Analysis of oxygen isotopes in water from Tagish Lake together with Mössbauer Spectroscopy suggest some similarities to the CI group of meteorites but also suggest differences in the extent of parent body hydrothermal alteration

Baker, Lee

Bland, P. A.

Franchi, Ian

Pillinger, Colin

Wright, Ian

English

Baker, L

Franchi, IA

Bland, PA

Wright, IP

Pillinger, CT

Spiral - Imperial College Digital Repository

Insight into the Evolution of the Tagish Lake Carbonaceous Chondrite by Analysis of the Oxygen Isotopic Composi-tion of Extracted Water and Mössbauer Spectroscopy.  L. Baker, I. A. Franchi, P. A. Bland, I. P. Wright and C. T. Pill-inger. Planetary and Space Science Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA,UK. (l.Baker@open.ac.uk).Introduction:  Elucidating the processes that haveoccurred on primitive meteorite parent bodies and un-derstanding the relationship between different classesof primitive meteorites has been dominated in recentyears by research into the processes of low tempera-ture alteration [1-3]. The results of such alteration areexhibited particularly clearly in the highly altered matrixmaterial in carbonaceous chondrites [4]. Tagish Lake, acarbonaceous chondrite consisting of over four hun-dred recovered fragments, was observed to fall in Brit-ish Colombia on Jan 18th 2000 [5]. Initial analyses of thismeteorite have resulted in its classification (basedmainly upon mineralogy, oxygen isotopes and bulkchemical composition) as being intermediate betweenCM and CI meteorites [5] and suggest that Tagish Lakemay be one of the most primitive Solar System materialsyet studied. We have measured the oxygen isotopiccomposition of water extracted from samples of TagishLake using a recently developed stepped pyroly-sis/fluorination system [6] and have found that theresulting yield and isotopic data more closely resem-bles patterns derived from CI meteorites than thosefrom CM samples. However, Mössbauer spectroscopysuggests that anhydrous silicates may be much moreabundant in Tagish Lake than in CI meteorites.Analytical Technique: Measurements of δ17O andδ18O were made on water released by pyrolysis ofcrushed, bulk meteorite samples of 1-6 mg, using a con-tinuous-flow, isotope ratio mass spectrometer (Delta C,Finnigan Mat). The samples used in this study werefrom a ~5g piece of pristine material which had beenmaintained at sub zero temperatures since initial impact.Sub-samples were removed while still frozen and wereloaded within one hour of warming to room tempera-ture. Water evolved from the samples was swept by aflow of purified helium directly onto the in-line fluori-nating agent (CoF3), releasing oxygen. Unwanted reac-tion products, mainly HF, were removed from the flowby chemical and cryogenic traps. After cryofocussing,the effluent passed through a capillary GC column forpurification before mass spectrometer analysis. Sam-ples were heated in a step-wise manner from room tem-perature to 800oC, with steps of either 33 or 50oC de-pending upon the anticipated yield. The heating dura-tion at each step was five minutes. The 1σ precision ofthe data, as determined by repeat analysis of standardwaters and an internal solid standard, are ±0.30 ‰(δ18O) and ±0.14 ‰ (∆17O). A Mössbauer spectrum wasrecorded from a sample of ~100mg sealed in a teflonbag at 298K [7].Results: The Mössbauer spectrum showed a num-ber of interesting features. We observe componentsconsistent with magnetite, anhydrous silicates, sul-phides and Fe-bearing clays. Given that Tagish Lakecontains 19.3wt% Fe [5], these absorptions may beconverted into wt% of iron bearing phases. Based onthis analysis, Tagish contains 8.4wt% magnetite, similarto CIs (Orgueil has 10.6wt%, Alais 7.7wt% magnetite[8]) and the CM-like meteorite Essebi (8.9wt% [8]), butmuch higher than other CMs, typically ~1wt%. Anhy-drous silicates are much more abundant in Tagish thanin CI meteorites, approaching CM values, whilst Fe-bearing clays are proportionately lower in abundancethan either CM or CI. Although the absorption isstrongly overlapped, sulphides appear to be signifi-cantly more abundant in Tagish compared with eitherCM or CI meteorites, in contrast to the findings of [5],suggesting that the bulk of the sulphides may be pres-ent as a fine-grained matrix phase.The water yield found during our stepped pyrolysisanalyses was 3.5-4 wt%, approximately half that of CMmeteorites and only about 1/3 of that from CI samples.The lower yield is in good agreement with the Möss-bauer data suggesting that a larger proportion of thematrix consists of anhydrous silicate material ratherthan hydrated silicates. While releasing a much smallerquantity of water than the CIs the release profile (seefigure) is otherwise similar to CI samples with twopeaks in yield at about 150-200oC