In the present work, we have measured iron isotope compositions of a group of Martian meteorites to establish a baseline Fe-isotope fractionation pattern in the case of high-temperature igneous rocks from Mars

Anand, M.

Grady, M. M.

Mullane, E.

Russell, S. S.

English

Open Research Online 

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsFe isotopic composition of Martian meteoritesConference or Workshop ItemHow to cite:Anand, M.; Russell, S. S.; Mullane, E. and Grady, M. M. (2005). Fe isotopic composition of Martian meteorites. In:36th Lunar and Planetary Science Conference, 14-18 Mar 2005, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1859.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukFE ISOTOPIC COMPOSITION OF MARTIAN METEORITES M. Anand (M.Anand@nhm.ac.uk), S.S. Russell, E. Mullane, M.M. Grady Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK   Introduction The scientific community has actively debated for quite some time the possibility that conditions were once favorable for life to originate and evolve on Mars. However, as we know it, for life to survive, the pres-ence of liquid water is essential. The present day cli-mate on Mars is inhospitable for sustaining any life form. However, recent data from twin rovers, Spirit and Opportunity, and Mars Express spacecraft have strengthened previous interpretations and suggestions that Mars was once a warmer and wetter place [e.g., 1]. Until we have a returned sample mission from Mars, one of the ways to look for past evidence of aqueous activity on Mars is through mineral, chemical and isotopic studies of primary and secondary minerals in Martian meteorites. The focus of this research is on understanding the action and effects of water on Mars by using fractionation of iron isotopes (which can be significant during low temperature aqueous processes) to trace the alteration of primary to secondary minerals by water on Mars. Another key aspect of the project is to first establish a baseline Fe-isotope fractionation pattern in the case of high-temperature igneous rocks from Mars that are available to us in the form of Mar-tian meteorites. Once the baseline Fe isotope fractiona-tion in these rocks are established, it will be possible to determine any Fe isotopic variation because of low-temperature aqueous processes including biogenic ac-tivities. Previous research has recognized an evaporite sequence preserved within some of the Martian meteor-ites [e.g., 2]. This finding highlighted different altera-tion processes affecting Martian meteorites and Mars, and has important implications for the timing, extent and duration of hydrous activity at the planet’s surface.  Methodology and Samples:  The goals of the pro-ject are pursued through a detailed study of the bulk, elemental and isotopic composition of components within Martian meteorites. Iron is an important con-stituent of the rock-forming minerals that are produced at the elevated temperatures experienced during magma genesis. Iron has four stable isotopes (54, 56, 57 and 58) and is also an important constituent of sec-ondary minerals produced by alteration of primary minerals, occurring in clay minerals and carbonates. Kinetic, equilibrium and nuclear processes fractionate the isotopes, in the same way as is commonly observed for light elements. Iron isotopes have also been shown to be fractionated by biological processes, and thus have potential in providing bio-signatures in a sample.  Previous to this study, a suite of 8 Martian meteorites were analysed for their whole-rock Fe isotope compo-sition using an MC-ICP-MS technique, which indicated limited but distinct fractionation patterns compared to terrestrial and lunar samples [3]. In the present work, we have measured iron isotope compositions of a group of Martian meteorites to ascertain variation in δ56Fe and δ57Fe. Six samples (Shergotty, Zagami, Los Angeles, Sah Al Uhaymir 005, Nakhla and Chassigny) were analysed. ~0.2g rock powder dissolution followed a two-step HF-HClO4-HCl method [4]. Digests were purified using anion exchange chromatography, which separated out Fe, Cu and Zn. This procedure has been detailed elsewhere [4]. Fe-isotope compositions (δ56Fe and δ57Fe) were measured on a fixed resolution (m/ m = 500) MC-ICP-MS (IsoProbe, GV Instruments, U.K.) with respect to the Fe-isotope standard IRMM-014, using the sample-standard bracketing method [4]. Cor-rection for 54Cr on 54Fe is made by monitoring the 53Cr signal and applying an on-line mathematical correction at mass 54. Blank subtraction is undertaken off-line. The effect of 40Ar16OH background at mass 57 is re-duced by analyzing solutions at approximately 10ppm concentration and sample and standard solutions are concentration matched to within 5%. Typical errors are less than 0.05 per mil (2 sigma) in most cases.  Lunar and Planetary Science XXXVI (2005) 1859.pdfFe-isotope Fractionation: All of the Martian me-teorites analysed plot within a restricted range on a 3-isotope plot (diamond symbols in Fig. 1). The inset in Figure 1 shows the spread in the measured δ56Fe and δ57Fe ratios in our samples with 2 sigma errors. The red circles in Figure 1 are all the Fe isotope data available on various planetary materials and have been compiled from various sources [3,4-14]. Figure 2 is a bar dia-gram illustrating the reported range in δ56Fe among various planetary materials, including our data. The Fe-isotopic range in our samples is 0.00 to 0.02 ‰ for δ56Fe and 0.00 to 0.04 ‰ for δ57Fe (Table 1, Fig 1), similar to those reported for terrestrial igneous rocks [13]. Furthermore, the Fe-isotope data for Martian me-teorites are tightly grouped and fall on the mass frac-tionation line already described for a range of other solar system samples [3,4-14] (Fig 1). The similarity and narrow range in iron isotope fractionation within the Martian meteorites, analysed in the present study, with terrestrial igneous rocks suggests that Earth and Mars were derived from a homogenous solar nebula with respect to Fe-isotope composition, consistent with previous studies [6]. Further Fe-isotope measurement on primary and secondary minerals in the Martian me-teorites is in progress. Suitable terrestrial analogues, such as some Proterozoic lava flows and sedimentary rocks, are also being studied for their Fe isotopic signa-tures.  References: [1] Squyres et al. (2004), Science, 306, 1698-1703. [2]  Bridges J. C. and Grady M. M., (2000) Earth Planet. Sci. Lett. 176, 267-279; [3] Poitrasson et al., (2004) Earth Planet. Sci. Lett. 223, 253-266. [4] Mullane et al. (2003) In: Holland & Tanner (Eds.) Plasma Source Mass Spec., Royal Soc. Chem. 351-361. [5] Mul-lane et al. (2003) LPSC XXXIV, Abs. #1027. [6] Zhu et al. (2001) Nature 412: 311. [6] Mullane et al. (2004) LPSC XXXV, Abs. #1015. [7] Mullane et al. (2004) Met Soc, Abs. #5147. [8] Mul-lane et al. (2004) Met Soc, Abs. #5148. [9] Mullane et al. (2003) LPSC XXXIV, Abs. #1027. [10] Wiesli et al. (2003) Earth Planet. Sci. Lett. [11] Zhu et al. (2000) Earth Planet. Sci. Lett. 200, 47-62. [12] Beard  et al (2003) Chem. Geol., 195, 87-117. [13] Beard et al (1999), Science, 285, 1889-1892.   Lunar and Planetary Science XXXVI (2005) 1859.pdf

Fe isotopic composition of Martian meteorites

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Fe isotopic composition of Martian meteorites

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