Nakhla and Lafayette delta 7Li values for samples and extracts (4.1-14.2ï¿½) are consistent with brine evaporation. Relatively 13C-poor siderite in Lafayette suggests more than one carbon source was sampled

Baker, L.

Bridges, J. C.

James, R. H.

Pearson, V. K.

Verchovsky, A. B.

Wright, I. P.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsLithium and carbon isotopic fractionations between thealteration assemblages of Nakhla and LafayetteConference or Workshop ItemHow to cite:Bridges, J. C.; James, R. H.; Pearson, V. K.; Baker, L.; Verchovsky, A. B. and Wright, I. P. (2005). Lithiumand carbon isotopic fractionations between the alteration assemblages of Nakhla and Lafayette. In: 36th Lunar andPlanetary 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/1758.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.ukLITHIUM AND CARBON ISOTOPIC FRACTIONATIONS BETWEEN THE ALTERATION ASSEMBLAGES OF NAKHLA AND LAFAYETTE.  J. C. Bridges1, R. H. James2, V. K. Pearson1,  L. Baker1, A. B. Verchovsky1 and I. P. Wright1 1PSSRI, Open University, Milton Keynes MK7 6AA, UK j.bridges@open.ac.uk. 2Dept. of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK.     The nakhlites experienced a fluid alteration event on Mars leading to the rapid crystallization at low tem-perature (e.g. 25-150oC) of siderite, clay and other salts in fractures within olivine and interstitial sites [1-2].  In order to help constrain the origin and evolution of the brine we present new Li-isotopic, C and Thermal Gas Analysis data for Nakhla and Lafayette samples.   Techniques: Whole rock samples of Nakhla (BM 1911, 369) and Lafayette (BM 1959, 755) were pow-dered using an agate mortar and pestle.  We also used Nakhla pyroxene separates (>95% pure) and olivine-enriched samples of lower purity.   Samples for Li-isotope analysis were dissolved in HF/HNO3, and passed through a cation exchange col-umn to separate Li [3]. Li-isotope ratios were measured using a Nu instruments multicollector ICP-MS [4]. A sub-sample of Nakhla was subjected to sequential leaching procedure: the sample was ultrasonicated in water to dissolve soluble salts, and the residue was then leached with weak (1N) acetic acid. The residue from this procedure was dissolved in HF/HNO3.  Li isotope ratios are reported as δ7Li, where  δ7Li ‰={[(7Li/6Li)SAMPLE/(7Li/6Li)STANDARD] -1}*1000.   Carbon yield and isotopes of whole samples of Nakhla (7.85 mg) and Lafayette (6.63 mg) were ana-lysed by stepped-combustion mass spectrometry [5].  Samples were heated to 1300ºC in 100ºC steps from 0-500ºC and 1000-1300ºC.  Heating between 500-1000ºC was in 50ºC steps.  The same procedure was applied to Nakhla olivine-rich (10.44 mg) and pyrox-ene (10.55 mg) samples.  Lafayette was heated in 50ºC steps from 300-500ºC and 25ºC steps from 500-700ºC.  In order to remove traces of carbonates in the pyroxene sample an aliquot (9.53 mg), was washed in 0.1M HCl prior to combustion. Powders of both meteorites were pyrolysed using a combined thermogravimetric and evolved gas analysis system in a He carrier gas from 20-1000oC, with CO2 release monitored. Results:  Li-Isotopes.  Table 1. shows our results which include the first bulk δ7Li values for martian meteorites.  The δ7Li value for bulk Nakhla is within error of that for bulk Lafayette, and similar to those of terrestrial ultramafic rocks [6].   The Nakhla water-leach (+14.2 ‰) reflects the δ7Li of soluble salts known to be in this meteorite.  However, the Nakhla acid-leach has a similar composition to the igneous phases implying that the siderite does not contain frac-tionated Li-isotopes.  The detectable fractionation be-tween Nakhla pyroxene and olivine-rich samples (0.5‰) suggests that the olivine contains a 7Li-rich alteration phase, which may be the nontronite-like clay present in cracks e.g. [1] or intramineral fractionation [6].    Table 1.  Li-isotopes in Nakhlite Samples  δ7Li ‰ 1σ error Nakhla bulk 4.5 0.2 Nakhla px 3.7 0.3 Nakhla ol-rich 5.1 0.3 Lafayette bulk 4.1 0.3 Nakhla water leach 14.2 0.5 Nakhla acid leach 3.7 0.5 Nakhla bulk (after leaches) 3.8 0.2 δ7Li is reported relative to the NIST L-SVEC standard [7].    Carbon.  Our results (Fig. 1a-c) show the bulk of carbon released <400oC, which in addition to terrestrial contamination may include some martian organics [8,9].  The δ13C excursions at ~450oC mark the com-bustion of siderite [10]: with Lafayette wr showing δ13C = -12 ‰ at this point and the Nakhla olivine-rich sample δ13C = +22‰.  This suggests a difference of  approximately 35‰, between the siderite compositions within the Nakhla olivine (which contains siderite along veins) and Lafayette (wr) but exact differences cannot be determined without Lafayette mineral sepa-rates.  However Jull et al. [11] have reported δ13C val-ues from acid-extracts of Nakhla of up to +50‰ that was thought to be carbonate and similarly Wright et al. [10] reported an acid extract in Lafayette of +1‰.   