and 550oC. CM mete-orites also exhibit 2 peaks in yield, however, the secondpeak is at the distinctly lower temperature of ~350oC.The range of δ18O values measured in Tagish Lake is fargreater than that found in either CM or CI samples,spanning about 27‰ and having a plateau value ataround +22‰. Despite the range, the majority of waterwas enriched in heavy isotopes and the bulk mean of+13.5‰ is approximately 10‰ heavier than for either CIor CMs [6].Discussion: The apparent similarities between theCI meteorites and Tagish Lake perhaps suggest a ge-netic link between the two. Typical CI samples producecomplex δ18O patterns suggesting multiple water reser-voirs. In contrast Tagish Lake produces a much simplerLunar and Planetary Science XXXII (2001) 1703.pdfEvolution of Tagish Lake.  L. Baker et al.pattern that may indicate mixing between two comp o-nents, one dominant at low temperature with a δ18Obetween –4 and 0‰ and a second, dominant at highertemperatures with a δ18O close to +22‰. The lack of acomplex δ18O release pattern may result from signifi-cantly less terrestrial weathering of the lower tempera-ture minerals. In Tagish Lake samples the second peakin yield is somewhat larger than in CIs while the firstpeak is noticeably smaller, perhaps again, reflecting therelatively short period of time available for terrestrialalteration. The greatest similarity between Tagish Lakeand the CI class of meteorites is in the ∆17O values.These were found to be very similar to those of CIsamples, with lower temperature steps lying close tothe terrestrial fractionation line and higher temperaturesteps rising to a poorly defined plateau of about +1‰(see figure, top). The presence of terrestrial ∆17O valuesfor steps up to ~250oC in CI samples had been pre-sumed to be the result of isotopic exchange of low tem-perature phases with terrestrial water. However, thepresence of this effect in Tagish Lake samples that hadremained frozen since impact suggests that valuesclose to the TF line may in fact be indigenous or thatexchange with terrestrial oxygen occurs very quicklyand extensively, possibly even at sub-zero tempera-tures.A significant difference between CIs and TagishLake is the offset in the ∆17O values of Tagish Lakewater and silicates. These display a large differenceapproaching 2‰ with high temperature water rising to+1‰, while the bulk silicate values are -0.7 to -1‰ [5].Models developed in recent years to account for thecomposition of silicates and fluids on the parent bodiesof these meteorites have assumed isotopic startingcompositions for both rock and fluid reservoirs. Fluidswere assumed to have large positive values (both for∆17O and δ18O) and pre-cursor silicates to have largenegative values for both parameters [1-3]. Subsequentmixing and hydrothermal activity within the nebula andas part of a parent body then produced the range ofisotopic values observed today. During reaction be-tween the two reservoirs extreme values were moder-ated and final isotopic compositions were largely con-trolled by the different proportions of oxygen presentin fluid and silicate reservoirs and how near to isotopicequilibrium the final mineral assemblage was. The par-ent material of CI meteorites appears to be have beendominated by fluids during alteration, with the finalassemblage approaching isotopic equilibrium. Thisresulted in the final silicate and water isotopic composi-tions possessing positive values of ∆17O separated byonly ~0.7‰. CM meteorites suggest that parent mate-rial was apparently further from isotopic equilibriumwith a difference of about 1.4‰ between the ∆17O val-ues of the silicate and water components. The relativelylarge negative ∆17O values retained by the silicates (~-2.5‰) suggests that rock was the dominant source ofoxygen during parent body hydrothermal activity.  Ifsuch models are to successfully account for the is o-topic composition of the silicate and water portions ofTagish lake then reactions between the two must havefallen far short of attaining equilibrium.References::[1] Clayton R.N. and Mayeda T.K. 1984. EPSL. 67,151-161. [2] Rowe M.W. et al. 1984. GCA. 58, 5341-5347.[3] Young E.D. et al. 1999. Science. 286. 1331-1335. [4]Bunch T.E. and Chang S. 1980. GCA. 44. 1543-1577. [5]Brown P.G. et al. 2000. Science 290. 320-325. [6] BakerL. et al. 1998. LPS XXIX. [7] Bland P.A. et al. 1999 Me-teoritics & Planet. Sci., 34, A12.  [8] Bland P.A. et al.2001, in prep.Figure 1. Water yield and oxygen isotopic data froma stepped pyrolysis experiment with a 4mg sample ofTagish Lake.0100200300400500600700800-4048121620240 100 200 300 400 500 600 700 800Tagish lake Yield H2O (ppm oC-1)δδ18O SMOW (‰)Temperature (oC)-1.5-1-0.500.511.50 10 20 30 40 50 60 70 80∆∆17O (‰)TF LineLunar and Planetary Science XXXII (2001) 1703.pdf

