Авторы: Sun X.,b, Tang Q., Sun W.

2007 г.

Geochim.Cosmochim.Acta

        We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6–10 lm (mostly 6 lm) and about 50–75 lm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 lm, with most around 1.5 lm, and lengths that vary dramatically from 6 to 50 lm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of 30–150 lm and widths of 10 to 50 lm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 lm and lengths of about 60–70 lm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th–U-total Pb isochron method (CHIME) yields an U–Pb isochron age of 202 ± 28Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO2-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO2-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems.

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