Lunar and Planetary Science XXXI 1717.pdf OXYGEN THREE-ISOTOPE MEASUREMENTS ON THE FIRST ARTIFICIAL METEORITES (THE ESA ‘STONES’ EXPERIMENT): CONTRASTING BEHAVIOUR OF DOLOMITE AND A SIMULATED MARTIAN SOIL. M. F. Miller1, A. Brack 2, P. Baglioni3, R. Demets3, I. A. Franchi1, G. Kurat4 and C. T. Pill- inger1, 1Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom (e-mail correspondence: [email protected]), 2Centre de Biophysique Moleculaire, CNRS, Rue Charles Sadron 45071 Orléans cedex 2, France. (e-mail correspondence: [email protected]), 3ESA Director- ate of Manned Spaceflight and Microgravity, ESTEC, Postbus 299, NL-2200 AG Noordwijk, The Netherlands, 4Naturhistorisches Museum, Postfach 417, A-1014 Wien, Austria. Introduction: The ‘Stones’ experiment is an cemented by gypsum (mixing proportion 80:20). The autonomous investigation of the Microgravity space samples were attached to the heat shield of Foton-12, flight program of the European Space Agency (ESA). which was launched from Pletsetsk (Russia) on 9th It is led by Dr A Brack (Principal Investigator) and is September 1999 and flew for 16 days before re-entry. designed to assess physical, chemical and isotopic Nominal orbital parameters were: perigee 220km; apo- modifications experienced by meteorites during their gee 380km; inclination 62.8°. The landing site was passage through the Earth’s atmosphere, by measure- near the Kazakhstan/Russian border. Unfortunately, the ments made on geological samples attached to the heat basalt sample was lost during flight. However, the shield of a recoverable Foton space capsule. Such arti- dolomite and ‘Martian soil analog’ survived and were ficial meteorites offer a unique means of identifying successfully recovered. changes that accompany atmospheric entry, by compar- Oxygen isotopes: We report here some prelimi- ing well-defined materials with original (control) non- nary results of oxygen three-isotope measurements on flown samples. The principle objective is to address the recovered dolomite and ‘Martian soil analog’ sam- the problem of why, amongst the 14 examples of SNC ples, together with comparable data on the (post-flight) meteorites so far identified, none are of a sedimentary Foton-12 heat shield and the respective control sam- rock type (or, indeed, appear to originate from the sur- ples. For reference purposes, oxygen isotopic data on face of the parent body), despite evidence that the most the distinctive fusion crust and interior of an LL6 or- probable parent body (Mars) was characterized by a dinary chondrite fall (Appley Bridge meteorite) are also warm and wet climate earlier in its history. By con- presented. Measurements were undertaken at the Open trast, most lunar meteorites identified to date are rego- University, using infra-red laser-assisted fluorination lith samples, in accord with their ejection from the (by BrF5) of 1-2 mg of sample. All samples were laser lunar surface by impact. heated to fusion, in vacuo, prior to fluorination. After 17 In the case of Mars, the present-day surface is cov- purification, the resulting O 2 was analyzed for d O and ered both by impact-generated regolith and by sedi- d18O using a dual inlet mass spectrometer of high reso- mentary rocks; the latter presumably deposited by lution (DM/M~250) and abundance sensitivity. This flowing or still water. Thus, the presence of detrital system provides dual oxygen isotope ratio measure- deposits and evaporites would be expected. Further- ments to very high precision, such that the offset be- more, groundwater might be expected to compact loose tween different mass-dependent fractionation lines is sediments and regolith by filling pore space with <±0.02‰. Full details of the experimental procedure evaporitic minerals. Such sedimentary hard rocks are given by Miller et al. [1]. should be among the Martian meteorites, but none The development of fusion crusts on meteorites have yet been identified. It is postulated that consoli- during their passage through the Earth’s atmosphere is dated sedimentary rocks may have survived ejection universally recognized, but fusion crusts and their im- from the Martian surface, but did not survive terrestrial plications for the atmospheric reprocessing of extrater- atmospheric entry because of decrepitation of the ce- restrial materials have not been extensively studied [2]. menting matrix (most probably a sulfate). Further- In particular, isotopic effects resulting from reaction more, carbonate-rich sedimentary rocks may never have with atmospheric do not appear to have been docu- developed a fusion crust during terrestrial atmospheric mented hitherto. It has recently been shown [3], how- entry: thermal decrepitation of carbonates is likely to ever, that atmospheric O2 is characterized by a mass- produce a surface texture that is not recognizable as a independently fractionated component, resulting from fusion crust. stratospheric photochemical processes and involving