Phase equilibria in subducting basaltic crust: implications for H2O release from the slab
Phase equilibria in subducting basaltic crust: implications for H2O release from the slab
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DOI：
10.1016/s0012-821x(03)00305-4
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发表时间：
2003-09
影响因子：
5.3
通讯作者：
Juliette F. Forneris;J. Holloway
中科院分区：
文献类型：
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作者：
Juliette F. Forneris;J. Holloway
俯冲板块释放的流体导致板块上方的地幔楔形部分融化，进而导致地球表面的弧形火山活动。板块的部分水化玄武岩层是这些流体的潜在来源，也是深层H2O的主要储集层。在压力-温度(P-T)空间约束该体系中水化相的稳定域和脱水反应的位置，对于描述和定量洋壳向上覆地幔楔体的流体释放是至关重要的。在2.2-3.4 Gpa和625-750°C的范围内进行了实验，以确定与俯冲带有关的条件下饱和的天然玄武岩中的相平衡。实验持续时间通常为1个月，尽管一些实验是以较短的运行持续时间(1-2周)重复的，以确定潜在的亚稳定相。使用天然大洋中脊玄武岩(MORB)玻璃和矿物种子的混合物作为起始材料。氧逸度的缓冲范围为±1.3个对数单位的镍-本辉石(NiNiO)。结果表明，钙闪石(钙闪石)在2.2~2.4 Gpa范围内稳定。在2.6 GPa时，它被钠质角闪石(接近端员蓝闪石)所取代，在625℃时稳定在3 Gpa。这种高压组合构成了天然角闪石榴辉岩的真实模拟，也是首次从玄武岩成分的岩石中合成蓝闪石。与以前对玄武岩成分的研究结果相反[A.R.Pawley，J.R.Holloway，Science 260(1993)664-667；S.Poli，AmJ.Sci。293(1993)1061-1107；S.Poli，M.W.施密特，J.地球物理。第100号决议(1995年)22299-22314；M.W.施密特，S.Poli，地球。SCI。让我们来吧。163(1998)361-379]，在本文研究的压力-温度范围内，类氯仅作为亚稳定相存在。在早期的实验中，由于运行时间较短，类绿泥石的亚稳定性是造成这种差异的最可能的解释，并表明类绿泥石在俯冲带玄武岩层的整体脱水过程中不起重要作用。当压力高于角闪石的稳定场时，闪锌矿/斜长石在645℃以上成为稳定的水合相，而方钠铝石在较低的温度下稳定。本研究确定的橄榄石和钠铝石反应的位置表明，对于中温俯冲带，板块的玄武岩层在90~110公里深度内将完全脱水。
Fluids released from subducting slabs induce partial melting of the mantle wedge above the slab, which in turn is responsible for arc volcanism at the Earth’s surface. The partially hydrated basaltic layer of the slab is a potential source of these fluids and a major reservoir for H2O at depth. Constraining the stability domains of hydrous phases and the position of the dehydration reactions in this system in pressure–temperature (P–T) space is essential to describe and quantify the fluid release from subducting oceanic crust into the overlying mantle wedge. Experiments were conducted in the ranges of 2.2–3.4 GPa and 625–750°C to determine phase equilibria in an H2O-saturated natural basalt at conditions relevant to subduction zones. The experimental duration was typically 1 month, although some experiments were replicated with a shorter run duration (1–2 weeks) in order to identify potentially metastable phases. A mixture of a natural mid-ocean ridge basalt (MORB) glass and mineral seeds was used as the starting material. Oxygen fugacity was buffered within ±1.3 log units of nickel-bunsenite (NiNiO). The results obtained show that a calcic amphibole (barroisite) is stable from 2.2 to about 2.4 GPa. At 2.6 GPa, it is replaced by a sodic amphibole (near end-member glaucophane), which is stable up to 3 GPa at 625°C. This high-pressure assemblage constitutes a true analog of a natural amphibole-bearing eclogite and the first synthesis of glaucophane from a rock of basaltic composition. As opposed to the results of previous studies on basaltic compositions [A.R. Pawley, J.R. Holloway, Science 260 (1993) 664–667; S. Poli, Am. J. Sci. 293 (1993) 1061–1107; S. Poli, M.W. Schmidt, J. Geophys. Res. 100 (1995) 22299–22314; M.W. Schmidt, S. Poli, Earth Planet. Sci. Lett. 163 (1998) 361–379], chloritoid is present only as a metastable phase in the pressure–temperature range investigated here. Metastability of chloritoid in earlier experiments, due to short run duration, is the most likely explanation for this difference, and suggests that chloritoid does not play an important role in the overall dehydration process of the basaltic layer in subduction zones. At pressures above the stability field of amphibole, zoisite/clinozoisite becomes the stable hydrous phase at temperatures above 645°C, whereas lawsonite is stable at lower temperatures. The positions of the zoisite-out and lawsonite-out reactions determined in this study indicate that, for an intermediate temperature subduction zone, the basaltic layer of the slab would be completely dehydrated between 90 and 110 km depth.