurements; outside of this range, isotope measurements are inaccurate but corrections are possible when standards and samples are spiked at a similar ratio.

The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion

The Neoarchean Stillwater Complex, one of the world’s largest known layered intrusions and host to a rich platinum-group element deposit known as the J-M Reef, represents one of the cornerstones for the study of magmatic processes in the Earth’s crust. A complete framework for crystallization of the Stillwater Complex is presented based on the trace element geochemistry of zircon and comprehensive U–Pb zircon–baddeleyite–titanite–rutile geochronology of 22 samples through the magmatic stratigraphy. Trace element concentrations and ratios in zircon are highly variable and support crystallization of zircon from fractionated interstitial melt at near-solidus temperatures in the ultramafic and mafic cumulates (Ti-in-zircon thermometry1⁄4 980–720 C). U–Pb geochronological results indicate that the Stillwater Complex crystallized over a 3 million-year interval from 2712 Ma (Basal series) to 2709 Ma (Banded series); late-stage granophyres and at least one phase of post-emplacement mafic dikes also crystallized at 2709 Ma. The dates reveal that the intrusion was not constructed in a strictly sequential stratigraphic order from the base (oldest) to the top (youngest) such that the cumulate succession in the complex does not follow the stratigraphic law of superposition. Two distinct age groups are recognized in the Ultramafic series. The lowermost Peridotite zone, up to and including the G chromitite, crystallized at 2710 Ma from magmas emplaced below the overlying uppermost Peridotite and Bronzitite zones that crystallized earlier at 2711 Ma. Based on the age and locally discordant nature of the J-M Reef, the base of this sequence likely represents an intrusion-wide magmatic unconformity that formed during the onset of renewed and voluminous magmatism at 2709 Ma. The thick anorthosite units in the Middle Banded series are older (2710 Ma) than the rest of the Banded series, a feature consistent with a flotation cumulate or ‘rockberg’ model. The anorthosites are related to crystallization of mafic and ultramafic rocks now preserved in the Ultramafic series and in the lower part of the Lower Banded series below the J-M Reef. The Stillwater Complex was constructed by repeated injections of magma that crystallized to produce a stack of amalgamated sills, some out-of-sequence, consequently it does not constitute the crystallized products of a progressively filled and cooled magma chamber. This calls into question current concepts regarding the intrusive and crystallization histories of major open-system layered intrusions and challenges us to rethink our understanding of the timescales of magma processes and emplacement in these large and petrologically significant and remarkable complexes. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 153 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 1, 153–190 doi: 10.1093/petrology/egy024 Advance Access Publication Date: 28 February 2018

A Temperature-Composition Framework for Crystallization of Fractionated Interstitial Melt in the Bushveld Complex from Trace Element Systematics of Zircon and Rutile

The near-solidus crystallization history of the Paleoproterozoic Bushveld Complex, the world’s largest layered intrusion, has been investigated using the in situ trace element geochemistry (LA-ICP-MS) of accessory minerals that crystallized from late, highly fractionated pockets of interstitial melt in layered cumulates and from granitic magmas in felsic roof rocks. Zircon with simple to complex sector zoning occurs in mafic–ultramafic rocks in interstitial pockets that contain quartz–biotite–plagioclase and local granophyric intergrowths. Chondrite-normalized rare earth element patterns are typical of igneous zircon and Ti is negatively correlated with Hf in most samples. Ti-in-zircon thermometry of the cumulates (T1⁄4 950–730 C) records the onset of zircon saturation through to the solidus, with notably cooler temperatures determined for Upper Zone and roof rock zircon (T1⁄4 875–690 C). Forward modelling of proposed Bushveld parental magmas using rhyolite-MELTS consistently yields similar temperatures for zircon saturation (800–740 C) from highly fractionated melts ( 5–20% remaining melt) with late-stage, near-solidus mineral assemblages similar to those observed in the rocks. Anomalously high and variable Th/U (2–77) in zircon from orthopyroxenites in the Critical Zone, including those associated with the PGE-rich UG2 chromitite and Merensky Reef in the Upper Critical Zone, can be related to U loss from the fractionated interstitial melt during exsolution of late, oxidized Cl-rich fluids. In addition to zircon, rutile occurs throughout the Critical Zone of the Bushveld Complex in two different textural settings, as interstitial grains with quartz and zircon and with chromite, each with distinctive chemistry. Euhedral rutile needles found in interstitial melt pockets have relatively high HFSE concentrations (Nb1⁄4 1000–20 000 ppm; Ta1⁄4 100–1760 ppm), high Zr-in-rutile temperatures (1000–800 C), and are magmatic in origin. Rutile associated with chromite, either as rims or inclusions, is strongly depleted in HFSE (Nb <1000 ppm; Ta <100 ppm) and in Cr and Sc relative to magmatic rutile, and represents a sub-solidus exsolution product of Ti from chromite. Exploring the near-solidus evolution of mafic layered intrusions such as the Bushveld Complex using the trace element chemistry of accessory minerals provides a novel approach to constraining the late stages of crystallization from highly fractionated interstitial melts in these petrologically significant intrusions.