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.
Across Earth, there are regions that expose rocks that traveled from the surface of Earth to the mantle (100 km depth) and then returned back to Earth’s surface. This proposal evaluated if the return to Earth’s surface was driven by buoyancy generated by two-stage partial melting of continental crust material, first at mantle depths and then as the rocks were coming back up to the surface. We investigated rocks exposed in Papua New Guinea (PNG), as they are the youngest (ca. 5 million years old) exposure of these unique rocks that went to the ultrahigh-pressures of the mantle. This study proved that there was abundant partial melt present with the PNG rocks, and U-Pb isotopic dating of the mineral zircon constrained the timing of the different melt generations and the timing in which the rocks were at mantle depths. The ages suggest that there was early melting that occurred while the rocks were at high pressure. This, along with rifting of continental crust probably drove the initial rise of the rocks from mantle depths. Our investigations also showed that melting continued as the rocks came to the surface. The overall buoyancy generated through the production of melt likely aided in the rapid return (~3 million years) of these rocks from mantle depths to Earth’s surface. I am a junior, female faculty member, and this project has provided salary and research support to me, helping to jumpstart my research program at the University of Nevada, Reno. In addition, the dissertation of my graduate student, Joel DesOrmeau, is focused on the research described in this grant. Joel has received a variety of different training: field, writing (through the publication of one manuscript thus far), oral presentation (through talks and posters at the AGU and GSA meetings), and laboratory training. In addition, this project has brought together scientists from the U.S., New Zealand and Papua New Guinea, promoting significant international collaboration, including lectures to the local villages in Papua New Guinea.
