Alteration Assemblages in the Nakhlites: Variation with Depth on Mars
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Meteoritics & Planetary Science 45, Nr 12, 1847–1867 (2011) doi: 10.1111/j.1945-5100.2010.01123.x Alteration assemblages in the nakhlites: Variation with depth on Mars H. G. CHANGELA1 and J. C. BRIDGES1,2* 1Space Research Centre, Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK 2Scientific Associate, Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK *Corresponding author. E-mail: [email protected] (Received 12 October 2009; revision accepted 28 August 2010) Abstract–Secondary mineral assemblages in the nakhlite meteorites, Lafayette, Governador Valadares (GV), Nakhla, Yamato (Y)-000593 ⁄ Y-000749 have been studied using scanning electron microscopy, transmission electron microscopy, and electron probe micro analysis. The different nakhlites have distinctive secondary assemblages in their olivine grains and mesostases, showing compositional fractionation correlated with their relative depths below the Martian surface. Fracture-filled veins in Lafayette at the bottom of the pile consist of a siderite-phyllosilicate-Fe oxide-hydrated silicate gel assemblage. Corresponding veins in Nakhla and GV further up the pile are predominantly a siderite-gel assemblage, with additional evaporites including gypsum. Y-000593 ⁄ Y-000749 veins are dominated by gel. The gel’s Mg ⁄ (Mg + Fe) ratio decreases from Lafayette (0.37) to GV (0.32), Nakhla (0.24), and Y-000593 (0.15). We suggest that hydrothermal fluid flowed up this depth profile, initiated by melting of buried H2O–CO2 ice. Our results show a complex mix of Fe-rich phyllosilicate within the veins and mesostasis of Lafayette with d-spacings of 0.7–1.1 nm suggesting a mixture of smectite and serpentine. The phyllosilicate formed at close to neutral pH, £150 °C. We also suggest that water rock ratios (W ⁄ R) of 1–10 occurred in Lafayette with smaller values for the other nakhlites. This is reflected in the volume of alteration minerals: 10% of olivine in Lafayette to 3% in Nakhla. Textural evidence of rapid cooling, together with the W ⁄ R and likely fluid velocities, suggest that the secondary assemblages formed quickly, e.g., within months. A model is proposed in which the secondary assemblages formed in an impact-induced hydrothermal system terminated by precipitation of the gel and evaporation of soluble salts. INTRODUCTION These are present in fractures not only within olivine but also within the mesostasis (Bridges and Grady The nakhlites are clinopyroxenites thought to have 2000). The preterrestrial, Martian origin of the veins has crystallized as cumulates within a thick basic-ultrabasic been established because Nakhla and Lafayette veins lava flow or shallow (100 m) intrusion on Mars are seen to be truncated by the fusion crust (Gooding 1.3 Ga (e.g., Treiman et al. 1993; Lentz et al. 1999; et al. 1991; Treiman et al. 1993). The secondary mineral Bridges and Warren 2006). Fe-rich olivine grains make assemblages vary in abundance and composition up approximately 10% volume of all but one of them between the nakhlites but include siderite, phyllosilicate, (Miller Range [MIL] 03346, which has no cumulus and salts. They have been dated by K-Ar within the olivine). Within the eight known nakhlites (Nakhla; time range £670 Ma (Swindle et al. 2000) and so this Governador Valadares [GV]; Lafayette; Northwest constrains the timing of the hydrothermal alteration to Africa [NWA] 998; NWA 817; NWA 5790, MIL 03346, a relatively recent time in Mars’ history. and the paired Yamato [Y]-000593 ⁄ Y-000749 ⁄ Y- The secondary mineral assemblage was derived 000802), there are secondary mineral assemblages (e.g., from a brine percolating through the nakhlite parent Ashworth and Hutchinson 1975; Gooding et al. 1991; body (Bridges and Grady 2000; Bridges et al. 2001). Bridges et al. 2001; Gillet et al. 2002; Treiman 2005). This is, for instance, consistent with the varying Ca 1847 Ó The Meteoritical Society, 2011. 1848 H. G. Changela and J. C. Bridges content of the siderite between Lafayette and the other All five nakhlite samples were initially analysed with nakhlites (Bridges and Grady 2000). They formed at scanning electron microscopy (SEM). The Phillips XL30 low temperature (£150 °C) and rapidly, to form ESEM and FEI Sirion Field Emission Gun (FEG)-SEM metastable mineral compositions prior to final of UL’s Advanced Microscopy Centre (AMC) were evaporation of the fluid. Some secondary minerals used for this. Alteration products within the olivine within the nakhlites may however, be terrestrial, e.g., were imaged in more detail and further analyzed by sulphates within the Antarctic finds, Y-000593 ⁄ energy dispersive X-ray spectroscopy (EDX). The FEG- Y-000749 (Gooding 1986; Kuebler et al. 2004). SEM and ESEM are equipped with a PGT EDX system In this article, we report the results of a transmission and Oxford Instruments INCA spectrometer. EDX electron microscopy (TEM) and electron microprobe spectra and X-ray maps were