^1,2,3Vincenzo Stagno, ⁴Luca Bindi, ⁵Changyong Park, ⁶Sergey Tkachev, ⁶Vitali B. Prakapenka, ^1,7H.-K. Mao, ¹Russell J. Hemley, ⁸Paul J. Steinhardt,¹Yingwei Fei
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
²Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
³Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
⁴Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
⁵HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, U.S.A.
⁶Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁷Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, P.R. China
⁸Department of Physics and Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544, U.S.A.

Icosahedrite, the first natural quasicrystal with composition Al63Cu24Fe13, was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P-T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P-T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solid-solid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures.

Reference
Stagno V, Bindi L, Park C, Tkachev S, Prakapenka VB, Mao H-K, Hemley RJ, Steinhardt PJ,Fei Y (2015) Quasicrystals at extreme conditions: The role of pressure in stabilizing icosahedral Al63Cu24Fe13 at high temperature. American Mineralogist 100, 2412-2418
Link to Article [doi: 10.2138/am-2015-5412]

Copyright: The Mineralogical Society of America

¹Vivian Z. Sun, ¹Ralph E. Milliken
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA

Clay minerals on Mars have commonly been interpreted as the remnants of pervasive water-rock interaction during the Noachian period (>3.7 Ga). This history has been partly inferred by observations of clays in central peaks of impact craters, which often are presumed uplifted from depth. However, combined mineralogical and morphological analyses of individual craters have shown that some central peak clays may represent post-impact, possibly authigenic processes. Here we present a global survey of 633 central peaks to assess their hydrous minerals and the prevalence of uplifted, detrital, and authigenic clays. Central peak regions are examined using high-resolution CRISM and HiRISE data to identify hydrous minerals and place their detections in a stratigraphic and geologic context. We find that many occurrences of Fe/Mg clays and hydrated silica are associated with potential impact melt deposits. Over 35% of central peak clays are not associated with uplifted rocks, thus caution must be used when inferring deeper crustal compositions from surface mineralogy of central peaks. Uplifted clay-bearing rocks suggest the martian crust hosts clays to depths of at least 7 km. We also observe evidence for increasing chloritization with depth, implying the presence of fluids in the upper portions of the crust. Our observations are consistent with widespread Noachian/Early Hesperian clay formation, but a number of central peak clays are also suggestive of clay formation during the Amazonian. These results broadly support current paradigms of Mars’ aqueous history while adding insight to global crustal and diagenetic processes associated with clay mineral formation and stability.

Reference
Sun VZ, Milliken RE (2015) Ancient and recent clay formation on Mars as revealed from a global survey of hydrous minerals in crater central Peaks. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004918]

Published by arrangement with John Wiley&Sons