Hematite Formation in Gale Crater

Introduction: Secondary minerals identified by Mars Science Laboratory (MSL), together with their sedimentological and stratigraphic context, provide an unprecendented opportunity to constrain the nature of martian fluids and habitability. One of the main targets for the MSL mission within the new Extended Mission is the Hematite Ridge on the north, lower slopes of Mt. Sharp (Aeolis Mons). This was identified by CRISM, with clay and sulfate-rich mineralogy in other parts of Mt Sharp [1,2] and is a 200 m wide layer extending 6.5 km northeast-southwest [3]. After landing in August 2012, Curiosity has identified clay and Fe oxides within fine-grained sediments along the traverse to Hematite Ridge. ChemCam analyses show the overall basaltic composition of the sediments (Fig. 1). The Sheepbed member is a mudstone of basaltic chemical composition with ~15% smectite, ~50% igneous minerals, and ~35% X-ray amorphous material [4]. The observed magnetite is considered to be of authigenic origin [5]. In previous work we showed that dissolution of approximately 70:20:10 % amorphous material, olivine and whole rock in an open system within the Sheepbed Member mudstone can explain the smectite and magnetite abundances identified by CheMin XRD at the John Klein and Cumberland sites [6]. More recently, at the Kimberley drill site, CheMin has identified ~10% magnetite with some hematite [7]. Here we show thermochemical models for the formation of Fe oxide enrichments, and the ferric oxide hematite in particular, within Gale sediments. This provides an insight into the formation conditions of the Hematite Ridge layer during diagenesis or other alteration stages. This model will be tested once we have a full mineral assemblage from Hematite Ridge. Method: We use ChemCam (PLS1), APXS and CheMin analyses and sedimentological observations of the Gale sediments [4,8,9] to guide the input parameters of our thermochemical model. We have used CHIM-XPT [10] to perform the model runs for a variety of compositional, T, W/R values and initial fluid compositions. The bulk composition is assumed to be basaltic (Fig. 1). Here Water/Rock ratio W/R is the ratio of incoming fluid to reacted rock. Results: 1. The Effect of Water/Rock Ratio. In general, the model based on dissolution of 70% amorphous phase, 20 % olivine and 10 % whole rock [6], produces precipitates that are enriched in Fe, Al, and S compared to the original rock. This effect can be more pronounced at higher W/R (Fig. 2). High W/R runs e.g. >1000 also predict the precipitation of ferric oxide at the expense of ferric silicates or other Fe oxides. Repeated weathering/leaching cycles such as can occur at the surface or along fluid conduits, will increase the effect. For example, if the alteration assemblage formed by incongruent dissolution of Portage soil (with 70 % amorphous phase, 20 % olivine and 10 % whole rock), is subject to another fluid event, the newly precipitated assemblage (at W/R 1000) contains 27 % goethite, 44 % serpentine, and 22 % clay with minor pyrite and apatite (Fig. 3).