The oxygen isotopic composition of water from carbonaceous chondrites suggests that close to isotopic equilibrium was attained on CI and CM parent bodies. This is more consistent with a closed system than one with fluid flow. Tagish Lake does not display such equilibrium

Baker, L.

Franchi, Ian

Pillinger, Colin

Wright, Ian

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe oxygen isotopic composition of water extractedfrom carbonaceous chondritesConference or Workshop ItemHow to cite:Baker, L.; Franchi, Ian; Wright, Ian and Pillinger, Colin (2001). The oxygen isotopic composition of waterextracted from carbonaceous chondrites. In: Eleventh Annual V.M. Goldschmidt Conference, 20-24 May 2001, HotSprings, Virginia, 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/gold2001/pdf/3610.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.ukThe Oxygen Isotopic Composition of Water Extracted from Carbonaceous Chondrites.  L. Baker, I. A. Fran-chi, I. P. Wright and C. T. Pillinger. Planetary and Space Sciences Research Institute, The Open University, WaltonHall, Milton Keynes, MK7 6AA, UK. (l.Baker@open.ac.uk).Introduction: Some hypotheses regarding the originand movement of water in the Solar System as a wholeare based upon models which utilise the D/H ratio ofterrestrial and extra-terrestrial materials [e.g. 1,2].However, these have provided little insight into thesubsequent interaction that has taken place betweenwater reservoirs and the solid portions of primitiveSolar System material, e.g. meteorite parent bodies.Understanding the evolution of oxygen isotopic reser-voirs on meteorite parent bodies has the potential toprovide greater insight into the sources and effects ofSolar System water. Such insight has thus far beenhampered by the inherent difficulty of measuring theoxygen isotopic compositions of the necessarily smallquantities of water present in meteoritic samples. De-spite this, numerous attempts at modeling parent bodyevolution using measured silicate and carbonate com-positions [3-5] have allowed first order estimates of theoxygen isotopic composition of pre-cursor water. Nev-ertheless the requirement for direct measurements ofoxygen isotopes in water is undiminished. Using a re-cently developed stepped pyrolysis/fluorination system[6] we have measured the oxygen isotopic compositionof water extracted from a range of whole-rock samplesof CI and CM chondrites. The results of the analysesherein allow discrimination between contaminant waterof terrestrial origin, evolved at low temperature steps,with that of indigenous water released subsequentlyand provides new insight into the sources and interac-tion of water in meteorite parent bodies.Analytical Technique: Measurements of δ17O andδ18O were made on water released by pyrolysis ofcrushed, bulk meteorite samples of 1-4 mg. This waterwas swept by a flow of purified helium directly onto anin-line fluorinating agent (CoF3), releasing oxygen foranalysis using a continuous-flow, isotope ratio massspectrometer (Delta C, Finnigan Mat). For details see[6]. The 1σ precision of the data, as determined byrepeat analysis of standard waters and an internal solidstandard, are ±0.30 ‰ (δ18O) and ±0.14 ‰ (∆17O,where ∆17O=δ17O-0.52δ18O).Results: Water yields from all samples were be-tween 3.5 and 15 wt%. In all cases water released atlow temperatures had oxygen isotopic compositionscharacteristic of a terrestrial origin, with ∆17O valuesclose to 0‰. At temperatures above 350oC ∆17O valuesindicated that the released water had an extraterrestrialorigin. The peak release temperature of this indigenouscomponent occurs at a higher temperature in the CIsthan in the CMs. δ18O values in water from CM sam-ples displayed a relatively simple release pattern whilstthose from CI samples produced a more complex pat-tern, indicative of multiple reservoirs. ∆17O valuesfrom CI samples (after removal of terrestrial water)were positive (~+0.6‰), possessing values close tothose measured in bulk samples and matrix. ∆17O val-ues from CM samples were, like the CM matrix andsilicate, negative (-0.6 to-1.3‰). Water from theTagish Lake meteorite shows similarities to bothgroups but does not comfortably fit into either. It has arelatively low water content and displays a simple δ18Opattern over a wide range of values. Its ∆17O values aremuch more positive than whole rock and other solidcomponents.Discussion. The geochemical effects of extrater-restrial water are exhibited particularly clearly in thehighly altered matrix material found in carbonaceouschondrites [3]. Phases such as carbonates and phyllo-silicates retain a record of the isotopic composition ofthe fluids within which they formed. Comparison ofisotopic data from water originating from phyllosili-cates with oxygen isotopic data from carbonate phases[7] suggests that for each of the CI and CM samples thetwo groups of minerals lie close to a single mass frac-tionation line. Unless deposition of carbonate and for-mation of phyllosilicates occurred simultaneously thisis more compatible with a closed system than one in-volving fluid flow in which the isotopic composition ofthe liquid continually changes with time. In the CMmeteorite Cold Bokkeveld this line displays a largernegative ∆17O value than either Murchison or Murrayindicating either a greater level of equilibration be-tween fluid and anhydrous silicate (consistent with themineral alteration index of [8]) or a slightly lower wa-ter/rock ratio during reaction. This assumes that sili-cates initially had large negative ∆17O values (~-5.2‰)[3] and water possessed large positive ∆17O val-ues (>+2‰). Carbonates and water from Tagish lakedo not show a consistent ∆17O value, implying a lack ofequilibrium and possibly fluid flow during alteration.References: [1] Drouart et al. (1999) Icarus 140,129-155. [2] Robert et al. (2000) Space Sci. Rev. 92201-224. [3] Clayton and Mayeda (1984) EPSL. 67,151-161. [4] Rowe M.W. et al. (1994). GCA. 58, 5341-5347. [5] Young E.D. et al. (1999). Science. 286.1331-1335. [6] Baker L. et al. (1998). LPSC XXIX. [7]Benedix et al. (2000) LPSC XXXI. [8] Browning et al.(1996) GCA 60, 2621-2633.Eleventh Annual V. M. Goldschmidt Conference  (2001) 3610.pdf

The oxygen isotopic composition of water extracted from carbonaceous chondrites

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The oxygen isotopic composition of water extracted from carbonaceous chondrites

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