Diffusion-driven chromium isotope fractionation in ultramafic cumulate minerals: Elemental and isotopic evidence from the Stillwater Complex

Bai, Yang; Su, Ben‐Xun; Xiao, Yan; Chen, Chen; Cui, Meng‐Meng; He, Xiaoyun; Qin, Liping; Charlier, Bernard

doi:10.1016/j.gca.2019.07.052

Cited by 26 publications

(25 citation statements)

References 78 publications

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“…Such hydrous fluids could easily enhance chemical diffusion between silicate phases and chromite (Shimizu and Okamoto, 2016;Bai et al, 2019). Some components in the hydrous fluids might also be partitioned into olivine around chromite and even into chromite itself, which are later expelled in the form of mineral exsolutions (e.g., Liang et al, 2018).…”

Section: Effects Of Hydrous Fluids On Chromite Surfacesmentioning

confidence: 99%

The occurrence, origin, and fate of water in chromitites in ophiolites

Robinson

Chen

et al. 2020

American Mineralogist

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We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that clinopyroxene and olivine in chromitites have H2O contents of 801–366 and 53–17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxene-bearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.

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Section: Effects Of Hydrous Fluids On Chromite Surfacesmentioning

confidence: 99%

The occurrence, origin, and fate of water in chromitites in ophiolites

Robinson

Chen

et al. 2020

American Mineralogist

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“…The variations in Cr isotopes are reported using the notation: Most recent analyses on mantle xenoliths and mineral separates have shown that Cr isotopes are fractionated during partial melting (Xia et al 2017, Farkas et al 2013, Bai et al 2019.…”

Section: Chromium Isotopesmentioning

confidence: 99%

“…Our measurements predict that during partial melting, the silicate melts will be enriched in light isotopes, and the residues will be enriched in heavy isotopes. Recent measurements of ocean island basalts (Shen et al 2020, Bonnand et al 2020 and in mantle peridotites (Xia et al 2017, Shen et al 2018, Bai et al 2019 show that residues tend to be 0.05 ‰ heavier than products. Furthermore, the results on the time series experiments demonstrate that in non-equilibrium partial melting reactions, Cr isotope fractionations are driven by kinetic reaction and the melts are enriched in light isotopes.…”

Section: Implications For Cr Isotopes At High Temperaturementioning

confidence: 99%

Redox control on chromium isotope behaviour in silicate melts in contact with magnesiochromite

Bonnand

Bruand

Matzen

et al. 2020

Geochimica et Cosmochimica Acta

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Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr 2+ , Cr 3+ and Cr 6+ ) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 < logfO2 < −4), this study presents the first experimental petro-isotopic investigation of Cr isotope fractionation and reveals clear systematics between Cr concentration, isotopic compositions and fO2. Two series of experiments were performed to study (a) the dissolution of a natural magnesiochromite into Cr-free silicate melts (series A) and (ii) the crystallisation of magnesiochromite from Cr-doped silicate melts (series B). In agreement with previous studies, the Cr solubility in the silicate melts at equilibrium with magnesiochromite is strongly controlled by oxygen fugacity. Melts produced at low fO2 are enriched in Cr compared to more oxidised melts. In series A experiments, the Cr isotopic composition of silicate melts are lighter than the initial chromite starting material. The experiments, performed at −12 < logfO2 < −6, reveal that Cr isotopic compositions of the silicate melts are correlated with fO2. This demonstrates that, as for the Cr solubility, Cr isotopes are sensitive to fO2 and could be used to track changes in redox conditions in hightemperature processes. Furthermore, the Cr isotopic compositions of silicate melts that are reacted under more oxidising conditions (logfO2 > −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions.

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“…This results in compositional variations and modifications in minerals via re-equilibration and interaction (Pagé et al 2011). Further interaction or chemical diffusion may also occur between crystallized minerals and interstitial liquids (Raedeke and McCallum 1984;Lenaz et al 2012) and between subsolidus mineral phases, such as olivine and chromite during solidification and cooling (Jackson 1961;McCallum 2002;Bai et al 2019). The extent of such interactions depends largely on the spatial migration of the melts; O' Driscoll et al (2009) proposed downward infiltration of a melt during the formation of layers in such intrusions, whereas others have argued for upward-percolation of the melts (Kaufmann et al 2018) owing to compaction of the underlying crystal pile (Irvine 1980) or a temperature gradient-driven flux (Latypov et al 2008).…”

Section: Introductionmentioning

confidence: 99%

“…In this study, we conducted in situ analyses of major and trace elements and Li and O isotopes of major silicate minerals from the Ultramafic Zone of the Stillwater Complex following petrographical and mineralogical investigations. These datasets, together with the Cr isotope data from the same samples in Bai et al (2019), are used to identify elemental and isotopic variations in different rock types and to constrain potential melt/fluid activity as well as chemical interactions between various components.…”

Section: Introductionmentioning

confidence: 99%

Petrogenesis of the Ultramafic Zone of the Stillwater Complex in North America: constraints from mineral chemistry and stable isotopes of Li and O

Bai

Cui

et al. 2020

Contrib Mineral Petrol

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To investigate the petrogenesis of cyclic units in layered intrusions, we examined chromitite, dunite, poikilitic harzburgite and bronzitite from the Ultramafic Zone of the Stillwater Complex and measured stable isotopes of Li and O in their major minerals. The Li isotopes in olivine range from 4 to 26‰ in δ 7 Li with uniform Li contents of 1-3 ppm, whereas orthopyroxene and clinopyroxene have Li contents of 0.5-5 ppm and 4-8 ppm, and δ 7 Li ranges of −13 to 7‰ and −14 to −6‰, respectively. The δ 18 O values vary from 4.91 to 5.72‰ in olivine, from 5.11 to 5.87‰ in orthopyroxene, and from 4.64 to 5.86‰ in clinopyroxene. For a given sample, olivine displays more variable and higher δ 7 Li but lower δ 18 O values than orthopyroxene, indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene. The general Li and O isotopic compositions are interpreted as the result of re-equilibration between interstitial liquids, from which pyroxenes crystallized, and cumulus minerals. The inter-mineral and inter-sample isotopic variations correlate with mineral assemblages, crystal sizes and major and trace element compositions, revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma. Abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids, which collected on chromite surfaces and were later released from chromite seams, played an additional, critical medium of chemical exchange between minerals in the chromitites.

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Diffusion-driven chromium isotope fractionation in ultramafic cumulate minerals: Elemental and isotopic evidence from the Stillwater Complex

Cited by 26 publications

References 78 publications

The occurrence, origin, and fate of water in chromitites in ophiolites

The occurrence, origin, and fate of water in chromitites in ophiolites

Redox control on chromium isotope behaviour in silicate melts in contact with magnesiochromite

Petrogenesis of the Ultramafic Zone of the Stillwater Complex in North America: constraints from mineral chemistry and stable isotopes of Li and O

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