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Clays in acid environments on Mars

Final Report Summary - ACID-MARS (Clays in acid environments on Mars)

The objectives of the project are:
1) To identify acid-alteration environments in Riotinto that served as analogue for Martian environments.

2) To understand the processes that took place in Riotinto and fully characterize the samples collected there, preparing a set of near-infrared data that can be compared with spectra from Mars.

3) To use the collected data in conjunction with data from Mars to establish the type of environments and processes that generated the Martian rocks.

Work performed since the beginning of the project

1) The fieldtrip in Riotinto with Prof. Nieto (University of Huelva) and Ms Vega (EMED-Tartessus Ltd., now called Atalaya Mining Plc.) allowed us to understand generally the types of rocks available and the processes that had generated them. A complete set of samples was collected.

2) The samples were prepared for analysis by crushing and milling.

3) The mineralogical study took place using X-ray diffraction in several steps. First, all samples (approx. 50) were analysed for their complete mineralogy. From these data, a selection was made (31 specimens) of the samples that represented the progress of acid alteration, avoiding the use of samples that were too similar. Then the fraction less than 2 micro-metres was separated for specific investigation of the clays. This fraction prepared as oriented mounts and investigated as air-dried and saturated with ethylene-glycol, using X-ray diffraction. The following step was the modelling of X-ray diffraction patterns simulating the experimental ones. 62 patterns were modelled (air-dry and saturated with ethylene-glycol) using ClaySim from MDI. The modelling provided the mineralogy of the clays, identified mixed-layered phases and provided the full quantification of clays in the investigated samples.

4) The chemical analyses of the selected 31 samples were carried out in three different sets. First, the composition of the major elements in the fraction less than 2 micro-metres was obtained by dissolution and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Second, the major elements of the whole fraction of the selected samples was carried out with ICP-AES. Third, the trace elements of the whole fraction of the samples was obtained with inductively coupled plasma mass spectrometry (ICP-MS).

5) The mid-infrared analyses of the selected samples were carried out at the Natural History Museum, in transmission mode. This investigation allows a good structural characterization of the clays and is a link with the near-infrared data described below.

6) The near-infrared analyses of the selected 31 samples were carried out in Brown University, USA, using the standard procedures for the reflection mode advised by NASA and which are common among planetary scientists. Brown University is the depositary of the near-infrared spectral library of reference minerals.

7) The above results have been analysed and interpreted by the research team. This involved two trips of the Fellow to California for discussions with one of the collaborators (Dr. J. Bishop, SETI Institute).

Main results achieved so far

1) The Fellow has received a thorough training in clay mineralogy and geochemistry, particularly in the alteration processes of clay minerals. He has learned to use modelling of X-ray diffraction patterns, an essential tool in the investigation of complex clay systems. He has acquired a robust, state-of-the-art knowledge of Martian Geology and Geochemistry and training in the interpretation of near- and mid-infrared spectra of clay minerals and other mineral typically present with them. Finally, he has received training on how to correlate mineral assemblages, mineral processes and geological scenarios through discussions with the research team.

2) Environments of progressive acidic alteration have been identified and described, with special emphasis on the clay minerals. Inferences have been extracted for similar environments on Mars. The main characteristics of these environments have been specified and can be used for their characterization on Mars.

3) The characterization of clay minerals has confirmed that mixed-layered clays are abundant in acidic environments and they are to be expected in similar Martian environments. Our near-infrared data will be very helpful to identify properly the mineral assemblages on Mars because mixed-layering complicates spectral interpretation.

4) Our study has suggested that vermiculite should be widespread on Mars, given the typical Martian conditions and rocks. This addresses an ongoing discussion about the identity of certain clays on Mars that appear to be vermiculite, and their origin, which has not been understood so far. An article on this issue is now prepared to be submitted for publication.

Expected final results and their implications

1) The use of our near-infrared data of complex clay mixtures that have been thoroughly characterized will be a great asset for the identification of clay assemblages and environments on Mars using remote sensing for many years from now. These spectra will replace the use of some of the spectra in the library from samples that are not well characterized and introduce errors in the interpretation of spectra from Mars. Our spectra will also widen the choice of reference data for comparison, which is essential because single clay minerals present spectral differences due to chemical and structural variations. The spectra are archived at Brown University and are available to all.

2) We aim at developing models of acidic environments on Mars using the data from the Earth analogue in Riotinto. This will include: mode of alteration, intensity of alteration and expected mineral suits.

The implications of these results can be summarized in two points: a) The information of the project refers to hydrous processes on Mars and thus sheds light on the mode of existence of water on Mars, an essential contribution to the investigation of past or present Martian life. b) The project is contributing and will contribute to our knowledge of Mars geological evolution and, through comparative planetary evolution, to understanding Earth past and future processes.