Hawai’i and Gale Crater: a Mars Analogue Study of Igneous, Sedimentary, Weathering, and Alteration Trends in Geochemistry

Introduction: Sedimentary rocks in Gale Crater on Mars indicate a varied provenance with a range of alteration and weathering [1, 2]. Geochemical trends identified in basaltic and alkalic sedimentary rocks by the Alpha Particle X-ray Spectrometer (APXS) on the Mars rover Curiosity represent a complex interplay of igneous, sedimentary, weathering, and alteration processes. Assessing the relative importance of these processes is challenging with unknown compositions for parent sediment sources and with the constraints provided by Curiosity’s instruments. We therefore look to Mars analogues on Earth where higher-resolution analyses and geologic context can constrain interpretations of Gale Crater geochemical observations. We selected Maunakea (AKA Mauna Kea) and Kohala volcanoes, Hawai’i, for an analogue study because they are capped by post-shield transitional basalts and alkalic lavas (hawaiites, mugearites) with compositions similar to Gale Crater [1, 3]. Our aim was to characterize Hawaiian geochemical trends associated with igneous processes, sediment transport, weathering, and alteration. Here, we present initial results and discuss implications for selected trends observed by APXS in Gale Crater. Methods: We sampled fresh and altered lavas from basaltic, hawaiitic, and mugearitic flows on Maunakea and Kohala. Weathering profiles were sampled at roadcuts where a range of unaltered to highly weathered material was exposed. To evaluate sedimentary mixing, we collected outwash sand and gravel samples at various elevations. To emulate APXS measurements, bulk powders were prepared from cm-scale hand samples and analyzed by standard XRF methods. Sampling cinder cones and elevations >3700 m was not permitted; we report previous results on unaltered and altered materials from these sites [4], and from an acid sulfate alteration profile of a Kilauea basalt [5]. Results: Igneous compositions fall into the postshield compositional classes of transitional basalt, hawaiite, and mugearite, with alkali content increasing in the younger, more evolved flows. Outwash sediment spans the same range, but also fills the igneous compositional gap between basalts and hawaiite/mugearites. Weathering and alteration trends fall into two groups defined by [4]: hydrolytic and acid sulfate. Hydrolytic alteration is apparent in weathering profiles and palagonitized tephra, and is characterized by mobilization of K 3 Na > Ca > Mg > Si and conservation of Ti, Al, and Fe (Fig. 1a, b, c). Volatile content is high (LOI >1 wt%) but SO3 content is low (<0.2 wt%). Acid sulfate alteration leads to low pH mobilization of Mg, Mn, Fe, Al, and Ca, while Si and Ti are retained (Fig. 1d, e) and SO3 is usually added (>1 wt%). Potassium tends to accumulate in secondary sulfates (Fig 1d). In both alteration regimes, variable leaching of some elements indicates open systems.