NORTHWEST AFRICA 10416 AMONG THE OLIVINE-PHYRIC SHERGOTTITES: SIMILAR BUT DISTINCT (Invited Presentation)

The olivine(ol)-phyric shergottites consist of a suite of approximately two dozen rocks out of the ~90 currently known meteorites from Mars. These samples are characterized by mm-scale olivine phenocrysts set in a finer groundmass of pyroxene, plagioclase, and accessory minerals including spinel (chromite, titanomagnetite), ilmenite, pyrrhotite, merrillite and apatite (e.g., Bridges and Warren 2006, J. Geol. Soc.). Their texture results from the crystallization of olivine phenocrysts at depth, followed by eruption and emplacement in lava flows at the Martian surface. Detailed petrologic studies of these meteorites demonstrate that a) they are variably enriched in incompatible elements (e.g., LREE), with enrichments corresponding to higher liquidus oxygen fugacity (fO₂; e.g., Herd et al. 2013, LPSC; Castle and Herd In Press), and b) many show an increase in fO₂ with crystallization, possibly resulting from oxidative degassing during eruption and emplacement (e.g., Shearer et al. 2013, GCA). Correlations observed in bulk REE, V/Sc and fO₂ strongly suggest that the ol-phyric shergottites are derived from mantle sources that are distinct from those that produced the basaltic (i.e., olivine-absent) shergottites (e.g., Herd et al. 2013). In support of this are experimental studies which demonstrate the difficulty in producing melts matching basaltic shergottites through fractionation of ol-phyric parental melts (Castle and Herd In Press).

The NWA 10416 ol-phyric shergottite is ‘typical’ in terms of igneous mineralogy, bulk REE and fO₂, and is a reduced, depleted example of this suite (Herd et al. 2016, LPSC). An increase in fO2 with crystallization is suggested by the compositions of groundmass Fe-Ti oxides (Herd et al. 2016). However, this sample is unique in that olivine cores are stained red-brown as a result of alteration to iddingsite and laihunite. In contrast olivine rims and some grains within the groundmass show no evidence of having been subject to the same type of alteration (Herd et al. 2016). Plagioclase has only partially been transformed to maskelynite (Walton et al. 2016, LPSC) and in places the maskelynite has been preferentially altered to amorphous/cryptocrystalline beidellite. Thus, the petrogenesis of NWA 10416 may include magma mixing, two shock events and two secondary alteration events.