We have measured the abundance and stable isotopic composition of magmatic carbon extracted from a suite of shergottites. The results confirm previous findings that primordial carbon on Mars is isotopically lighter than that of the Earth

Grady, Monica

Wright, Ian

English

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsCarbon reservoirs on Mars: Constraints from MartianmeteoritesConference or Workshop ItemHow to cite:Grady, Monica and Wright, Ian (2003). Carbon reservoirs on Mars: Constraints from Martian meteorites. In:34th Lunar and Planetary Science Conference, 17-21 Mar 2003, 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/lpsc2003/pdf/1312.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.ukCARBON RESERVOIRS ON MARS: CONSTRAINTS FROM MARTIAN METEORITES.  Monica M. Grady1 and I. P. Wright2, 1Dept. of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K. (mmg@nhm.ac.uk), 2PSSRI, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. (i.p.wright@open.ac.uk).   Introduction:  The search for life on Mars is a sub-ject of enormous scientific and public interest; it is anticipated that remote missions to Mars over the com-ing few years will garner the requisite data that will allow the presence, or otherwise, of martian biological carbon to be determined. The only previous attempt to detect the presence of life in martian soil, made by in-struments on the Viking landers of 1976, yielded am-biguous results [1]. In the three decades since Viking, analysis of carbon in martian meteorites has given in-sight to the complexities of carbon in martian rocks: it is clear that carbon from several reservoirs (mantle, crust, atmosphere) is present. Unless we have a full understanding of these reservoirs, their relative abun-dances and isotopic compositions, then interpretation of data acquired directly at Mars' surface will be diffi-cult to interpret. Carbon in martian rocks:  There are several con-tributions to the carbon budget in martian meteorites (not including potential martian biological material): primary indigenous carbon dissolved in minerals (sili-cates, phosphates); secondary carbonates produced by surficial aqueous activity; atmospheric CO2 trapped during ejection from the martian surface; spallogenic carbon produced during irradiation in space and or-ganic contamination following arrival on Earth. It is assumed that the first two of these components will be present in martian rocks analysed in situ; it remains to be seen whether there is an additional reservoir emanating from a martian biosphere. Many measurements have been made of the abun-dance and stable isotopic compositions of secondary carbonates in martian meteorites, particularly in ALH 84001 and Nakhla, the two specimens in which they can most readily be identified. Broadly speaking, sev-eral tens to hundreds of ppm 13C-enriched carbonate (δ13C up to around +60‰) occur in ALH 84001 and the nakhlites [2]. Carbon dioxide in Mars' atmosphere is also 13C-enriched, although its actual δ13C value is poorly-defined: measurements of the upper atmosphere made by Viking have large errors (δ13C ~ 0 ± 50‰; ref. 3), and data from martian meteorites only gives a lower limit (δ13C > 30‰; ref. 4). The occurrence of 13C-enriched carbon in the atmosphere and in secondary carbonates led to the interpretation that the carbonates were produced by low temperature aqueous processes on Mars' surface, by dissolution of atmospheric CO2 in percolating fluids [4, 5]. Thus a link between two res-ervoirs, crustal and atmospheric, has been established. In contrast, identification of magmatic (mantle) carbon in martian meteorites is even less well-constrained than that of crustal or atmospheric carbon. This is because planetary igneous rocks contain only low levels of pri-mary magmatic carbon. Attempts to measure magmatic carbon in martian meteorites by combustion are ham-pered because the temperature at which it is thought to combust is between that at which carbonate decom-poses and that at which trapped atmospheric CO2 is released.  Method:  It is against this backdrop that we have continued our programme of analysis of carbon in mar-tian meteorites by high resolution stepped combustion-mass spectrometry (HR-SC-MS). We are aiming to produce an internally consistent and complete suite of data from as many individual martian meteorites as possible, spanning the complete range of compositions and crystallization histories. Sample preparation in-volves washing whole-rock powders in HCl; this not only removes carbonates (both martian and from terres-trial weathering), but etches the silicates and reduces contributions from spallogenic carbon and trapped at-mospheric CO2. The washed grains are then analysed by HR-SC-MS, using the recently refurbished MS-86 static vacuum system at the Open University. Typical blanks run on the system are less than 10 ng carbon, with δ13C around -20‰. Results and Discussion:  We have started the pro-gramme with analyses of meteorites from the most nu-merous sub-group of martian samples, the shergottites. The past five years have seen a tripling of the number of martian meteorites recognized on Earth; by far the most rapid increase in numbers has been in the basaltic sub-group of shergottites. As of 31st December 2002, there were 14 individual shergottites, 10 of which are classified as basaltic. The increasing number of sher-gottites has also increased the number of ejection events from Mars' surface; dating of the shergottites indicates that lava flows with a range of crystallization ages (approx. 