Insight into the evolution of the Tagish Lake carbonaceous chondrite by analysis of the oxygen isotopic composition of extracted water and Mössbauer spectroscopy.

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsInsight into the evolution of the Tagish Lakecarbonaceous chondrite by analysis of the oxygenisotopic composition of extracted water and Mo¨ssbauerspectroscopyConference or Workshop ItemHow to cite:Baker, Lee; Franchi, Ian; Bland, P. A.; Wright, Ian and Pillinger, Colin (2001). Insight into the evolution ofthe Tagish Lake carbonaceous chondrite by analysis of the oxygen isotopic composition of extracted water andMo¨ssbauer spectroscopy. In: 32nd Lunar and Planetary Science Conference, 12-16 Mar 2001, 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/lpsc2001/pdf/1703.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.ukInsight into the Evolution of the Tagish Lake Carbonaceous Chondrite by Analysis of the Oxygen Isotopic Composi-tion of Extracted Water and Mössbauer Spectroscopy.  L. Baker, I. A. Franchi, P. A. Bland, I. P. Wright and C. T. Pill-inger. Planetary and Space Science Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA,UK. (l.Baker@open.ac.uk).Introduction:  Elucidating the processes that haveoccurred on primitive meteorite parent bodies and un-derstanding the relationship between different classesof primitive meteorites has been dominated in recentyears by research into the processes of low tempera-ture alteration [1-3]. The results of such alteration areexhibited particularly clearly in the highly altered matrixmaterial in carbonaceous chondrites [4]. Tagish Lake, acarbonaceous chondrite consisting of over four hun-dred recovered fragments, was observed to fall in Brit-ish Colombia on Jan 18th 2000 [5]. Initial analyses of thismeteorite have resulted in its classification (basedmainly upon mineralogy, oxygen isotopes and bulkchemical composition) as being intermediate betweenCM and CI meteorites [5] and suggest that Tagish Lakemay be one of the most primitive Solar System materialsyet studied. We have measured the oxygen isotopiccomposition of water extracted from samples of TagishLake using a recently developed stepped pyroly-sis/fluorination system [6] and have found that theresulting yield and isotopic data more closely resem-bles patterns derived from CI meteorites than thosefrom CM samples. However, Mössbauer spectroscopysuggests that anhydrous silicates may be much moreabundant in Tagish Lake than in CI meteorites.Analytical Technique: Measurements of d17O andd18O were made on water released by pyrolysis ofcrushed, bulk meteorite samples of 1-6 mg, using a con-tinuous-flow, isotope ratio mass spectrometer (Delta C,Finnigan Mat). The samples used in this study werefrom a ~5g piece of pristine material which had beenmaintained at sub zero temperatures since initial impact.Sub-samples were removed while still frozen and wereloaded within one hour of warming to room tempera-ture. Water evolved from the samples was swept by aflow of purified helium directly onto the in-line fluori-nating agent (CoF3), releasing oxygen. Unwanted reac-tion products, mainly HF, were removed from the flowby chemical and cryogenic traps. After cryofocussing,the effluent passed through a capillary GC column forpurification before mass spectrometer analysis. Sam-ples were heated in a step-wise manner from room tem-perature to 800oC, with steps of either 33 or 50oC de-pending upon the anticipated yield. The heating