Dating antarcic meteorites
The augite crystals (deep blue, in a, e, i) are often zoned, with an outer rim of ferroan pigeonite.
Olivine crystals are also present, but less common. The mesostasis also contains crystals of titanomagnetite (Tmag), iron sulphide (Fe, S) and chlorine-bearing apatite.
The filtering mechanism identified by the Manchester team indicates that sub-layers of ice, tens of centimetres beneath the surface of a MSZ, potentially contain iron-rich meteorites that have been kept in a preserved state for thousands of years.
As potassium and chlorine are both dominantly hosted in the mesostasis, the argon gas derived from radiogenic decay of potassium and chlorine gas from apatite will be closely associated in location and thermal behaviour during a We analysed unirradiated fragments of each of the six nakhlites, with our data yielding concordant cosmogenic exposure ages, and a weighted mean of 10.7 ± 0.8 Ma (2σ, 10% uncertainty on production rates; Fig. The indistinguishable cosmogenic exposure ages attest to the fact that the nakhlites can be launch-paired, indicating they were ejected from a single point source on the Martian surface by a single impact event. 3 and Supplementary Table 3) were used to correct the Ar cosmogenic exposure ages overlap within uncertainty, which is consistent with all of the nakhlites being sourced from the same impact.
Blue data points were analysed at Lawrence Livermore, and red data points were analysed at SUERC.
Images from the other meteorites (MIL 03346, Yamato 000593 and Yamato 000749) are in Supplementary Fig. Panels a, e, i display the distribution of aluminium (brown), iron (green) and magnesium (blue); panels b, f, j show potassium (red); panels c, g, k illustrate the distribution of phosphorus (blue, concentrated in apatite) and sulphur (magenta, in iron sulphide), as well as the backscattered electron intensity in grey; and panels d, h, l show the distribution of chlorine (green, in apatite).
These images illustrate that the nakhlite meteorites are igneous rocks, dominated by euhedral augite crystals set in a fine-grained mesostasis.