Session F5: Geophysics and Planetary Science III

Measuring the Melting Curve of Iron at Super Earth Core Conditions

The melting transition is critical to the Earth's evolution and habitability, as the latent heat from solidification of the inner core helps drive the magneto-dynamo in the liquid outer-core, which creates a magnetosphere that protects Earth's surface from harmful charged particles emitted from the Sun. As thousands of extrasolar planets are being discovered, an important question is whether a super Earth could have a solidifying iron inner core at pressures of 5-30 Mbar. At the National Ignition Facility (NIF), we have begun a campaign to experimentally address this question, where we shock the iron samples into the liquid state and then subsequently shocklessly compress the iron along a high temperature adiabat until the sample reaches a well defined peak pressure. At this point, we probe the iron sample with x-ray diffraction and test for solidification. We will present initial results on the melting curve of iron up to 5.5 Mbar and also the phase of iron below the melting curve. These results provide us with the stable phase of iron at the pressure and temperature conditions that are found within the Earth's inner core and the cores of Super Earths. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Forsterite and Enstatite Shock Temperatures: Implications for Planetary Impact Melting

We present experimental results on enstatite and forsterite to probe extreme conditions in the laboratory in order to examine melting and vaporization of rocky planet mantles upon shock and release. Flyer plate impact experiments are carried out on the Z-Machine at Sandia National Laboratory. Planar, supported shock waves are generated in single crystal samples, permitting observation of both compressed and released states. Shock velocity of the sample is measured using laser interferometry, and the pressure and particle velocity are derived through impedance matching to the aluminum flyer. Temperature of the shocked state is measured with a streaked visible spectrum and calibrated with a quartz standard, mounted downrange from the sample. Preliminary analysis shows that current equation of state models underestimate the entropy gain, which suggests that for shock pressures above 250 GPa, a higher degree of impact vaporization will be reached. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation for the U.S. DOE's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Phase transitions and melting on the Hugoniot of Mg2SiO4 forsterite: new diffraction and temperature results

The phase transitions of forsterite under shock were studied by x-ray diffraction and pyrometry. Samples of 2 mm thick, near-full density (\textgreater 98{\%} TMD) polycrystalline forsterite were characterized by EBSD and computed tomography and shock compressed to 50 and 75 GPa by two-stage gas gun at the Dynamic Compression Sector, Advanced Photon Source, with diffraction imaged during compression and release. Changes in diffraction confirm a phase transition by 75 GPa. In parallel, single-crystal forsterite shock temperatures were taken from 120 to 210 GPa with improved absolute calibration procedures on the Caltech 6-channel pyrometer and two-stage gun and used to examine the interpretation of superheating and P-T slope of the liquid Hugoniot.~This work performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, supported in part by LLNL's LDRD program under grants 15-ERD-012 and 16-ERD-010. The Dynamic Compression Sector (35) is supported by DOE / National Nuclear Security Administration under Award Number DE-NA0002442. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Caltech lab supported by NSF EAR-1426526.   

In-situ x-ray diffraction of a shock-induced phase transition in fluorite, CaF$_{\mathrm{2}}$

The difluorides are an important class of ionic compounds that show extensive polymorphism under both static and dynamic loading. In this study, the shock-induced phase transitions in CaF$_{\mathrm{2\thinspace }}$were investigated by in situ x-ray diffraction measurements in plate impact experiments carried out with the two-stage gas gun at the Dynamic Compression Sector of Argonne National Laboratory. Single-crystal samples in (100) and (111) orientations were shock loaded to pressures between 32 GPa to 70 GPa. The particle velocities at the interface between the sample and a LiF window were measured by VISAR and PDV. Synchrotron x-ray diffraction data were recorded at 153.4 ns intervals using a four-frame detector. The measured diffraction patterns show a high degree of sample texturing at all pressures. We observe evidence for a transition to a high-pressure phase followed by reverse transformation at late times during release. This study provides the first direct constraints on the high-pressure lattice structure of fluorite under shock compression.