Experiments were conducted on a natural basalt (with 5 wt.% added H2O) at 1.0―2.5 GPa and 900―1100℃. Experimental products include partial melts (quenched glasses) + residual mineral assemblages of amphibolite or e...Experiments were conducted on a natural basalt (with 5 wt.% added H2O) at 1.0―2.5 GPa and 900―1100℃. Experimental products include partial melts (quenched glasses) + residual mineral assemblages of amphibolite or eclogite. Electron microprobe and LAM-ICP-MS were used to determine major and trace element compositions of these quenched melts, respectively. Major ele- ment compositions of all the melts are tonalitic- trondhjemitic, similar to adakite. Their trace element characteristics are controlled by coexisting residual minerals. Signatures of adakite such as high Sr/Y, low HREE and negative Nb-Ta anomaly, etc. are present only in the melts coexisting with residual assemblages containing rutile and garnet (rutile-bearing eclogite or rutile-bearing amphibole-eclogite). Garnet leads to HREE depletion in melts, whereas rutile controls Nb and Ta partitioning during the partial melting and causes negative Nb-Ta anomaly in melts. Therefore, in addition to garnet, rutile is also a necessary residual phase during the generation of adakite or TTG magmas to account for the negative Nb-Ta anomaly of the magmas. The depth for the generation of adakite/TTG magmas via melting of metabasalt must be more than about 50 km based on the approximate 1.5 GPa mini- mum-pressure for rutile stability in the partial melting field of hydrous basalt.展开更多
基金This work was supported by Macquarie University,the National Natural Science Foundation of China(Grant No.40172029,40373035)the Chinese Academy of Sciences(Grant Nos.KZCX3-SW-152,GIGCX-04-03,GIGCX-03-04).
文摘Experiments were conducted on a natural basalt (with 5 wt.% added H2O) at 1.0―2.5 GPa and 900―1100℃. Experimental products include partial melts (quenched glasses) + residual mineral assemblages of amphibolite or eclogite. Electron microprobe and LAM-ICP-MS were used to determine major and trace element compositions of these quenched melts, respectively. Major ele- ment compositions of all the melts are tonalitic- trondhjemitic, similar to adakite. Their trace element characteristics are controlled by coexisting residual minerals. Signatures of adakite such as high Sr/Y, low HREE and negative Nb-Ta anomaly, etc. are present only in the melts coexisting with residual assemblages containing rutile and garnet (rutile-bearing eclogite or rutile-bearing amphibole-eclogite). Garnet leads to HREE depletion in melts, whereas rutile controls Nb and Ta partitioning during the partial melting and causes negative Nb-Ta anomaly in melts. Therefore, in addition to garnet, rutile is also a necessary residual phase during the generation of adakite or TTG magmas to account for the negative Nb-Ta anomaly of the magmas. The depth for the generation of adakite/TTG magmas via melting of metabasalt must be more than about 50 km based on the approximate 1.5 GPa mini- mum-pressure for rutile stability in the partial melting field of hydrous basalt.