Samples and flight details: To address these is- O2, O3 and CO2. Of particular interest, therefore, is to sues, the ‘Stones’ experiment involved the attachment assess the extent to which a terrestrial atmospheric of three different rock types to the heat shield of a re- oxygen signal may be identified in the flown ‘Stone’ coverable Foton capsule. These were: a basalt; a dolo- samples and in the fusion crust of the LL6 ordinary mite (fine-grained, sedimentary Ca-Mg carbonate, with chondrite sample, analyzed for comparison. minor quartz) and a ‘synthetic Martian soil’. The latter In terrestrial rocks and minerals, oxygen isotope consisted of comminuted basalt (<5mm grain-size) fractionations 17O/16O and 18O/16O follow a mass- Lunar and Planetary Science XXXI 1717.pdf OXYGEN ISOTOPE MEASUREMENTS ON ARTIFICIAL METEORITES: M. F. Miller, A. Brack, P. Baglioni, R. Demets, I. A. Franchi, G. Kurat and C. T. Pillinger dependent relationship given by: ‘Stones’ - Dolomite sample. It was observed that a 17 16 17 16 l Rs/ Rr = ( Rs/ Rr) significant proportion of the leading face of the post- where xR refers to xO/16O, subscripts s and r represent flight dolostone had apparently been ablated away. sample and reference materials, respectively, and Furthermore, this surface was white in color, whereas a l » 0.52 [4,5]. In terms of the delta notation, in which cross-section through the sample revealed distinct oxygen isotope values are conventionally reported, the stratification into a ‘grey’ zone (interior) which, in above expression is usually approximated to: turn, preceded a darkly-colored section, located adja- 17 18 cent to the heat shield. For present purposes, duplicate d O = 0.52 . d O sub-samples were taken from all three zones. It should Departures from the terrestrial mass-dependent frac- 17 17 be noted that direct measurement of d O in carbonates tionation line are thence quantified in terms of D O, is not feasible using laser-assisted fluorination proce- which is defined as d17O - 0.52 .d17O [6]. dures; CO2 is evolved during sample heating and its Results: 17 associated d O value cannot be determined without the Reference materials. For 11 analyses of isotopic complexity of converting the CO2 into O2. Therefore, reference materials (consisting of NBS-28 and an ob- 17 d O measurements are undertaken on the residual ox- sidian, secondary standard), undertaken at the same 17 ide, formed by extended heating of the carbonate, and time as the ‘Stones’ samples, a mean D O value of assuming that the carbonate-to-oxide conversion con- 0.026 ± 0.032 ‰ (1s ) was obtained, on the basis of the forms to mass-dependent fractionation behavior. In the ‘exact’ mass-dependent fractionation relationship be- 17 18 case of the ‘Stones’ dolomite sample, significant con- tween d O and d O, and using a slope value (l ) of version to CaO and MgO will have occurred during the 0.5248, as measured for silica samples at the Open 17 heating that accompanied re-entry of the capsule University. The D O of atmospheric O2 was measured through the Earth’s atmosphere. using tank oxygen gas: five replicate analyses gave a Isotopic measurements revealed that the ‘white’ value of -0.323 ± 0.032 ‰ (1s ). For the atmospheric 18 18 17 (leading edge) was depleted in O (d O 30.0 ± 1.5 ‰) oxygen O calculations, the same value was used 18 D l relative to the other two zones (d O 35.7 ± 0.2 ‰). as for the terrestrial silicates. Interestingly, all zones displayed isotopic exchange ‘Stones’ - ‘Martian soil analog’. Basalt grains and with atmospheric oxygen, as evidenced by a mean gypsum from the control (non-flight) samples pre- 17 17 D O value of -0.241 ± 0.075 ‰ (1s ). Note that this dictably gave D O values of essentially zero. (Basalt value is comparable to that observed for the silica crust gave a value of 0.058‰; duplicates of the gypsum formed on the sample holder. cement gave -0.009 and -0.022‰ respectively.) A very Appley Bridge meteorite. A mean difference of limited quantity of post-flight material was available, 17 0.214‰ was found between the respective D O values consisting of a few small grains and a fine powder, of the distinctive fusion crust (1.151 ± 0.053‰, 1s which made it virtually impossible to assess whether 18 precision, four replicates) and interior (1.348 ‰, any fusion crust had formed. Results of d O meas- 1.382 ‰; duplicate measurements) of this meteorite. urement confirmed that the coarser grains consisted 18 Isotopic exchange between the fusion crust and atmos- primarily of the basalt (initial d O ~ 5.8‰) whereas the pheric oxygen during infall is implicated. fine-grained material consisted largely of the gypsum 18 17 Conclusions: The preliminary findings presented matrix (initial d O ~ 8.2‰). No change in D O from here show that, whereas a meteorite consisting primar- zero was detectable in either fraction, however, within ily of carbonate minerals would not be expected to the limits of experimental precision.