165 - 475 Myr) are being sampled [6]. As well as constraining the isotopic composition of carbon reservoirs on Mars, a goal of the current re-search is to look at the abundance and isotopic compo-sition of magmatic volatiles in martian rocks, in order Lunar and Planetary Science XXXIV (2003) 1312.pdfCARBON ON MARS:  M. M. Grady and I. P. Wright  to obtain a picture of whether Mars' volatile inventory has evolved over the past 500 million years or so. Preliminary carbon isotope results from six shergot-tites are given in Figure 1. Abundance data are not plotted, but are approx. 10-20 ppm. The isotope data are more tightly constrained than previous reports for magmatic carbon in martian meteorites [7]. Even so, the primary carbon reservoir on Mars still appears to be isotopically light, and similar to basalts from the Moon and the HED parent-body. It is the Earth that seems to be the outlier. This distinction between Earth and Mars reinforces the idea that martian rocks retain the signature of primordial carbon, whilst terrestrial mantle rocks have been contaminated through many millennia of crustal recycling.  Earth [11]MantleAtmosphereMarineMoon [10]HED [9]SNC phosphates [8]ZagamiShergottySaU 005QUE 94201LEW 88516ALHA 77005-40 -30 -20 -10 0 10To Mars atms & carbonates (> +30‰)δ13C (‰)  Fig. 1. The isotopic composition of carbon in vari-ous Solar System reservoirs. Martian meteorite data are for acid-washed whole-rock powders, determined by HR-SC-MS; error bars are ± 1σ. References for the other ranges are given below.  The same carbon isotope data are plotted against crystallisation age (where known) in Figure 2. There is potentially a relationship of decreasing δ13C with in-creasing age, but it is not well-defined, and we do not, as yet, attach any significance to it. However, addi-tional analyses of older rocks might serve to define the relationship if magmatic degassing was a significant process during Mars' history. On the limited dataset currently available, there does not appear to be any difference in carbon isotopic composition (or carbon abundance) between the basaltic and lherzolitic sher-gottites, perhaps indicating the homogeneity of the martian mantle, despite the lack on convective tecton-ics.  -30-28-26-24-22-20150 200 250 300 350Crystallisation Age (Myr)ALHA 77005LEW 88516ShergottyZagamiQUE 94201δ13C(‰) Fig. 2. Relationship between crystallisation age (from ref. 6) and the isotopic composition of magmatic carbon in shergottites. Symbols: triangle, basalt; circle, lherzolite.  Summary:  primordial carbon in basalts that crys-tallised in the latest epoch of Mars' history are more 12C-enriched than their terrestrial counterparts. This is rather unfortunate from an experimental viewpoint, as it places the data firmly in the range exhibited by ter-restrial organisms. We are satisfied that our experimen-tal technique is capable of distinguishing between re-sidual terrestrial contamination and martian magmatic carbon. However, if martian organisms have a similar carbon isotopic composition to their terrestrial rela-tives, then without a complete understanding of the interrelationships between different carbon reservoirs on Mars, interpretation of data acquired at Mars' sur-face might be problematic.  References: [1] Klein H. P et al. (1976) Science, 194, 99-105. [2] Jull A. J. T. et al. (1995) Meteoritics, 30, 311-318. [3] Nier A. O. et al. (1976) Science, 194, 68-70. [4] Carr R. H. et al. (1985) Nature, 314, 248-250. [5] Romanek C. S. et al. (1994) Nature, 372, 655-657. [6] Nyquist L. E. et al. (2001) Sp. Sci. Rev., 96, 105-164. [7] Grady M. M. et al. (1997) JGR, E102, 9165-9173. [8] Goreva J. et al. (2002) LPS XXXIII, Abstract #2014. [9] Grady M. M. et al. (1997) MAPS, 32,863-868. [10] Des Marais D. J. (1983) GCA, 47, 1769-1781. [11] Hoefs J. (1987) Stable Isotope Geo-chemistry, Springer 3rd Ed. Lunar and Planetary Science XXXIV (2003) 1312.pdf

Carbon reservoirs on Mars: Constraints from Martian meteorites

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Carbon reservoirs on Mars: Constraints from Martian meteorites

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