High‐Mg# Olivine, Clinopyroxene and Orthopyroxene Reference Materials for <i>In Situ</i> Oxygen Isotope Determination

A number of bulk‐rock and mineral Fe and Mg isotope data have been increasingly reported from ophiolitic rocks, but their anomalous fractionations are still not well addressed. This study presents new Fe, Mg, and O isotopes and published Li isotope data of olivine and/or chromite in chromitite, dunite, and harzburgite from the Pozantı‐Karsantı ophiolite (southern Turkey) to reveal the mechanisms of inter‐ and intra‐mineral isotope fractionation during chromitite formation. In the studied samples, both olivine and chromite have highly variable δ56Fe ranging from −0.125‰ to 0.101‰ and −0.261‰ to 0.191‰, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with δ26Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ26Mg values ranging from −0.02‰ to 0.45‰. The δ18O values of olivine range from 4.65‰ to 5.86‰ and co‐vary with δ7Li (−5.43‰ to 16.65‰), δ56Fe, and δ26Mg, particularly at lithological scale. These isotopic features cannot be explained solely by subsolidus diffusion between olivine and chromite, or by partial melting and fractional crystallization. Instead, they require fluids as a diffusive medium for high efficient element exchange between chromite and olivine. The fluids were likely the hydrous fluids that were collected on chromite grain surfaces during crystallization and released later from chromite piles during precipitation. The fluid‐olivine reaction preferentially transferred light Fe from olivine grains resulting in elevated δ56Fe, but caused small O and Mg isotopic variations in the olivine. The Li diffusion depends on Li concentration gradient between olivine and fluid and olivine/fluid ratio, which can account for large δ7Li variations in olivine between different rock types. As these reactions proceeded, they induced elemental and isotopic variations in the fluids. These fluids could continuously penetrate the harzburgites through the dunite envelopes around the chromitites, producing some of the observed anomalous geochemical features of the harzburgites. Thus, we conclude that the hydrous fluids acted as critical media for ion exchange between chromite and olivine and for significant modification of the host harzburgites.

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“…(2010) and Tang et al. (2015, 2019). The δ 18 O and published δ 7 Li values are shown in Table 3.…”

Section: Methodsmentioning

confidence: 94%

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

Chen

Xiao

et al. 2021

JGR Solid Earth

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“…There is still lack of accessible clinopyroxene reference material for Sr isotope microanalysis. Despite a few published papers about in situ Sr analysis of clinopyroxene, the shortage of reference materials hinders the development of in situ Sr isotope measurements for clinopyroxene (Waight et al, 2002 ; Bizzarro et al, 2003 ; Schmidberger et al, 2003 ; Hart et al, 2005 ; Sun et al, 2012 ; Xu et al, 2013 ; Su et al, 2015 ; Tong et al, 2016 ; Deng et al, 2017 ; Tang et al, 2019 ).…”

Section: Introductionmentioning

confidence: 99%

Natural Clinopyroxene Reference Materials for in situ Sr Isotopic Analysis via LA-MC-ICP-MS

Zhao

Roux

et al. 2020

Front. Chem.

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Clinopyroxene is a major host mineral for lithophile elements in the mantle lithosphere, and therefore, its origin is vital for constraints on mantle evolution and melt generation. In situ Sr isotopic measurement of clinopyroxene has been available since the recent development of laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) in the 2000s. Therefore, there is an increasing demand for natural clinopyroxene reference materials for Sr isotope microanalysis. In this contribution, we present six natural clinopyroxene reference materials from South Africa (JJG1424) and China (YY09-47, YY09-04, YY09-24, YY12-01, and YY12-02) for Sr isotope microanalysis. The Sr content of these clinopyroxenes ranges from 50 to 340 μg g−1, which covers most natural clinopyroxene compositions. Homogeneity of these potential reference materials were investigated and evaluated in detail over a 2-year period using 193-nm nanosecond and 257-nm femtosecond laser systems coupled to either a Neptune or Neptune Plus MC-ICP-MS. Additionally, the major and trace element of these clinopyroxenes were examined by electron probe microanalyzer (EPMA) as well as solution and laser ICP-MS. The in situ87Sr/86Sr values obtained for the six natural clinopyroxene reference materials agree well with data obtained using the thermal ionization mass spectrometer (TIMS) method. The Sr isotopic stability and homogeneity of these clinopyroxenes make them potential reference materials for in situ Sr microanalysis to correct instrumental fractionation or as quality control materials for analytical sessions. The new Sr isotope data provided here might be beneficial for microbeam analysis in the geochemical community.

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“…The analytical spot size was approximately 20 µm squared (10 µm beam size + 10 µm raster). The signals of 16 O and 18 O were simultaneously collected by two Faraday cups using a multi-collection mode. A normal electron gun was used to compensate for the charge effect.…”

Section: Methodsmentioning

confidence: 99%

“…The matrix effects associated with Fe-Mg-exchangeable minerals (such as olivine, dolomite, and garnet) have been reported [14][15][16]. Because of matrix effects, matrixmatched reference materials or a suitable fractionation law to calibrate IMF is required during SIMS oxygen isotope measurement [17,18]. As for beryl, the iron content could vary from 0.1 to 3 wt.% [19].…”

Section: Introductionmentioning

confidence: 99%

Beryl Reference Materials for In Situ Oxygen Isotope Determination

Ling

Liu

et al. 2021

Crystals

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The mineral beryl (Be3Al2(SiO3)6) has the most abundant phase with industrial value for extracting a critical metal—beryllium. Due to multi-stage, fluid-induced growth, individual beryl grains may yield complex geochemical records, revealing variations in the oxygen isotopes of the fluids from which they crystallize. Secondary ion mass spectrometry (SIMS) with high sensitivity and high spatial resolution represents a good tool for in situ isotopic analysis. SIMS oxygen analyses require matrix-matched reference materials to calibrate instrumental mass fractionations during measurement. In this work, the oxygen isotope homogeneities of six beryl samples with different compositions (BS1, BS2, BS3, BS4, BS5, and BS6) were documented by SIMS. These samples’ recommended oxygen isotope compositions were characterized by laser fluorination isotope ratio mass spectrometry (IRMS). This study suggests that there is no matrix effect related to composition variation in beryl SIMS oxygen isotope analysis. The recommended δ18O values of the four reference materials, BS1, BS2, BS4, and BS5, were 15.01 ± 0.34‰ (2 standard deviations, 2 SD), 7.53 ± 0.16‰ (2 SD), 2.38 ± 0.14‰ (2 SD), and 10.72 ± 0.44‰ (2 SD), respectively. Therefore, BS1, BS2, BS4, and BS5 are recommended as suitable reference materials for in situ mineral beryl oxygen isotope microanalysis.

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High‐Mg# Olivine, Clinopyroxene and Orthopyroxene Reference Materials for In Situ Oxygen Isotope Determination

Cited by 23 publications

References 29 publications

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

Natural Clinopyroxene Reference Materials for in situ Sr Isotopic Analysis via LA-MC-ICP-MS

Beryl Reference Materials for In Situ Oxygen Isotope Determination

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