Origin of spinel-bearing peridotite from oceanic core complexes

在许多大洋核杂岩中，不含斜长石、含尖晶石的地幔橄榄岩直接产于海底。在ODP航段153上采集的橄榄岩样品为蛇纹岩化（50- 100%），并且轻稀土和其他微量元素严重贫化，表明它们经历了约10-20%的熔融损失。这种海底岩石类型的成因仍然是高度投机性的，尽管它对我们理解海洋构造过程具有巨大的意义。尖晶石的存在需要在950 °C下至少8千巴的平衡压力。然而，超过20千巴的显著更高的压力是可能的。这意味着尖晶石橄榄岩的折返深度在25公里和可能超过70公里之间，这与以前提出的4至7公里的折返形成强烈对比。将对这些岩石应用地热变温技术（传统的热压力测定法和相图模拟），以推断其平衡深度和压力-温度演化。Ar-Ar年龄测定将被用来试图限制这些岩石的折返速率和冷却历史。部分熔融事件的时间是负责剩余的橄榄岩的地球化学特征的争论。它可能发生在最近的隆升或元古代。为了解决这一争议，并确定部分熔融的时间，计划进行Sm-Nd，Rb-Sr和Pb同位素研究。总的目标是区分不同的岩石圈成分，并限制原岩来源的年龄。有了这些新的数据，再加上推断的PT演化，我们的目标是分配这些岩石的来源和构造演化的元古代和/或最近的岩浆和构造变质事件。最终，预期的结果将提高我们对深海橄榄岩的认识，其起源与现代地球全球洋脊系统中目前的部分熔融不一致。

In many oceanic core complexes plagioclase-free, spinel-bearing mantle peridotite occurs directly on the seafloor. Peridotite samples collected on ODP leg 153 are variably serpentinised (50- 100%) and are strongly depleted in light rare earth and other trace elements, indicating that they experienced some 10-20% melt loss. The genesis of this rock type on the ocean floor has remained highly speculative in spite of its enormous significance for our understanding of oceanic tectonic processes. The presence of spinel requires equilibration pressures of at least 8 kbar at 950 °C. Significantly higher pressures in the excess of 20 kbar are however possible. This implies an exhumation of the spinel peridotite from depths between 25 and possibly more than 70 km, which is in strong contrast to the 4 to 7 km of exhumation that have been suggested previously. Geothermobaromeiric techniques (conventional thermobarornetry and phase diagram modelling) will be applied to these rocks to infer their equilibration depth and their pressure-temperature evolution. Ar-Ar age dating will be used in an attempt to put constraints on the exhumation rate and cooling history of these rocks. The timing of the partial melting event that is responsible for the residual geochemical character of the peridotite is debated. It could have occurred during recent uplift or in Proterozoic times. In order to resolve this controversy and to determine the timing of partial melting it is planned to carry out a Sm-Nd, Rb-Sr and Pb isotope study. The overall goal is to distinguish between different lithospheric components and to constrain the age of the source for the protoliths. With such new data and, together with the inferred PT evolution, we aim to assign the source and tectonic evolution of these rocks to Proterozoic and/or recent magmatic and tectonometamorphic events. Ultimately, the expected results will improve our understanding of abyssal peridotite whose origin is inconsistent with current partial melting in the global ridge system, of modern Earth.