Melt Inclusions, Keys to Understand Silicon Isotopes Behavior at Small Scale.
Abstract
In this study, we analyze δ30Si in euhedral olivine crystals and their hosted melt inclusions (OHMIs) from three samples from the Mid-Atlantic Ridge, in order to better understand the behavior of δ30Si at small scale in magmatic system. Indeed, to date, only bulk δ30Si have been reported for Mid-Ocean Ridge Basalt (MORB) samples with a range from -0.36 to -0.22‰ [1].
Si isotopes were first measured in OHMIs with a Cameca IMS1280HR (SIMS). A calibration using optical basicity was used to correct for instrumental mass fractionation [2]. Average δ30Si in OHMIs from the three samples is -0.48 ± 0.39‰ (2sd). Importantly, δ30Si variability could be up to 2‰ within a sample, which is 4 times larger than in bulk MORB [<0.5‰; 1]. In order to verify this variability, δ30Si were also measured in the same OHMIs, as well as in four olivine crystals using laser-ablation-MC-ICP-MS following the protocol published in [3]. OHMIs analyzed by laser ablation display a variability of 0.54‰, by far smaller compare to SIMS measurements but close to bulk MORB data. The average δ30Si of all OHMIs measured by LA-MC-ICP-MS is -0.47 ± 0.17‰ (2sd), consistent with SIMS, but slightly lower than average MORB glasses from the Atlantic (-0.27 ± 0.06‰ (2sd); [4]). The four host olivine crystals (Fo90) have an average δ30Si of -0.34 ± 0.17‰ (2sd). The different δ30Si variability measured in OHMIs by two different in situ methods suggest that despite careful calibrations (R2 > 0.99), an analytical bias still needs to be identified for SIMS δ30Si measurements in glasses. The lower δ30Si compare to MORB obtained by both methods are suggested to be real. Indeed, the three standards used during the LA-MC-ICP-MS session (BHVO-2, BIR and BCR) gave consistent results with literature. The Si isotope fractionation between olivine and OHMIs varies from -0.26 to 0.0‰, similar to theoretical equilibrium (Δ30Siolivine-melt = -0.15‰ [5]). This does not suggest boundary layer effect as a possible process for the low δ30Si values measured in OHMIs. These results will be discussed as a possible tool to investigate silicon isotope fractionation between melt and minerals or during magmatic differentiation. REFERENCES [1] Poitrasson, 2017, RIMG 82 [2] Tissandier et Rollion-Bard, 2017, RCMS 31 [3] Guitreau et al., 2020, JAAS [4] Savage et al., 2010, EPSL 295 [5] Savage et al., 2011, GCA 75- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV025...08L
- Keywords:
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- 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRY;
- 1105 Quaternary geochronology;
- GEOCHRONOLOGY;
- 8434 Magma migration and fragmentation;
- VOLCANOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY