Cubanite in Stardust: from Indicator of the Thermal and Aaqueous Processing from Comets

Eve L. Berger and Dante S. Lauretta
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ

In 2006, Stardust returned with samples of Comet Wild 2, becoming the first successful U.S. robotic sample return mission. Among the identified minerals is cubanite (CuFe₂S₃) [1]. Prior to this discovery, this mineral was unique to the CI-chondrite meteorites among extraterrestrial samples. Meteoritic cubanite formed during hydrothermal alteration on an asteroid that concentrated Cu in a process similar to the formation of terrestrial Cu-sulfide ore deposits [2]. Its discovery in the Stardust sample population suggests that similar hydrothermal cells occur on comets, which concentrate base and precious metals.

At 210°C, the crystallographic structure of cubanite undergoes an irreversible transition from orthorhombic to cubic [3-6]. Identification of the low-temperature form in Stardust and CI chondrites places an upper limit on the formation temperature. We have initiated a study to determine the origin of this material. Although much work has been done on the Cu-Fe-S phase relations at high temperatures [7], stability fields at low temperatures (<300°C) have not been fully investigated. Through thermodynamic modeling and experimental simulations, the formation mechanism and stability fields of cubanite are determined. This work will shed light on Cu-sulfide formation during hydrothermal circulation in comets, asteroids, and on Earth. It is the first step in understanding extraterrestrial ore formation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas