Hydrogen isotopic compositions of pyroxenes in peridotite xenoliths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain ...Hydrogen isotopic compositions of pyroxenes in peridotite xenoliths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain and among the different grains from the same sample. The lack of correlation between the D/H ratios and the hydrogen contents indicates that the hydrogen isotopic compositions are inherited from their mantle source. Combining with the δD values of coexisting amphiboles, it is inferred that the Nushan mantle experienced at least a two-stage metasomatic event. One was responsible for formation of amphiboles with extremely variable δD values, and the other was probably responsible for the high δD values (up to ?20‰) of some clinopyroxenes from peridotites, clinopyroxene and mica megacrysts. High δD values point to a subduction-related fluid being involved in one metasomatic event. The primary δD values (?90‰ to ?140‰) of the Nushan pyroxenes, together with data gleaned from the literature, suggest that the D/H ratios of the nominally anhydrous mineral reservoir might have differed from that of the other mantle hydrogen in being relatively depleted in D.展开更多
in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogene-...in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogene-ity in d 18O values, with variation in different rocks from 8.5 to +9.7 and within one sample from 2 to 12. No measurable difference in d 18O was observed between proto-lith magmatic (detrital) zircons and metamorphic recrystal-lized zircons within analytical uncertainties from the ion micro-probe measurements. This indicates that the meta-morphic zircons have inherited the oxygen isotopic composi-tions of protolith zircons despite the HP to UHP metamor-phism. According to their protolith ages from zircon U-Pb in situ dating by the same ion micro-probe, two groups of oxy-gen isotope composition are recognized, with one having d 18O values of 6—7 for old protolith of 1.9—2.5 Ga ages and the other 0—2 for young protolith of 0.7—0.8 Ga ages. The latter anomalously low d 18O values of zircons in-dicate that the magma has had the obvious involvement of meteoric water when forming the young protolith of high-grade metamorphic rocks. This may be correlated with the snowball Earth event occurring in South China and the world elsewhere during the Neoproterozoic.展开更多
基金supported by the National Natural Science Foundation of China(Grants 49803002 and 40473007)Program for New Century Excellent Talents in University(NCET)the CAS-CNRS-CNR cooperative project.
文摘Hydrogen isotopic compositions of pyroxenes in peridotite xenoliths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain and among the different grains from the same sample. The lack of correlation between the D/H ratios and the hydrogen contents indicates that the hydrogen isotopic compositions are inherited from their mantle source. Combining with the δD values of coexisting amphiboles, it is inferred that the Nushan mantle experienced at least a two-stage metasomatic event. One was responsible for formation of amphiboles with extremely variable δD values, and the other was probably responsible for the high δD values (up to ?20‰) of some clinopyroxenes from peridotites, clinopyroxene and mica megacrysts. High δD values point to a subduction-related fluid being involved in one metasomatic event. The primary δD values (?90‰ to ?140‰) of the Nushan pyroxenes, together with data gleaned from the literature, suggest that the D/H ratios of the nominally anhydrous mineral reservoir might have differed from that of the other mantle hydrogen in being relatively depleted in D.
基金supported by the National Natural Science Foundation of China(Grants Nos.40033010 and 40273028)the State Key Basic Research Project(Grant No.G1999075503)
文摘in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogene-ity in d 18O values, with variation in different rocks from 8.5 to +9.7 and within one sample from 2 to 12. No measurable difference in d 18O was observed between proto-lith magmatic (detrital) zircons and metamorphic recrystal-lized zircons within analytical uncertainties from the ion micro-probe measurements. This indicates that the meta-morphic zircons have inherited the oxygen isotopic composi-tions of protolith zircons despite the HP to UHP metamor-phism. According to their protolith ages from zircon U-Pb in situ dating by the same ion micro-probe, two groups of oxy-gen isotope composition are recognized, with one having d 18O values of 6—7 for old protolith of 1.9—2.5 Ga ages and the other 0—2 for young protolith of 0.7—0.8 Ga ages. The latter anomalously low d 18O values of zircons in-dicate that the magma has had the obvious involvement of meteoric water when forming the young protolith of high-grade metamorphic rocks. This may be correlated with the snowball Earth event occurring in South China and the world elsewhere during the Neoproterozoic.
