| Autor: |
Wang, Changhong, Zhang, Zhaochong, Giuliani, Andrea, Demouchy, Sylvie, Thoraval, Catherine, Krmíček, Lukáš, Bo, Hongze, Zhang, Wanfeng, Xia, Xiaoping |
| Předmět: |
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| Zdroj: |
Journal of Geophysical Research. Solid Earth; Dec2022, Vol. 127 Issue 12, p1-20, 20p |
| Abstrakt: |
Water enters Earth's mantle via subduction of hydrated oceanic slab and largely returns to the ocean‐atmosphere system through arc volcanism. However, the extent to which H2O is transferred into the deep mantle is poorly constrained. Here, we address this question by combining mineral chemistry and bulk‐rock geochemistry data for aillikites related to the deep mantle plume which generated the Permian Tarim large igneous province (NW China). The water contents of olivine phenocrysts are 75–168 ppm H2O and positively correlated with Ti contents. These results, combined with infrared hydroxyl peaks at 3,572 and 3,525 cm−1, suggest that H is mainly present in the form of Ti‐clinohumite‐like point defects. Hydrogen concentration profiles across olivine reveal that H loss during decompression was limited to the outermost rims, and yield dehydration durations of 15–417 min. Based on the water contents of the highest‐Fo olivine cores, the water contents of the primitive aillikite melts and their mantle source are estimated as 1.6–4.7 wt% and 150–1,200 ppm H2O, respectively. 3He/4He ratios (5.31–5.84 Ra) of olivine phenocrysts are slightly lower than MORBs and suggest involvement of recycled slab containing U (and hence radiogenic 4He) in the plume source. This interpretation is consistent with Pb isotope compositions of the aillikites which are intermediate between PREMA (Prevalent Mantle) and EM (Enriched Mantle) compositions. These lines of evidence combined with the depleted Sr‐Nd isotopes and moderately radiogenic Os isotopes of the aillikites suggest that water in these rocks derived from a plume source marginally contaminated by deeply subducted hydrated material. Plain Language Summary: Water enters Earth's mantle via subduction of water‐bearing oceanic crust and largely returns to Earth's surface through arc volcanism above subduction zones. However, how much surface‐derived water travels to the deep Earth locked in subducted material is poorly constrained. Here, we address this question by combining mineral and bulk‐rock geochemical data for carbonate‐rich and water‐rich volcanic rocks genetically linked to the Tarim mantle plume (convective upwelling of thermally anomalous mantle). Olivines in these rocks contain high water contents that are positively correlated with Ti due to a coupled mechanism of incorporation in the olivine structure. Water content profiles across olivine reveal that water loss from olivine during magma ascent was limited to the outermost rims. The calculated water contents of the mantle source of these rocks based on water concentrations in the olivine cores are higher than typical H2O contents in the upper mantle. In combination with bulk‐rock Pb‐Sr‐Nd‐Os isotope data of these volcanic rocks and new He isotope compositions for these olivines, these data suggest that the deep mantle source of these rocks was polluted by subducted water‐bearing oceanic crust. The elevated water contents in mantle source help explain the occurrence of voluminous magmatism of large igneous province. Key Points: Olivines in the Wajilitag aillikites show high water contents (75–168 ppm H2O)Hydrogen loss in olivines during decompression was limited to the outermost rims, and yields dehydration durations of 15–417 minH2O in these rocks derived from a plume source slightly contaminated by recycled hydrous crustal material [ABSTRACT FROM AUTHOR] |
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