dura-tion at each step was five minutes. The 1s precision ofthe data, as determined by repeat analysis of standardwaters and an internal solid standard, are ±0.30 ‰(d18O) and ±0.14 ‰ (D17O). A Mössbauer spectrum wasrecorded from a sample of ~100mg sealed in a teflonbag at 298K [7].Results: The Mössbauer spectrum showed a num-ber of interesting features. We observe componentsconsistent with magnetite, anhydrous silicates, sul-phides and Fe-bearing clays. Given that Tagish Lakecontains 19.3wt% Fe [5], these absorptions may beconverted into wt% of iron bearing phases. Based onthis analysis, Tagish contains 8.4wt% magnetite, similarto CIs (Orgueil has 10.6wt%, Alais 7.7wt% magnetite[8]) and the CM-like meteorite Essebi (8.9wt% [8]), butmuch higher than other CMs, typically ~1wt%. Anhy-drous silicates are much more abundant in Tagish thanin CI meteorites, approaching CM values, whilst Fe-bearing clays are proportionately lower in abundancethan either CM or CI. Although the absorption isstrongly overlapped, sulphides appear to be signifi-cantly more abundant in Tagish compared with eitherCM or CI meteorites, in contrast to the findings of [5],suggesting that the bulk of the sulphides may be pres-ent as a fine-grained matrix phase.The water yield found during our stepped pyrolysisanalyses was 3.5-4 wt%, approximately half that of CMmeteorites and only about 1/3 of that from CI samples.The lower yield is in good agreement with the Möss-bauer data suggesting that a larger proportion of thematrix consists of anhydrous silicate material ratherthan hydrated silicates. While releasing a much smallerquantity of water than the CIs the release profile (seefigure) is otherwise similar to CI samples with twopeaks in yield at about 150-200oC and 550oC. CM mete-orites also exhibit 2 peaks in yield, however, the secondpeak is at the distinctly lower temperature of ~350oC.The range of d18O values measured in Tagish Lake is fargreater than that found in either CM or CI samples,spanning about 27‰ and having a plateau value ataround +22‰. Despite the range, the majority of waterwas enriched in heavy isotopes and the bulk mean of+13.5‰ is approximately 10‰ heavier than for either CIor CMs [6].Discussion: The apparent similarities between theCI meteorites and Tagish Lake perhaps suggest a ge-netic link between the two. Typical CI samples producecomplex d18O patterns suggesting multiple water reser-voirs. In contrast Tagish Lake produces a much simplerLunar and Planetary Science XXXII (2001) 1703.pdfEvolution of Tagish Lake.  L. Baker et al.pattern that may indicate mixing between two comp o-nents, one dominant at low temperature with a d18Obetween –4 and 0‰ and a second, dominant at highertemperatures with a d18O close to +22‰. The lack of acomplex d18O release pattern may result from signifi-cantly less terrestrial weathering of the lower tempera-ture minerals. In Tagish Lake samples the second peakin yield is somewhat larger than in CIs while the firstpeak is noticeably smaller, perhaps again, reflecting therelatively short period of time available for terrestrialalteration. The greatest similarity between Tagish Lakeand the CI class of meteorites is in the D17O values.These were found to be very similar to those of CIsamples, with lower temperature steps lying close tothe terrestrial fractionation line and higher temperaturesteps rising to a poorly defined plateau of about +1‰(see figure, top). The presence of terrestrial D17O valuesfor steps up to ~250oC