Possible Ni-rich Mafic-ultramafic Magmatic Sequence in the Columbia Hills: Evidence from the Spirit Rover

Introduction: The Spirit rover landed on geologic units of Hesperian age in Gusev Crater. The Columbia Hills rise above the surrounding plains materials, but orbital images show that the Columbia Hills are older [1, 2]. Spirit has recently descended the southeast slope of the Columbia Hills doing detailed measurements of a series of outcrops. The mineralogical and compositional data on these rocks are consistent with an interpretation as a magmatic sequence becoming increasingly olivine-rich down slope. The outcrop sequence is Larry’s Bench, Seminole, Algonquin and Comanche. The “teeth” on the Rock Abrasion Tool (RAT) wore away prior to arrival at Larry’s Bench; the data discussed are for RAT brushed surfaces. APXS data: The compositional data show systematic trends related to location. The compatible elements Mg, Cr (Fig. 1) and Ni increase, while incompatible elements Al, Ca, Ti (Fig. 1) and P decrease in outcrops measured during the downhill traverse. These trends are reflected in CIPW normative mineralogy (on a Sand Cl-free basis with assumed Fe/total Fe = 0.056). Down slope, normative plagioclase+orthoclase decrease, while olivine+low-Ca pyroxene increase; Comanche target Palomino has ~77% normative olivine+low-Ca pyroxene. Bulk rock mg# (using total Fe) also increases in the sequence from ~48 in Larry’s Bench to ~65 in Comanche. The structural attitudes of the outcrops with respect to the slope are poorly constrained at present; we do not know whether down slope equates to down section. The total elevation change across the sequence is ~28 m. Nor do we have control on possible lateral changes in units. Nevertheless, the systematic compositional trends observed in these outcrops are most simply interpreted as resulting from a series of related rocks. These trends are consistent with the lower southeast slope of Columbia Hills consisting of a sequence of mafic-ultramafic magmatic rocks, either a single emplaced unit that underwent crystal accumulation or a series of related units showing systematic compositional variations. At present we cannot distinguish whether the rocks were emplaced as intrusions or on the surface. Caveat Emptor: Because we are unable to grind rock surfaces, the APXS data will be influenced by alteration rinds if present. In addition, because of sur1 2 3 4 5