Evidence for Multiple Diagenetic Episodes in Ancient Fluvial-Lacustrine Sedimentary Rocks in Gale Crater, Mars

Abstract

The Curiosity rover's exploration of rocks and soils in Gale crater has provided diverse geochemical and mineralogical data sets, underscoring the complex geological history of the region. We report the crystalline, clay mineral, and amorphous phase distributions of four Gale crater rocks from an 80-m stratigraphic interval. The mineralogy of the four samples is strongly influenced by aqueous alteration processes, including variations in water chemistries, redox, pH, and temperature. Localized hydrothermal events are evidenced by gray hematite and maturation of amorphous SiO₂ to opal-CT. Low-temperature diagenetic events are associated with fluctuating lake levels, evaporative events, and groundwater infiltration. Among all mudstones analyzed in Gale crater, the diversity in diagenetic processes is primarily captured by the mineralogy and X-ray amorphous chemistry of the drilled rocks. Variations indicate a transition from magnetite to hematite and an increase in matrix-associated sulfates suggesting intensifying influence from oxic, diagenetic fluids upsection. Furthermore, diagenetic fluid pathways are shown to be strongly affected by unconformities and sedimentary transitions, as evidenced by the intensity of alteration inferred from the mineralogy of sediments sampled adjacent to stratigraphic contacts.

Keywords: Mars; XRD; diagenesis; mineralogy.

Figure 1

Stratigraphic column of rock units studied by Curiosity from landing through Sol 1577. The strata are generally flat‐lying, allowing us to correlate elevation with stratigraphy. Drill hole locations discussed in this manuscript are shown in white stars.

Figure 2

MAHLI images of the drill holes of (a) Oudam; (b) Marimba; (c) Quela; and (d) Sebina.

Figure 3

CheMin diffraction patterns of the Oudam sandstone and Marimba, Quela, and Sebina mudstones. Pattern intensities have been normalized to reflect equal analysis durations. Major phases identified are plagioclase (Pl), hematite (Hem), anhydrite (Anh), gypsum (Gp), jarosite (Jar), opal‐CT (O), smectite (Sme), pyrophyllite (Prl), and Kapton (Kap).

Figure 4

SAM EGA H₂O profiles for Oudam, Marimba, and Quela.

Figure 5

BGMN models of dioctahedral (blue) and trioctahedral (green) smectite profiles for Marimba, Quela, and Sebina 02l smectite bands. Data are available through Bristow et al. (2018).

Figure 6

SAM EGA profiles for evolution of SO₂ with temperature.

Figure 7

Major events in the depositional and diagenetic history of rocks at the Oudam drill site. (1) Deposition of Hartmann's Valley (HV) basaltic sands; (2) lithification/burial resulting in dissolution of Fe‐Mg‐silicates to form Fe‐oxides and amorphous silica, erosion of HV, and deposition lithification/burial of Stimson (ST) sands; (3) hydrothermal fluids flow along the HV/ST contact causing dissolution of amorphous silica and reprecipitation as opal‐CT, Fe‐oxide/oxyhydroxide precipitation and/or recrystallization to gray hematite, formation of minor Fe‐pyrophyllite (or degradation of nontronite), and precipitation of anhydrite; (4) influx of low‐temperature groundwaters resulting in partial rehydration of anhydrite to gypsum and precipitation of minor Mg‐ and Fe‐sulfates; and (5) veinlet formation due to late‐stage fracturing and subsequent infilling by Ca‐sulfates can be concurrent with Step 4 if fractures were the result of volume expansion induced by the partial rehydration of anhydrite to gypsum. See Figure 1 for lithology legend.

Figure 8

The depositional and diagenetic history of rocks at Marimba, Quela, and Sebina. Basaltic sediments are deposited in a lacustrine environment resulting in the aqueous alteration of Fe‐Mg‐silicates to smectite and Fe‐oxides. Sediments are further altered due to episodic shallowing and evaporative events that increase over time. As a result, early diagenesis is marked by an increase in cation mobility and more oxic waters with higher sulfate concentrations. Late diagenesis is characterized by the influx of oxic and sulfate‐rich groundwaters (possibly associated with lithology transitions), augmenting the Ca‐sulfate and hematite abundances. Combined, early and late diagenesis resulted in an increase in Al‐smectites, Ca‐sulfates, and hematite from Marimba to Sebina. See Figure 1 for lithology legend.

Figure 9

Distribution of major oxides in the crystalline (left) and amorphous (right) components for Oudam, Marimba, Quela, and Sebina.

Figure 10

Crystalline, phyllosilicate, and amorphous material distributions for drilled samples analyzed by CheMin in Gale crater. Phase distributions shown are the maximum crystalline abundances (minimum amorphous abundance; Table S3) calculated for each sample. Data for samples stratigraphically below Oudam can be found in Vaniman et al. (2014) (Cumberland and John Klein), Rampe et al. (2017) (Confidence Hills, Mojave, Telegraph Peak, and Buckskin), and Morris et al. (2016) (Buckskin).