Miller Range Nakhlite (MIL)

Abstract

Secondary phases resulting from aqueous alteration in Miller Range nakhlites (MIL) can give clues to the paleoenvironment of Mars. However, similarities in weathering processes on Earth and Mars make distinguishing between alteration-type difficult. Paired thin sections MIL 03346(U) and MIL 090136(25) were investigated to distinguish Martian from terrestrial alteration phases. Using optical microscopy, augite clinopyroxene and altered olivine phenocrysts surrounded by a mesostasis glass indicated MIL formation from a parent basaltic magma on Mars. Backscatter scanning electron (BSE) and energy dispersive spectrometry (EDS) images obtained in a scanning electron microscope (SEM) displayed olivine hosted iddingsite veins and calcite precipitation, deemed Martian and terrestrial alteration respectively.

Mineralogical and chemical analysis suggest past anoxic conditions on Mars, where Fe-silicates became altered by an acidic fluid. On Earth, chemical weathering of Antarctic bedrock by an acidic fluid is proposed to have further altered MIL. Observed variability of alteration phases between MIL samples highlights the influence of thin section extraction and shelter from weathering on interpretations; which should be considered in future investigations of MIL.

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Introduction

Miller Range nakhlite (MIL) samples MIL 03346 and MIL 090136 were discovered in Antarctica and host both Martian and terrestrial alteration. Such secondary phases result from aqueous processes and can reflect host-rock petrology, fluid composition and environmental conditions at the time of alteration. It is important to distinguish between alteration-types to ensure correct mineralogical and chemical analysis is being conducted for interpretation of the paleoenvironment of Mars.

Nakhlite Meteorites

The nakhlite meteorites are a group of Martian meteorites ejected from Mars at ~10.

75 Ma, falling to Earth within the last ten thousand years (Treiman, 2005). Currently, seven different nakhlite samples are known to exist on Earth (The Meteoritical Society, 2018), including Antarctic MIL 03346 and MIL 090136 (Udry et al., 2012). These igneous rocks formed from Martian basaltic magma at approximately 1.3 Ga (Cohen et al., 2017). They contain clinopyroxene and olivine phenocrysts within an interstitial mesostasis glass, composed of silicates and Ti-magnetite (Righter, 2018).

Martian Weathering

On Mars, ~620 Mya, the nakhlite lava became exposed at the surface to Martian weathering, resulting in alteration (Righter, 2018). An acidic fluid formed veins within the Miller Range (Hallis and Taylor, 2011), which chemically weathered olivine to "iddingsite", referred herein as: a poorly crystalline Si-enriched mixture of olivine alteration products (Bunch and Reid, 1975).

Terrestrial Weathering

On Earth, MIL are exposed to Antarctic weathering which creates conical etched pits in olivine crystals exposed at the surface (Velbel, 2009). Wind-blown sea spray can form gypsum and jarosite veins as well as an "Antarctic Varnish" of Si, Fe-rich sulphate layers on the outside of samples (Hallis and Taylor, 2011).

Miller Range Nakhlites

MIL 03346 and MIL 090136 were discovered in the Transantarctic Mountains ~200m apart (Fig.1). Their close proximity suggests a shared parent meteorite, with radio-isotopic chronometers dating cosmic ray exposure at ~10.75 Ma (Treiman, 2005). Unlike the Governador Valadares nakhlite, meteorite "fall" was unobserved, instead discovered by ANSMET (US Antarctic Search for Meteorite Program) as "finds" in 2003 and 2009 respectively (Righter, 2018). It is therefore assumed, MIL contains both Martian and terrestrial alteration (Udry et al., 2012).

Figure 1. Location map of MIL in the Transantarctic Mountains with inset of relation to Antarctica. MIL 03346 and MIL 090136 (triangle) were found ~200m apart in glacial ice between the East and West Antarctic ice sheets. The ice advances south towards the Ross Sea (>500km away) providing a potential source of sea spray.

Adapted from Udry et al. (2012) and GoogleEarth, 2019.

MIL 03346 is 10 x 6 x 5.5cm, weighing ~715g and MIL 090136 is 6 x 4.5 x 4cm, weighing ~171g (The Meteoritical Society, 2018). Figure 2 illustrates the black fusion crust and vugs (crystal-lined cavities) externally imposed as the meteorites entered Earth's atmosphere (The Meteoritical Society, 2018). Internally, the samples have a coarse-grained, dark-green crystalline matrix and a granular texture of homogenous interlocking phenocrysts (Fig.2) (The Meteoritical Society, 2018). Satterwhite and Righter (2004) report a mineral composition of abundant elongate clinopyroxene and minor olivine crystals within a fine-grained intercumulus mesostasis (Fig.2). Udry et al. (2012) calculated weighted modal abundance of 72.3% clinopyroxene, 3.9% olivine and 23.7% mesostasis.

