Sulfur isotope anomaly of igneous sulfides in the Belingwe komatiites implication for the onset of the subduction
Abstract
Earth's crustal material has subducted deep into the mantle and occasionally recycled back to the surface. Recently, the findings of mass-independent fractionation (MIF) sulfur from HIMU and EM I type OIBs revealed that these OIBs contains Archean crustal material (Cabral et al., 2013; Delavault et al., 2016). However, it is still uncertain when the MIF bearing crust subducted into the mantle. Here, we report high-precision multiple sulfur isotope analysis of Archean komatiite in order to test whether the recycling of the subducted crustal material took place even in Archean or before. We have measured one of the best-preserved Archean komatiites from 2.7 Ga Belingwe Greenstone Belt. The results of the isotope analysis show that some komatiite and related basalt samples showed negative MIF signature for Δ33S. All the S-MIF bearing samples show depleted REE pattern (Shimizu et al., 2005), indicating that assimilation of crustal material was minimal during the volcanism. Lead isochrones of all the samples are ~2.7 Ga (Shimizu et al., 2005), implying that chalcophile elements are not altered after the emplacement. In the MIF-bearing samples, sulfides mainly occur as fine disseminated pyrrhotite and chalcopyrite, crystallized after pyroxene spinifex or cumulate. The sulfides show no detectable Ni. The observed nickel depletion in the sulfides reflects the partitioning of Ni into olivine crystalized before the sulfides. All these features are consistent with the igneous origin of the S-MIF bearing sulfides, not introduced from secondary hydrothermal alteration. On the other hand, some non-MIF-bearing samples contain sulfides that do not seem to be crystalized during eruption and might not be igneous sulfide but alteration. Consequently, the observed S-MIF signature in the Belingwe komatiites are originated from the mantle. Therefore, our result infers that transfer of surface material into the mantle has already operated at least before 2.7 Ga.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR51A..07K
- Keywords:
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- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY;
- 8409 Atmospheric effects;
- VOLCANOLOGY