A comparative study of clay mineral authigenesis in terrestrial and martian lakes; an Australian example

Abstract

Clay mineral-bearing mudstones are a prominent component of ancient fluvial-lacustrine deposits, 100s of meters thick, documented by the Mars Science Laboratory (MSL) rover, in Gale crater, Mars. Most of the clay minerals documented by MSL are hypothesized to have formed in situ, at or close to the time of deposition ∼3.5 Ga ago, by aqueous alteration of basaltic detritus. Here we study the mechanisms, controls, and timescales of clay mineral authigenesis in a series of lakes with a wide range of water chemistries from the Western Volcanic District, Victoria, SE Australia, as an analog to the Gale crater mudstones. X-ray diffraction (XRD) analysis reveals that the sediments of most of the Western Volcanic District lakes studied contain mixtures of kaolinite, illite, mixed-layer illite-smectite (I-S), and dioctahedral smectite clay minerals. Comparisons of this mineral assemblage with regional soils and creek bedload material confirm previous assertions of significant inputs of detrital clay minerals into the lakes. A trioctahedral clay mineral phase is also detected, making up to 39 wt.% of bulk sediments. The abundance of trioctahedral clay minerals correlates with contemporary lake hydrology and proxies for past lake water Mg concentration. This indicates in situ formation of trioctahedral clay minerals by the uptake of Mg and Si from lake waters and pore fluids at rates determined by local physico-chemical conditions. Examination of crater lake sediments, where detrital clay mineral input is minimized, demonstrate that neoformed trioctahedral clay minerals are poorly crystalline trioctahedral smectites. Neoformation of trioctahedral smectites also occurs in lakes where detrital clay minerals are more abundant. However, an additional authigenic transformation process is indicated by the proportions of Mg and Si added to detrital clay minerals as well as evidence for the uptake of K from lake waters. The transformation process probably involved the incorporation of Mg into the octahedral sheets of detrital clay minerals, leading to irreversible uptake of K into interlayer sites (illitization). The distribution of trioctahedral smectites and radiocarbon ages from sediment cores show that clay mineral authigenesis occurred before sediment consolidation, on timescales of years to 100s of years. These results support syndepositional interpretations of analogous Mg-rich clay minerals documented by MSL, and their use as proxies for chemical conditions in ancient Gale lakes. In comparison with the Western Volcanic District lakes, clay mineral-bearing lacustrine mudstones from Gale crater exhibit only modest chemical weathering of basaltic detrital materials and rarely contain carbonate minerals in quantities detectable by XRD. These observations highlight significant differences in weathering regimes and regolith mineralogy on ancient Mars that could be linked to lake catchment geomorphology, climate, atmospheric CO₂ content, and the absence of biotic processes on Mars.