Two generations of exsolution in pyroxene, one thermal story for a mesosiderite

Seann McKibbin, Lidia Pittarello, Gang Ji, Dominique Schryvers, Vinciane Debaille, Philippe Claeys

Research output: Chapter in Book/Report/Conference proceedingMeeting abstract (Book)

Abstract

Introduction: Mesosiderites are meteoritic breccias which are stony-iron meteorites consisting of a mixture of metal chemically similar to IIIAB iron meteorites, and silicates which resemble those in Howardite Eucrite Diogenite (basaltic gabbroic) achondrites. The most accredited hypothesis for mesosiderite formation is an impact induced mixture of crustal and core material (with a possible minor mantle contribution) from one or more differentiated bodies. The mesosiderite meteorite Asuka 09545 (A 09545) was found during a joint Belgian-Japanese mission in Antarctica. This mesosiderite is highly equilibrated (B3) and contains large grains of pyroxene exhibiting two generations of exsolution lamellae. The study of these lamellae provides constraints on the thermal evolution of the silicate fraction in the sample. Methods: Preliminary investigations of a polished thin section from A 09545, provided by the Japanese National Institute of Polar Research (NIPR) has been undertaken using scanning electron microscope (SEM) and electron microprobe (EMPA) at the NIPR. Two foils, with size of ca. 5 μm x 5 μm and 8090 nm thickness, were prepared with focus ion beam (FIB) in selected locations: (1) at the boundary between the pyroxene host and the first generation lamellae; and (2) within this lamellae across a set of second generation of lamellae. These foils were investigated with a transmission electron microscope (TEM), equipped with energy dispersive X-ray spectroscopy (EDS) detector and a precession unit, at EMAT, University of Antwerp. Results: A 09545 pyroxene displays two generations of exsolution. The host pyroxene has composition Wo3En59Fs38 and has been identified by TEM as clinoenstatite (Monoclinic, P21/c). Thick (up to 25 μm) lamellae of augite (Monoclinic, C2/c) are present along a cleavage, with irregular margins and elongated shape, and have composition Wo42En41Fs17. Within these augite lamellae, a second generation of exsolution appears, with thin (ca. 300 nm) lamellae, parallel to [001], with composition Wo1En56Fs43 and also identified as clinoenstatite (Monoclinic, P21/c), but with very low Ca content (below detection limit). Discussion: Orthopyroxene (a low Ca pyroxene) hosting exsolved augite (a high-Ca pyroxene) is generally referred to as inverted pigeonite (a mid-Ca pyroxene), because it originates during cooling by breakdown of pigeonite crystallized at high temperature. Equilibrium temperatures have been calculated for pyroxene pairs using a multi element geothermobarometer, based on the combined content of Ca, Fe, Al, and Cr, in order to obtain a rough estimate of the exsolution conditions. Both exsolution generations are consistent with a temperature range of ca. 950-1100° C under low pressure conditions. Calculations based on the Fe/Mg ratio show that the second generation exsolution probably formed at slightly lower temperature than the first one. This is consistent with a low cooling rate during thermal metamorphism. Lattice parameters and the orientation of the lamellae also support our rough estimates for temperature and cooling rate. The host pyroxene is in fact monoclinic (i.e. a clinopyroxene), as well as the exsolved pyroxene in the lamellae of augite. Although orthorhombic geometry is expected for inversion of pigeonite at 950-1100°C, and orthorhombic pyroxene should also exsolve from augite, another inversion from orthoenstatite to clinoenstatite occurs at ca. 570°C at low pressure, during slow cooling conditions. Conclusion: In the historical literature, cooling of silicates above 500°C was generally considered to be faster than that of metal, which was extremely slow below 500°C. Metallographic textures indicate that the mesosiderite parent body underwent a uniquely slow cooling process. The presence of low temperature clinoenstatite in mesosiderites supports this history. Low-temperature clinoenstatite has been confirmed in a small number of mesosiderites, and A 09545 can be added to that list.
Original languageEnglish
Title of host publicationXII International Symposium on Antarctic Earth Science Abstract Volume
Pages530-530
Number of pages1
Publication statusPublished - 2015
EventXII International Symposium on Antarctic Earth Science - Goa, India
Duration: 13 Jul 201517 Jul 2015

Conference

ConferenceXII International Symposium on Antarctic Earth Science
Country/TerritoryIndia
CityGoa
Period13/07/1517/07/15

Fingerprint

Dive into the research topics of 'Two generations of exsolution in pyroxene, one thermal story for a mesosiderite'. Together they form a unique fingerprint.

Cite this