Donald Bowden

Known compositions of igneous material on Mars have been greatly expanded by the Curiosity rover’s exploration of Gale crater, which presents a range of basaltic and trachybasaltic compositions. Previous work has indicated that the range of compositions found in Gale crater may have evolved through fractionation of magma with similar chemistry to the Adirondack basalt found in Gusev crater. At the Kimberley and Bressay sites, the rover discovered two igneous samples which display unique textures among the Gale crater dataset, designated “Bindi” and “Askival”.
Geochemical data is taken from two instruments: the ChemCam laser induced breakdown spectrometry (LIBS) instrument, which uses a 1067nm laser to induce atomic emission from a target at typical ranges up to 3m, and the Alpha Particle X-Ray Spectrometer, which uses an alpha particle source to induce x-ray emission in contacted targets. We also use imagery from the MastCam stereo camera, the Mars Hand Lens Imager and ChemCam Remote Micro-Imager instruments.
The texture exhibited by these samples primarily consists of a light-toned phase with crystal sizes averaging >1cm. The secondary phase is dark in tone, and mainly occupies the interstitial spaces between the light-toned phase, with some large spaces equivalent in size to the light-toned crystals also occupied by this phase.
Due to the large grain sizes seen in these samples the ChemCam instrument functions as a geochemical microprobe, allowing us to examine the chemistry of individual crystals. Chemical data from the lighter phase is consistent with feldspar; however the Askival sample is enriched in silica, which correlates negatively with other major elemental oxides and positively with hydrogen. We interpret this as evidence of fluid alteration following crystallisation of Askival, leading to silicification of the feldspar phase. In contrast, Bindi’s light-toned phase has chemistry consistent with unaltered feldspar.
The darker phase, which has fewer LIBS target points sampling it, shows a range of iron and magnesium-rich chemistry, indicating it may be contain multiple mafic minerals. The point with the lowest silica content shows chemistry similar to the amphibole mineral ferrohastingsite.
Using the MELTS magmatic modelling software package, we simulate fractionation of magmas with three compositions found on Mars: the “Adirondack” basalt from Gusev crater, the “Johnnie” basalt from Gale crater, and basaltic clasts from the “Black Beauty” (NWA 7034) meteorite. We simulate cooling at a pressure of 5kbar, using 100ppm and 1000ppm water content. We compare the feldspar chemistry from our simulations to the chemistry found in Askival and Bindi using their albite fraction, which falls within the range 0.55 to 0.67, except for one outlier in Askival at 0.78. The feldspar fractionated from the Johnnie composition most closely matches the range of albite content found in the two cumulate samples, falling entirely within this range. Feldspar from the Black Beauty sample has lower albite fraction but does cross over into the range of Bindi and Askival. The Adirondack composition produces feldspar which is lower still in albite and does not overlap with the range seen in the cumulate samples.
Our results indicate that the Askival and Bindi cumulates likely formed from similar magmatic compositions to the basalt samples from Gale crater. In the context of possible evolution from a more primitive magma with chemistry closer to that of the Gusev crater basalts, our results indicate that the feldspar chemistry seen here may not form from a magma with higher calcium-sodium ratio. We are continuing investigation into these samples using a wider range of fractionation pressures, as well as investigation of the mafic phase using the Perple_X modelling package to model possible amphibole formation at lower temperatures.