ULTRAMAFIC SOILS: ANALOGUES FOR INCIPIENT WEATHERING ON MARS

X-ray amorphous material makes up 15-73% of material in all drilled samples measured to date within Gale crater, Mars. Despite its prevalence, little is known about the nature of this material beyond that it is Fe/Si-rich and contains incipient weathering products. As amorphous material is generally considered meta-stable, its presence in >3 Ga sedimentary rocks invites questions as to what environmental constraints allow for such longevity.

To better understand the implications of the presence of this Fe-rich amorphous material we investigated mineral and chemical evolution within terrestrial Fe-rich and Al-poor ultramafic soils of different ages within the Klamath Mountains of California (mediterranean climate), at Pickhandle Gulch, Nevada (desert climate), and in Newfoundland, Canada (subarctic climate). In-situ aqueous alteration in the wetter climates concentrates Fe into the clay-size fraction where amorphous material is typically found, while dry desert conditions promote accumulation of detrital material with little in-situ formation of Fe-containing material, amorphous or crystalline. Cool and wet climates appear to promote both the formation and longevity of Fe-rich amorphous material while warm and wet climates promote the formation of crystalline Fe-containing phases. Fe-rich amorphous material formation and persistence on Mars therefore are consistent with cool, water-containing conditions during formation followed by long term cold and dry conditions.

We also investigated olivine residence time within these soils to investigate olivine’s potential as an indicator of timing for water-rock interaction on Mars. Olivine persisted to ~12 ka in the Klamath Mountains and ~25 ka in the Tablelands. These ages are longer than previous estimates of olivine residence time (~10 ka) in soil chronosequences, and demonstrate a climatic variation with cooler climates promoting longer residence times even under wet conditions.