Heterogeneous mineral assemblages in martian meteorite Tissint as a result of a recent small impact event on Mars

Abstract

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene+ringwoodite±stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite+magnesiowüstite or clinopyroxene+magnesiowüstite. The pressure stabilities of these mineral assemblages are ∼15GPa and >19GPa, respectively. The ∼200-μm-wide margin of a thicker, mm-size (up to 1.4mm) shock melt vein contains clinopyroxene+olivine, with central regions comprising glass+vesicles+Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite+glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint⩾29–30GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19GPa required to transform olivine to silicate perovskite plus magnesiowüstite.Small volumes of shock melt (<100μm) quench rapidly (0.01s), whereas thermal equilibration will occur within 1.2s in larger volumes of melt (1mm2). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10–20ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at ∼1Ma was not exceptionally large.