Melt-induced Buoyancy: The Driving Force for Fast UHP Exhumation?

在超高压地幔中，浮力是一种常见的诱发模型，用于驱动俯冲物质从地幔深处向中地壳折返，在中地壳中获得中性浮力。该模型适用于不含明显证据表明熔体存在的超高压熔体;然而，在富含熔体的超高压岩石中，例如在巴布亚新几内亚，需要评估一种新的模型，其中岩石的快速折返是由大陆材料的两阶段部分熔融期间产生的浮力驱动的，首先在超高压条件下，然后在减压和超高压体返回地球期间？s表面。在这个项目中，正在对从巴布亚新几内亚收集的代表以前熔融物并与超高压岩石有各种不同结构关系的岩石的变形样品进行高精度锆石年代测定。从锆石和石榴石中测量微量元素，以确定锆石是否在石榴石和/或斜长石的存在下生长，将锆石日期与压力-温度条件联系起来，从而确定熔体是否存在于超高压条件下，或者在超高压岩石上升到表面的过程中。对该地区不同岩石单元的测绘将确定每一次熔体生成的体积，测绘和分析数据的结合将被用来量化熔体的浮力，来自大陆地壳的低密度岩石可以俯冲到100-150公里深的地幔中，在那里它们以高压或超高压矿物组合的形式结晶。这些高压/超高压岩石在地球表面被挖掘和发现。这些高密度岩石被挖掘出来的机制知之甚少。特别是不清楚它们是如何通过低密度地壳上升的。该项目探索了一种新的模型，其中超高压岩石部分熔化，这降低了它们的密度，降低了它们的体积粘度，并使它们上升到地壳中。

In ultrahigh-pressure terranes, buoyancy is a commonly evoked model for driving the exhumation of subducted material from mantle depths to the mid-crust, where neutral buoyancy is obtained. This model is adequate for ultrahigh-pressure terranes that do not contain significant evidence for the former presence of melt; however, in melt-rich ultrahigh-pressure terranes, such as in Papua New Guinea, a new model needs to be evaluated in which the rapid exhumation of ultrahigh rocks is driven by the buoyancy generated during two-stage partial melting of continental material, first at ultrahigh-pressure conditions and then during decompression and return of the ultrahigh-pressure body to Earth?s surface. In this project, high-precision zircon dating of variably deformed samples of rock collected in Papua New Guinea that represent former melt and are in a variety of different textural relationships to the ultrahigh-pressure rocks is being carried out. Trace elements are measured from zircon and garnet to determine if the zircon grew in the presence of garnet and/or plagioclase, tying the zircon date to the pressure-temperature conditions, and thus to determine whether the melt was present at ultrahigh-pressure conditions or later during ascent of the ultrahigh-pressure rocks back to the surface. Mapping of the different rock units in the field will determine the volume of each melt generation, and the combined mapping and analytical data is being used to quantify the buoyancy of the melt.Low-density rocks from the continental crust can be subducted to 100-150 kilometers into the Earth's mantle where they are recyrstallized in high pressure or ultrahigh pressure mineral assemblages. These high pressure/ultrahigh pressure rocks are exhumed and found at the Earth's surface. The mechanisms by which these high density rocks are exhumed are poorly understood. In particular it is not clear how they rise through the low density crust. This project explores a novel model in which ultrahigh pressure rocks partially melt, which reduces their density, decreases their bulk viscosity and allows them to rise into to the crust.