Alteration

Hallis and Taylor (2011) observed Si, Fe-rich "iddingsite" veins up to 50 ?m wide within olivine phenocrysts of MIL 03346. They were determined Martian alteration phases from descriptions of fusion crust truncation by Stopar et al. (2013). The poorly-crystalline veins formed from olivine alteration by an acidic S, Fe-rich fluid on Mars (Papike et al., 2006). Sulphates within the mesostasis of MIL 03346 have been suggested by Ling and Wang (2015) as direct deposits from a Martian Ca-rich fluid. Thin sections taken from the outside of MIL 03346 display olivine dissolution textures and etch pits (Velbel, 2016). Meanwhile, gypsum and jarosite associated with exposed surfaces and impact fractures are also observed and concluded by Hallis (2013) as terrestrial alteration by wind-blown Antarctic sea spray.

Figure 2. Hand specimen samples and photomicrographs of Miller Range nakhlites MIL 03346 and MIL 090136. Images taken using an optical microscope at x5 magnification in PPL ( Plain Polarised Light and XPL)  Cross Polarised Light. Optical images of MIL 03346(U) is unachievable due to the thickness of resin coating. Colour in MIL 090136(25) PPL is a malfunction of the equipment and should be ignored.

Fusion crust and vugs are visible on sample surfaces. Thin sections MIL 03346 and MIL 090136(25) display a dominance of elongate clinopyroxene phenocrysts alongside olivines and an intercumulus mesostasis. Adapted from Righter (2018).

The same Si, Fe-rich iddingsite veins were observed within olivine phenocrysts of MIL 090136 by Hallis and Taylor (2011), despite the discovery of the sample five years after MIL 03346. The pre-terrestrial alteration was again confirmed in an externally-derived thin section by Hallis (2013). Vein material was described as warped and vesiculated from fusion crust heating and suggested altered by a Martian acidic fluid by Papike et al. (2006). Narrow Fe-oxide veins (1-10 ?m wide) were observed along the edges of olivine hosted iddingsite alteration veins and traced back to Ti-magnetite grains in the mesostasis (Hallis and Taylor, 2011). Pre-terrestrial alteration was concluded based on association with Martian altered iddingsite veins and absence within externally-derived thin sections that would have been exposed to weathering on Earth (Hallis and Taylor, 2011). Hallis (2013) found jarosite in the olivine melt inclusions of some MIL 090136 internal-derived thin sections. Where Martian iddingsite veins penetrated the inclusions, a pre-terrestrial origin of sulphate precipitation was proposed (Hallis, 2013). However, research of terrestrial alteration within MIL 090136 is scarce, with sea-spray shelter and ice residence time insinuated as factors contributing to limited sulphate precipitation along sample edges and within fractures (Hallis, 2013).

Defining Martian from terrestrial alteration within MIL aids interpretation of associated environments. Currently, olivine-hosted iddingsite veins and in situ sulphate precipitation are proposed as Martian alteration products, which suggest meteorite infiltration by an acidic fluid on Mars. Terrestrial alteration is predominately recognised in externally-derived thin sections as surface dissolution of olivine crystals and sulphate precipitation along exposed edges and within fractures. Hallis (2013) believes deposition originates from Antarctic wind-blown sea spray salts. (The reader is referred to 8.0 Appendix for a full summary of published MIL secondary phase alterations) Further mineralogical and chemical analysis of thin sections MIL 03346(U) and MIL 090136(25) should confirm these findings. In addition, the chemistry of Martian waters and conditions of alteration on Mars could be interpreted on the basis of secondary phases from aqueous alteration.

Methodology

Thin section MIL 03346(U) is derived from an unknown location within sample MIL 03346. Thin section MIL 090136(25) has been cut from an external section of sample MIL 090136 and therefore, was exposed to weathering on Earth. Initial mineralogy is obtained using an optical microscope at x5 magnification. Plain polarized light (PPL) and cross polarized light (XPL) maps are constructed of each thin section to highlight areas of potential alteration. A more detailed analysis of mineralogy is attained through backscattered scanning electron (BSE) and energy dispersive spectrometry (EDS) images, obtained in a scanning electron microscope (SEM). MIL 03346(U) has been coated in carbon and MIL 090136(25) is coated in gold and carbon which improve conductivity of the samples in the SEM (Reimer, 2013).

Once mounted by conductive aluminium tape, a 20 keV voltage, 2.1 nA current of electrons is fired at the thin sections. Within vacuum conditions, the electrons focus into a beam (0.5nm diameter) and scan MIL thin sections in a raster (rectangle) pattern (Reimer, 2013). In BSE, atomic valences determine reflectance of electrons (Reimer, 2013). The signals are detected and amplified which provide mineralogical information of MIL thin sections (Reimer, 2013). In EDS, replacement of outer shell electrons by higher energy electrons causes X-ray release (Reimer, 2013). The wavelengths of these signals are detected and amplified, providing chemical information of MIL thin sections (Reimer, 2013).