Hydrothermal alteration in the Frankenstein gabbro Martian analogue: first models

Seidel, R. G. W.; Sherlock, S. C.; Schwenzer, S. P.; Bridges, J. C. and Kirnbauer, T. Hydrothermal alteration in the Frankenstein gabbro Martian analogue: first models. In: 2nd British Planetary Science Conference, 13-15 Jan 2020, Oxford, UK.

Abstract

Introduction: Analogue studies can improve our understanding of alteration processes on Mars where information from Martian surface missions or Martian meteorites is incomplete. This especially applies to Martian hydrothermal systems within basaltic host rocks, which are not represented in the meteorite record and thus are not available for in-detail studies in Earth-based labs, even though such systems are thought to be a common by-product of impact cratering on Mars [e.g., 1, 2] and have been observed by orbiter [3] and surface missions [4].

We present results of an ongoing petrologic and modelling study of a terrestrial analogue, the Frankenstein gabbro (Germany), to predict hydrothermal alteration in basaltic host rocks on Mars. Our aim is to constrain the likely reaction paths by which primary magmatic minerals are replaced, and to specify the expected properties of the hydrothermal fluids. Such information may be crucial in the interpretation of data gathered by Martian surface missions. In a next step, we aim to assess fluid habitability and identify mineral assemblages indicating the former presence of hydrothermal fluids suitable for potential Martian life, in order to inform the selection of sampling sites for future sample return missions.

The Frankenstein Gabbro. We use a gabbro from the Frankenstein massif in the Odenwald Mountains, Germany. This rock – chemically equivalent to basalt – formed in a magmatic arc setting at ~360 Ma BP [e.g., 5], and experienced at least three localized events of hydrothermal alteration at 170–180 Ma BP, 138 ± 8 Ma BP, and ~60 Ma BP [6, 7]. We focus on the 138 ± 8 Ma BP event, which is associated with hairline fault planes and veinlets with a complex secondary mineralogy. Our previous investigations [8] show a strong small-scale variability of mineral formation, depending on host minerals and water/rock ratio, a feature also seen in the hydrothermally-altered nakhlite group of Martian meteorites [e.g., 9].

Methods: Following our mineralogical characterization of Frankenstein gabbro samples [8], we use the CHIM-XPT thermochemical modelling software to reconstruct mineral reaction paths [10]. As input, we use previously published XRF bulk rock data [11], EMP data of single minerals, and a starting fluid based on observed mineral quantities and calculated element budgets of mineral replacement reactions.

Results: Initial results are summarized in Fig. 1, showing calculated secondary mineral assemblages for bulk rock, a pure plagioclase host, and a pure clinopyroxene host, at high and low water/rock ratios.

Discussion & Outlook: A trend seen in all models is the diminishing importance of chlorite as W/R decreases, with chlorite generally dominating at W/R > 10.000. This is consistent with observed chloritedominated mineral assemblages in Frankenstein gabbro fault planes, for which a high W/R is expected. The models thus show potential to match and inform petrologic observations. However, several important minerals predicted in the models (e.g., carpholite, zeolites) are not found in the Frankenstein alteration assemblage, while others are not expected to form at the modelled conditions (e.g., diopside [12]). Our next step will be to refine the models before applying them to Mars.

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