Origin of spinel-bearing peridotite from oceanic core complexes

在许多大洋核杂岩中，不含斜长石的、含尖晶石的地幔橄榄岩直接产于海底。在ODP153地段采集的橄榄岩样品具有不同程度的蛇纹岩化(50%-100%)，并且严重亏损轻稀土和其他微量元素，表明它们经历了大约10%-20%的熔融损失。海底这种岩石类型的成因一直是高度推测的，尽管它对我们理解海洋构造过程具有巨大的意义。尖晶石的存在要求在950°C下的平衡压力至少为8kbar，但是，超过20kbar的压力也是可能的。这意味着尖晶石橄榄岩将从25公里到70多公里的深度折返，这与之前提出的4到7公里的折返形成了强烈的对比。地热-压力测量技术(常规温压和相图模拟)将应用于这些岩石，以推断它们的平衡深度和压力-温度演化。将使用Ar-Ar年龄测定，试图对这些岩石的折返速度和冷却历史施加限制。部分熔融事件的时间对橄榄岩残留的地球化学性质有争议。它可能发生在最近的隆升时期或元古代。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。总体目标是区分不同的岩石圈成分，并限制原岩来源的年龄。根据这些新的资料，结合推断的PT演化，我们的目标是将这些岩石的源区和构造演化归属于元古界和(或)现代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理解，其起源与目前全球山脊系统中的部分熔融不一致，即现代地球。

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.