Channelized fluids in subducted continental crust: constraints from δD-δ~(18)O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

摘要

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD = -97 per thousand to -69 per thousnad). Quartz δ~(18)O values range from -2.5 per thousand to 9.6 per thousand fluid inclusions hosted in quartz have correspondingly low δ~(18)O values of -11.66 per thousand to 0.93 per thousand (T_h= 171.2~318.8°C). The low δD and δ~(18)O isotopic data indicate that protoliths of some CCSD HP-UHP meta?morphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ~(18)O of the quartz veins and fluid inclu?sions vary greatly with the drillhole depth. Lower δ~(18)O values occur at depths of -900-1000 m and -2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and sili?cates and is much more abundant in the quartz veins and silicate minerals than is hydro?gen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composi?tion. Therefore, different SD and δ~(18)O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation. Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure—temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P-T conditions within the stability field of law?sonite, dehydration and associated high-Si fluid release may have occurred as hydrous min?erals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the sur?face, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.