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Terrestrial alteration mineral assemblages in the NWA 10416 olivine phyric shergottite
DOI:
10.1016/j.gca.2020.03.026
Authors:
J. D.
Piercy
(University of Leicester)
,
J. C.
Bridges
(University of Leicester)
,
L. J.
Hicks
(University of Leicester)
,
J. L.
Macarthur
(University of Leicester; University of Manchester)
,
R. C.
Greenwood
(The Open University)
,
I. A.
Franchi
(The Open University)
Co-authored by industrial partner:
No
Type:
Journal Paper
Journal:
Geochimica Et Cosmochimica Acta
State:
Published (Approved)
Published:
April 2020
Abstract: We report on the alteration history of the olivine-phyric, highly depleted (HD) shergottite, Northwest Africa (NWA) 10416, paying particular attention to the origin of the aqueous alteration seen affecting the meteorite’s olivine megacrysts. The rock’s interior displays 1 mm, zoned, altered olivine megacrysts set in a groundmass of clinopyroxene, unzoned olivine, and interstitial plagioclase and maskelynite. Synchrotron micro X-ray diffraction (µ-XRD) and transmission electron microscopy (TEM) show that plagioclase and maskelynite have been partially replaced by kaolinite. The relict olivine megacryst cores display a unique concentric colouration for Martian meteorites, having central amber-coloured zones surrounded by a brown mantle zone, with the rims remaining clear and unaltered. This colouration is a result of fluid alteration and partial replacement, with hydration. TEM analysis revealed the ∼200 nm scale banded and largely amorphous nature of the alteration, but with some (∼ 20%) relict crystalline olivine patches. Although the coloured olivine zones show cation and anion site vacancies compared to stoichiometric olivine, a relict igneous compositional trend is preserved in the megacrysts, from Mg-rich altered cores (Mg# = 76) to unaltered stoichiometric rims (Fo53). Synchrotron Fe-K X-ray absorption near-edge structure (XANES) analysis revealed that the coloured zones of the megacryst have different Fe oxidation values. High ferric contents are present in the brown mantle zones (Fe3+/ΣFe ≤ 0.92) and the amber zones (Fe3+/ΣFe ≤ 0.30), whereas the clear rims are ferrous. This suggests alteration occurred in an oxidising environment and that the sharp contrast in colour of the megacryst (brown to clear) is a record of a relict fluid reaction front. In order to test the terrestrial or extraterrestrial origin of the alteration, olivine material from a shock-melt vein was analysed by TEM. The analysis revealed 0.952 nm curved d-spacing’s from clay alteration undisturbed by any shock effects, strongly suggesting a terrestrial origin. The d-spacing values most likely represent a collapsed saponite or vermiculite, showing that in some places olivine has been replaced by crystalline clay. Oxygen isotope analysis of bulk (Δ17O = 0.309 ± 0.009 (2σ) ‰) and amber-coloured megacryst material (= 0.271 ± 0.002 (2σ) ‰), are also consistent with terrestrial alteration. We propose a model in which, during the meteorite’s time in Northwest Africa, low-temperature, likely acidic, groundwater exploited fractures. The fluid altered the olivine megacrysts in a way that was controlled by the pre-existing, igneous compositional zonation, with Mg-rich olivine being more susceptible to alteration in this fluid environment. The plagioclase and maskelynite were also altered to a high degree. After the alteration event it is likely that NWA 10416 had a significant residence time in Northwest Africa, accounting for terrestrial calcite and the dehydration of some clay phases.
Subject Areas:
Earth Science,
Chemistry
Instruments:
I18-Microfocus Spectroscopy
Added On:
08/04/2020 10:17
Discipline Tags:
Earth Sciences & Environment
Mineralogy
Chemistry
Geology
Geochemistry
Technical Tags:
Spectroscopy
X-ray Absorption Spectroscopy (XAS)
X-ray Absorption Near Edge Structure (XANES)