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

在超高压地层中，浮力是驱动俯冲物质从地幔深处向中地壳挖掘的一个常见模型，在中地壳中获得中性浮力。这一模式适用于那些没有证据表明曾经存在熔体的超高压地体；然而，在富含熔体的超高压地体中，如巴布亚新几内亚，需要评估一种新的模式，在这种模式中，超高压岩石的快速挖掘是由大陆物质的两阶段部分熔融过程中产生的浮力驱动的，首先是在超高压条件下，然后是在减压和超高压体返回地球的过程中。年代的表面。在这个项目中，正在对巴布亚新几内亚收集的不同变形的岩石样品进行高精度锆石定年，这些岩石样品代表了以前的熔体，并且与超高压岩石具有各种不同的结构关系。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/或斜长石存在的情况下生长，将锆石日期与压力-温度条件联系起来，从而确定熔体是在超高压条件下存在，还是在超高压岩石上升回地表的过程中出现。野外不同岩石单元的测绘将确定每次熔体生成的体积，综合测绘和分析数据正在用于量化熔体的浮力。来自大陆地壳的低密度岩石可以俯冲到100-150公里的地幔中，在那里它们以高压或超高压矿物组合的形式再循环。这些高压/超高压岩石是在地球表面被挖掘出来的。人们对这些高密度岩石被挖掘出来的机制知之甚少。尤其不清楚它们是如何穿过低密度地壳上升的。这个项目探索了一种新的模型，在这种模型中，超高压岩石部分融化，从而降低了它们的密度，降低了它们的体积粘度，使它们能够上升到地壳中。

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.