made at 15 and 20 kV study of Nakhla, Lafayette, Y-000593, Y-000749, and accelerating voltage, respectively. Montages of five GV to document in more detail how the mineralogy of olivine grains from each of the nakhlites were produced the secondary mineral assemblages within olivine and loaded in Image Pro Plus software to calculate fractures and mesostasis varies between the nakhlites. volume fractions of the secondary assemblages within From this, we aim to further constrain the nature and each of the thin sections. origin of the hydrothermal alteration within the nakhlite Electron probe micro analysis (EPMA) was also parent rocks on Mars. In particular, we have studied the performed on the samples. A Cameca SX100 was used structure and varying composition of nakhlite in the Department of Mineralogy, NHM. The phyllosilicate and gel and show that the gel composition microprobe was operated at 20 kV accelerating voltage varies between the nakhlites. We have also attempted to with a beam current of 20 nA, using appropriate distinguish between a terrestrial overprint and Martian mineral standards to check calibration. Alteration secondary minerals in the Antarctic find Y-000749. products within the olivine veins were analysed in Mikouchi et al. (2003a, 2003b, 2006) suggested the phases with width approximately 5 lm or more in an probable relative original depths of the nakhlites on attempt to constrain interaction volumes within Mars by comparing their mineralogy and groundmass individual phases. textures. Bridges and Warren (2006) added to this After SEM-EDX and EPMA based characteriza- model using the composition of secondary assemblages. tion, cross sections of selected veins were prepared for In these models, MIL 03346 is nearest the surface, the TEM. A dual focussed ion beam (FIB-SEM) technique Yamato meteorites close to the top surface, followed by was used for TEM sample preparation, enabling in situ Nakhla, then GV with Lafayette at the bottom. sectioning of specific micron-sized areas of interest. An Treiman et al. (1993) suggested that in Lafayette the FEI Quanta 200 3D with a Ga+ ion beam was used at water to rock ratio and the temperatures could have UL; milling an approximately 70–110 nm thick wafer been higher compared to the rest of the nakhlites. We containing a section of the secondary assemblage(s). For consider our new results in the light of likely relative precise milling of the sections, the automated TEM depths of the nakhlites and position within a runscript was used with the Ga+ beam at 30 kV to hydrothermal system, perhaps associated with an impact generate a 0.1–0.2 lm thick wafer. The runscript steps event. We aim to provide a model to help explain the down milling currents from 5 nA to <100 pA. Vein formation of some recently identified phyllosilicate and regions chosen for extraction frequently contain some carbonate on the Martian surface (e.g., Ehlmann et al. void space between the olivine and veining assemblages. 2008; Mustard et al. 2008). Electron transparent wafers of these regions are therefore very fragile. To maintain their stability, the SAMPLES AND METHODS produced sections were imaged with snap shots at sequential stages of the milling sequence and the Polished thin sections of Y-000749 and Y-000593 runscript was aborted before the final milling currents have been provided by the National Institute of Polar were employed. Some Y-000593 ⁄ Y-000749 wafers were Research. A thin section of Nakhla (BM 1911, 369) and sectioned at 20 kV in an attempt to help preserve the polished resin blocks of GV (BM 1975, M16) and secondary features with lower voltages and currents. Lafayette (BM 1958, 775) were prepared at the Natural Prior to application of the runscript, the selected areas History Museum (NHM). Resin blocks of GV and in this study were capped with a 30 · 5 lm Pt layer of Lafayette were polished in oil in an attempt to initially approximately 150 nm thickness using the electron preserve soluble phases (Bridges and Grady 2000). A beam. An additional 2 lm thick cap was placed on top nontronite clay standard (Urgeirica Beira Alta, of this during application of the wizard script using the Portugal-MB 1972, 142) was also provided by the Ga+ beam. Both layers were deposited by the Pt NHM. hydrocarbon in the gas injection system (GIS). These Alteration assemblages in the nakhlites 1849 + capping layers protected the sample area from the Ga (a) ions during the milling and deposition processes. Wafers of length 15 and 4.5 lm width were produced. They were extracted and attached to an Omniprobe copper grid. Welding of the microneedle to the wafer and the wafer to the copper grid was performed with the FIB- SEM carbon GIS. Final ‘‘polishing’’ currents were employed manually after attachment to the grid for optimal electron transparency. The NHM nontronite standard was prepared by taking flakes scraped onto a carbon sticky pad, coated with 15 nm carbon and then inserted directly into the FIB-SEM for extraction and subsequent attachment to an Omniprobe grid.