in CI samples had been pre-sumed to be the result of isotopic exchange of low tem-perature phases with terrestrial water. However, thepresence of this effect in Tagish Lake samples that hadremained frozen since impact suggests that valuesclose to the TF line may in fact be indigenous or thatexchange with terrestrial oxygen occurs very quicklyand extensively, possibly even at sub-zero tempera-tures.A significant difference between CIs and TagishLake is the offset in the D17O values of Tagish Lakewater and silicates. These display a large differenceapproaching 2‰ with high temperature water rising to+1‰, while the bulk silicate values are -0.7 to -1‰ [5].Models developed in recent years to account for thecomposition of silicates and fluids on the parent bodiesof these meteorites have assumed isotopic startingcompositions for both rock and fluid reservoirs. Fluidswere assumed to have large positive values (both forD17O and d18O) and pre-cursor silicates to have largenegative values for both parameters [1-3]. Subsequentmixing and hydrothermal activity within the nebula andas part of a parent body then produced the range ofisotopic values observed today. During reaction be-tween the two reservoirs extreme values were moder-ated and final isotopic compositions were largely con-trolled by the different proportions of oxygen presentin fluid and silicate reservoirs and how near to isotopicequilibrium the final mineral assemblage was. The par-ent material of CI meteorites appears to be have beendominated by fluids during alteration, with the finalassemblage approaching isotopic equilibrium. Thisresulted in the final silicate and water isotopic composi-tions possessing positive values of D17O separated byonly ~0.7‰. CM meteorites suggest that parent mate-rial was apparently further from isotopic equilibriumwith a difference of about 1.4‰ between the D17O val-ues of the silicate and water components. The relativelylarge negative D17O values retained by the silicates (~-2.5‰) suggests that rock was the dominant source ofoxygen during parent body hydrothermal activity.  Ifsuch models are to successfully account for the is o-topic composition of the silicate and water portions ofTagish lake then reactions between the two must havefallen far short of attaining equilibrium.References::[1] Clayton R.N. and Mayeda T.K. 1984. EPSL. 67,151-161. [2] Rowe M.W. et al. 1984. GCA. 58, 5341-5347.[3] Young E.D. et al. 1999. Science. 286. 1331-1335. [4]Bunch T.E. and Chang S. 1980. GCA. 44. 1543-1577. [5]Brown P.G. et al. 2000. Science 290. 320-325. [6] BakerL. et al. 1998. LPS XXIX. [7] Bland P.A. et al. 1999 Me-teoritics & Planet. Sci., 34, A12.  [8] Bland P.A. et al.2001, in prep.Figure 1. Water yield and oxygen isotopic data froma stepped pyrolysis experiment with a 4mg sample ofTagish Lake.0100200300400500600700800-4048121620240 100 200 300 400 500 600 700 800Tagish lake Yield H2O (ppm oC-1)dd18O SMOW (‰)Temperature (oC)-1.5-1-0.500.511.50 10 20 30 40 50 60 70 80DD17O (‰)TF LineLunar and Planetary Science XXXII (2001) 1703.pdf

Insight into the evolution of the Tagish Lake carbonaceous chondrite by analysis of the oxygen isotopic composition of extracted water and Mössbauer spectroscopy

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Insight into the evolution of the Tagish Lake carbonaceous chondrite by analysis of the oxygen isotopic composition of extracted water and Mössbauer spectroscopy

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