Abstract Details
(2020) Metasomatism of Crust-Mantle Boundary by Melts Derived from Subducted Sedimentary Carbonates and Silicates
Hu J & Jiang N
https://doi.org/10.46427/gold2020.1079
03g: Room 1, Thursday 25th June 08:33 - 08:36
Jun Hu
View all 2 abstracts at Goldschmidt2020
View abstracts at 4 conferences in series
Neng Jiang View all 2 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
Neng Jiang View all 2 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
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Submitted by Jingao Liu on Wednesday 24th June 07:31
Hi Jun, very good data and story. I enjoyed. Quick questions: 1) Why do clinopyroxenites seem to have lower temperatures than both upper websterite and lower Cr-pyroxenites? 2) Is that chemically allowed to have so much carbonate-silicate melts into reaction to form clinopyroxenites? or it's open system? 3) from zircon ages, it seems have multiple formations. Could super enriched Sr-Nd isotopic compositions of clinopyroxenites derive from more anicent metasomatism? Thanks, Jingao
Hi Jinggao, thank you for your interest in our work and your good questions. Here are the answers to your questions: (1) We believe that the clinopyroxenites have lower temperatures (760-840 ºC) due to the metasomatism. In fact, we have constrained the P-T conditions of melting of the subducted sediments (~1.5 GPa with temperature of 750-850 ºC), which is identical to the P-T estimations of the clinopyroxenites. However, our contains on the P-T of sediment melting are not strong enough, so I do not talk about it in this time-limited presentation. PS. Our result (the estimated slab surface temperature of ~800 ºC at ~1.5 Gpa) is contradict with computational models but approximates the upper end of the temperature range derived from exhumed metamorphic rocks (Penniston-Dorland et al., 2015, EPSL). Shimoda et al. (1998, EPSL) also suggest that sediment melting might have taken place at ~1.5 GPa in their study of Setouchi high-Mg andesites. So, we believe that there is still a lot of controversy about the thermal models of the subduction zone and more work is needed to solve the temperature discrepancy between rocks and models. (2) The modeling calculation shows that metasomatism of the Al-websterites by 30-50% input of carbonate-silicate melts. I am not sure whether it is an open system, but I personally tend to think that the metasomatism have lasted for a long time as the zircons record the metasomatic events from 368 to 320 Ma. Therefore it is likely to be an open system. The clinopyroxenite are the products of long-term continuous metasomatism of Al-websterite with carbonate-silicate melts. (3) You are right that the zircons yield a wide range of ages from 92 to 1911 Ma. However, for the Hannuoba clinopyroxenites, their unusually high ?18O values require an 18O-rich metasomatic melt. Accordingly, zircons formed by this metasomatic event also should have high ?18O. Therefore, the zircons with Proterozoic and Mesozoic ages are considered not to be associated with the formation of the clinopyroxenites because their ?18O values are lower than 7.16‰. Only zircons with age of 320-368Ma have extremely high ?18O as the clinopyroxene and the whole rock.
Hi Jun, very good data and story. I enjoyed. Quick questions: 1) Why do clinopyroxenites seem to have lower temperatures than both upper websterite and lower Cr-pyroxenites? 2) Is that chemically allowed to have so much carbonate-silicate melts into reaction to form clinopyroxenites? or it's open system? 3) from zircon ages, it seems have multiple formations. Could super enriched Sr-Nd isotopic compositions of clinopyroxenites derive from more anicent metasomatism? Thanks, Jingao
Hi Jinggao, thank you for your interest in our work and your good questions. Here are the answers to your questions: (1) We believe that the clinopyroxenites have lower temperatures (760-840 ºC) due to the metasomatism. In fact, we have constrained the P-T conditions of melting of the subducted sediments (~1.5 GPa with temperature of 750-850 ºC), which is identical to the P-T estimations of the clinopyroxenites. However, our contains on the P-T of sediment melting are not strong enough, so I do not talk about it in this time-limited presentation. PS. Our result (the estimated slab surface temperature of ~800 ºC at ~1.5 Gpa) is contradict with computational models but approximates the upper end of the temperature range derived from exhumed metamorphic rocks (Penniston-Dorland et al., 2015, EPSL). Shimoda et al. (1998, EPSL) also suggest that sediment melting might have taken place at ~1.5 GPa in their study of Setouchi high-Mg andesites. So, we believe that there is still a lot of controversy about the thermal models of the subduction zone and more work is needed to solve the temperature discrepancy between rocks and models. (2) The modeling calculation shows that metasomatism of the Al-websterites by 30-50% input of carbonate-silicate melts. I am not sure whether it is an open system, but I personally tend to think that the metasomatism have lasted for a long time as the zircons record the metasomatic events from 368 to 320 Ma. Therefore it is likely to be an open system. The clinopyroxenite are the products of long-term continuous metasomatism of Al-websterite with carbonate-silicate melts. (3) You are right that the zircons yield a wide range of ages from 92 to 1911 Ma. However, for the Hannuoba clinopyroxenites, their unusually high ?18O values require an 18O-rich metasomatic melt. Accordingly, zircons formed by this metasomatic event also should have high ?18O. Therefore, the zircons with Proterozoic and Mesozoic ages are considered not to be associated with the formation of the clinopyroxenites because their ?18O values are lower than 7.16‰. Only zircons with age of 320-368Ma have extremely high ?18O as the clinopyroxene and the whole rock.
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