Acid-washed pyroxene from Nakhla (not plotted here) contains low C yields (<6 ppm C per temperature step) indicating the removal of almost all terrestrial and martian C-bearing phases. Pyrolysis. CO2 is released from both meteorites be-tween 175-600oC. Variations in release temperature may reflect textural and chemical differences between the siderite of the 2 meteorites. Organic components will also have been pyrolysed within this range but were indistinguishable with the sample sizes available.     Discussion:  Lafayette siderite has 22-27 mol% CaCO3 whereas Nakhla is 0.1-6 mol% [1,2].  This re-flects the passage of a brine through fractures in the nakhlite parent rocks, with Lafayette near the heat and fluid source and Nakhla closer to the surface with final Lunar and Planetary Science XXXVI (2005) 1758.pdfevaporation products (e.g. less Ca-rich siderite and more soluble salts) of the brine.  This relationship be-tween the 2 meteorites provides the context for inter-preting the Li-and C-isotopic fractionations. In a low-T, rapidly precipitated system such as found in the nakhlite secondary assemblage we assume that it can be considered a closed system for Li because it is unlikely that multiple crustal sources could easily exchange with the fluid.  Variation in the Li-isotope ratios of different components thus reflect Rayleigh fractionation.  Taking Nakhla pyroxene as the starting crustal composition and a range of likely isotopic frac-tionation factors for Li:  α (mineral-fluid) = 0.997–0.990 [12] With 90% brine evaporation (f=0.1), minerals with δ7Li of +11 to +27 ‰ would be precipitated across the range of likely α values.  For f=0.5, δ7Li = +6 to +11 ‰.  The f=0.1 values are consistent with the Nakhla water-leach composition and the presence of late-stage, water-soluble crystallisation products from the evapo-rating brine that are present in Nakhla but not found in Lafayette [1].   Although the mineral compositions and Li-isotopes may be modelled as the result of Rayleigh fractionation associated with progressive evaporation such a mecha-nism is inconsistent with an increase in δ13C values from Lafayette to Nakhla [13].  This difference might simply reflect a drop in brine T from Lafayette to Nak-hla (e.g. 150 to 25oC) within a system open to CO2 exchange between atmosphere and brine.  Carbonate compositions in equilibrium with an atmosphere of δ13C = +35‰ [14] would be +37‰ at 150oC and +45 ‰ at 25oC [15].  Clearly this does not explain the mag-nitude of the actual δ13C isotopic differences between the meteorites and the apparent lack of such 13C-rich siderite in Lafayette.  We have assumed that the source of the carbon is predominantly atmospheric, either through direct exchange of CO2(g) with the brine or through dissolution of pre-existing evaporitic deposits. However in order to explain the presumed light C-isotopic compositions of the Lafayette siderite it may be necessary to invoke the incorporation and mixing of other less 13C-rich material.  One possible source could be organics carried in the fluid although we have not been able to characterize it isotopically or by pyrolysis. Conclusions:  Mineral compositions and Li-isotopic values are consistent with the progressive frac-tionation of a brine from Lafayette to Nakhla.  Lafay-ette was nearest the fluid and heat source, which may have been impact, associated with fracturing in the nakhlites.  C-isotopic compositions were partially in-fluenced by falling T in an open CO2 (13C-rich atmos-phere)-H2O system but the relatively 13C-poor siderite in Lafayette suggests more than one carbon source was sampled.   References: [1] Bridges J. C. and Grady M. M. (2000) EPSL, 176, 267-279. [2] Bridges J. C. et al. (2001) Space Science Reviews, 96, 365-392.  [3] James R. H. and Palmer M. R. (2000) Chem. Geol., 166, 319-326.  [4] Kisakurek B. et al. (2004) Chem. Geol., 212, 27-44  [5] Wright I. P. and Pillinger C. T. (1989) USGS Bull., 1890, 9-34.  [6] Seitz H-M. et al. (2004) Chem. Geol., 212, 163-177. [7] Flesch G. D. et al. (1973) Int. J. Mass Spectrom. Ion Process, 12, 265-272. [8] Jull A. J. T. et al. (2000) GCA, 64, 3763-3772.  [9] Sephton M. A. et al. (2004) Astrophysical Journal, 612, 588-591.  [10] Wright I. P. et al. (1992) GCA, 56, 817-826.  [11] Jull A. J. T. et al. (1995) Meteoritics, 30, 311-318.  [12] Rudnick R. L. et al. (2004) Chem. Geol., 212, 45–57.  [13] Romanek C. S. et al. (1994) Nature, 372, 655-657.  [14] Carr R. H. et al. (1985) Nature, 314, 248-250.  [15] Chacko T. et al. (1991) GCA, 55, 2867-2882.        Fig. 1.  Stepped Combustion Carbon Analyses.    Lunar and Planetary Science XXXVI (2005) 1758.pdf

Lithium and carbon isotopic fractionations between the alteration assemblages of Nakhla and Lafayette

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Lithium and carbon isotopic fractionations between the alteration assemblages of Nakhla and Lafayette

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