The role of high- and low-temperature ocean crust alteration for the marine calcium budget

Calcium (Ca) is a key element for the understanding of the chemical evolution of the ocean and for the global climate on long geological time scales. This is because Ca is interacting with the carbon cycle and is a major constituent of continental weathering. Beside continental runoff, mid-ocean ridges are of quantitative importance for the marine Ca elemental and isotope budget. Variations of hydrothermal circulation of seawater through oceanic crust have been recognized to play a significant role for the oceanic Ca mass and isotope balance. Hydrothermal activity leads to a chemical alteration of the circulating seawater at low- and high temperatures during water-rock interaction, the formation of Ca-bearing minerals, and during phase separation. Within the framework of the subproject 'CARLA' in the 'Special Priority Program SPP 1144' Ca isotope ratios (d44/40Ca) in hydrothermal fluids sampled from the Logatchev hydrothermal field (15°N/45°W) and the Ascension area (4 11°S) have been investigated in detail in order to further constrain the global Ca cycling

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. Copyright 2006 by the American Geophysical Union

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 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 (T= 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 (T= 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 (Nb= 1000–20 000 ppm; Ta= 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 \u3c1000 ppm; Ta \u3c100 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

Evaluating Downhole Fractionation Corrections in LA-ICP-MS U-Pb Zircon Geochronology

Among the most significant challenges in maximizing the precision and accuracy of U-Pb zircon geochronology by LA-ICP-MS is minimizing the impact of downhole fractionation, the time-dependent evolution of Pb/U ratios caused mainly by complex differences in the volatility and chemical properties of elements as they are excavated from the ablation site. To produce meaningful dates for unknown materials, downhole fractionation is typically quantified in a reference zircon and a time-based correction factor subsequently employed to yield constant Pb/U in both standard and unknown zircons. This assumes that both the reference and unknown zircon exhibit similar downhole behaviour. As a test of this assumption, downhole fractionation trends were characterized and quantified in three common zircon reference materials (Plešovice, 337 Ma; Temora-2, 417 Ma; 91500; 1065 Ma) and in three low-U (\u3c 300 ppm) zircon samples with coherent U-Pb systematics from mafic intrusions (Laramie, 1436 Ma; Bushveld, 2057 Ma; Stillwater, 2710 Ma). Using an exponential downhole correction model based on each of the untreated zircon reference materials, the corrections were applied to each of the “unknowns” and the resulting time-dependent Pb/U ratios and final ages were compared. The effectiveness of pre-treatment protocols was also evaluated by comparing downhole fractionation trends for untreated grains, annealed grains, and grains that were annealed and leached (i.e., chemical abrasion). Each of the three zircon reference materials exhibited distinct downhole fractionation and their calculated correction factors had variable influence in correcting each of the unknowns. Application of the fit parameters either over-corrected or under-corrected the Pb/U ratios of the unknowns due to differences in slope for the different downhole rates at a given time. In most cases, annealing zircon prior to LA-ICP-MS analysis lessened the magnitude of U-Pb mass fractionation during laser ablation, whereas chemical abrasion did not significantly change ablation behaviour beyond simply annealing the grains. Based on the relative effectiveness of the downhole correction that was applied, the resulting U-Pb dates of zircon from the Precambrian mafic intrusions can vary significantly when compared to the CA-ID-TIMS ages established for these samples. The results of this study indicate that a robust downhole correction method for LA-ICP-MS U-Pb geochronology of zircon, particularly for application to magmatic zircon and igneous crystallization ages, involves characterization of downhole fractionation in different reference materials and then applying a correction to the unknown zircon based on a reference zircon that behaves similarly during ablation

‘The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-system Layered Intrusion’: a Reply to the Comment by Rais Latypov on Wall \u3cem\u3eet al\u3c/em\u3e. (J. Petrology, 59, 153–190, 2018)

We are pleased to have the opportunity to discuss the comments of Latypov (2019) on the interpretations and significance of the extensive U–Pb zircon–baddeleyite–rutile–titanite geochronological and zircon trace element dataset reported by Wall et al. (2018) from the Neoarchean Stillwater Complex in southern Montana (USA). Layered intrusions are the crustal repositories for minerals crystallized from mantle-derived magmas and have long fascinated geologists with their remarkable records of magmatic differentiation (Bowen, 1928; Wager & Brown, 1967; Parsons, 1987; Cawthorn, 1996; Charlier et al., 2015). Establishing the timescales of emplacement and crystallization of mafic intrusions (layered intrusions, dikes, sills) is critical to the assessment of a wide range of processes, including rates of mantle melting and magma transport through the crust, crystallization timescales in magma reservoirs and crystal mushes, relationships to mineralization, evolution of crustal magmatism, and potential links between magmatic degassing and global environmental impacts (e.g. LeCheminant & Heaman, 1989; Premo et al., 1990; Hamilton et al., 1998; Schwartz et al., 2005; Heaman et al., 2009; Mackie et al., 2009; Chamberlain et al., 2010; Scoates & Scoates, 2013; Burgess & Bowring, 2015; Scoates & Wall, 2015; Mungall et al., 2016; Burgess et al., 2017; Scoates et al., 2017). As a community, we now have the tools to precisely (i.e. significantly less than 1%) and accurately date individual, or even parts of, zircon grains that yield concordant U–Pb systematics (e.g. Mattinson, 2005; Schmitz & Kuiper, 2013; Schoene, 2014; Samperton et al., 2015). We can also now track the petrochronological evolution of individual zircon grains by integrating these dates with trace element geochemistry and Ti-in-zircon thermometry determined in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (e.g. Ferry & Watson, 2007; Grimes et al., 2009; Claiborne et al., 2010; Kylander-Clark et al., 2013; Samperton et al., 2015; Ver Hoeve et al., 2018)

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 thermometry=980–720℃). 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