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[PPS06-03] Redox conditions during aqueous alteration of CM chondrites: A combined study of Fe-Kα XANES and Fe Mössbauer spectroscopy
Keywords:XANES, Mössbauer spectroscopy, CM chondrites
Introduction: In CM chondrites, anhydrous phases such as olivine and pyroxene, Fe-Ni metal, Fe-Ni sulfide, and chondrules glass are replaced by hydrous phases, serpentine and tochilinite (e.g. [1] and references therein). Temperatures during aqueous alteration have been estimated to be from ~0oC to <170 oC [2]. However, compared with temperatures, redox conditions during aqueous alteration have not been understood well. Fe3+/ΣFe ratio of cronstedtite (Fe2+3Fe3+2(Fe3+2Si2)O10(OH)8 in matrix and chondrule rims were measured by TEM-EELS [3]. Two Fe Kα XANES studies measured Fe3+/ΣFe ratios of matrices and serpentine in CM chondrites, respectively [4, 5]. We performed a combined study of Fe Kα XANES and 57Fe Mössbauer spectroscopy to estimate Fe3+/ΣFe ratios of serpentine in CM chondrites more quantitatively.
Samples and methods: We used cronstedtite from two different localities (Aude, France and Bohemia, Czech). 57Fe Mössbauer spectroscopy was measured in a room temperature by using 370 MBq 57Co in Pd as a source at Shimane University. Forty and 200 mg aliquots of the samples were used as absorbers. Thin sections of Mighei, Bells, Murray, El Quss Ab Said, Nogoya, Sayama, LEW85311, Murchison, Y75293, Essebi, Cold Bokkeveld, and Y82042 were used for this study. To determine the degrees of aqueous alteration according to [6], we observed texture and mineralogy by using both an optical microscope and SEM at Ibaraki University. Chemical compositions of the serpentine-group minerals in chondrules were measured by electron microprobe at National Institute of Polar Research. Fe Kα XANES was measured at Beamline 4A, KEK-PF.
Results and discussion: It has been expected that Mg content in phyllosilicates increases as aqueous alteration proceeds [7, 8]. The average Fe*/(Mg+Fe*) ratio of serpentine in Sayama CM 2.1 was 0.35 and that of LEW 85311 CM 2.1 was 0.73 (Fe* =Fe2+ + Fe3+). However, it is hard to say that average Fe*/(Mg+Fe*) ratios of serpentine correlate positively with the degrees of aqueous alteration by considering large 1σ of each average value. We calculated Fe3+/ΣFe ratios of serpentine based on a combination of Fe Kα XANES and 57Fe Mössbauer spectroscopy. The average values of Fe3+/ΣFe of serpentine increase from 0.57 in LEW 85331 CM 2.1 to 0.71 in Sayama CM 2.6. However, similarly these variations may be statistically insignificant because the average values overlap by considering 1σ of these values. Ratios between integrated and area intensities of (Fe3+/ΣFe) in each meteorite do not show remarkable differences by considering 1σ. These results indicate that the redox state of aqueous alteration in CM chondrite parent body did not change considerably. Because standard deviation of Fe3+/ΣFe ratios within a meteorite is large, it is suggested that redox states may have been locally heterogeneous. Comparison between Fe*/(Mg + Fe*) and (Fe3+ + Al) / [(Fe3+ + Al) + (Si + Mg + Fe2+)] ratios revealed that Fe3+ and Al3+ cations in serpentine that replaced mesostasis of chondrules decreased as aqueous alteration proceeded. In contrast, Fe2+ and Mg2+ in the serpentine increased. This result supports the reaction of replacing elements proposed by [8]. Fe2+ and Mg2+ were probably supplied from phenocrysts in chondrules that replaced by serpentine. Fe3+ and Al3+ cations expelled from serpentine that replaced mesostasis were probably incorporated in abundant serpentine that were formed by replacement of phenocrysts as aqueous alteration proceeded.
References: [1] Brearley A. D., Jones R. (1998) RM 38, Chap. 3. [2] Keil K. (2000) PSS 48, 887-903. [3] Zega T. J. (2003) AM 88, 1169-1172. [4] Beck P. et al. (2012) GCA 99, 305-316. [5] Mikouchi T. et al. (2012) LPSC 43 #1496. [6] Rubin A. et al. (2008) GCA 71, 2361-2382. [7] Zolensky M. E. et al. (1989) Icarus 78, 411-425. [8] Browning L. B. et al. (1996) GCA 60, 2621-2633.
Samples and methods: We used cronstedtite from two different localities (Aude, France and Bohemia, Czech). 57Fe Mössbauer spectroscopy was measured in a room temperature by using 370 MBq 57Co in Pd as a source at Shimane University. Forty and 200 mg aliquots of the samples were used as absorbers. Thin sections of Mighei, Bells, Murray, El Quss Ab Said, Nogoya, Sayama, LEW85311, Murchison, Y75293, Essebi, Cold Bokkeveld, and Y82042 were used for this study. To determine the degrees of aqueous alteration according to [6], we observed texture and mineralogy by using both an optical microscope and SEM at Ibaraki University. Chemical compositions of the serpentine-group minerals in chondrules were measured by electron microprobe at National Institute of Polar Research. Fe Kα XANES was measured at Beamline 4A, KEK-PF.
Results and discussion: It has been expected that Mg content in phyllosilicates increases as aqueous alteration proceeds [7, 8]. The average Fe*/(Mg+Fe*) ratio of serpentine in Sayama CM 2.1 was 0.35 and that of LEW 85311 CM 2.1 was 0.73 (Fe* =Fe2+ + Fe3+). However, it is hard to say that average Fe*/(Mg+Fe*) ratios of serpentine correlate positively with the degrees of aqueous alteration by considering large 1σ of each average value. We calculated Fe3+/ΣFe ratios of serpentine based on a combination of Fe Kα XANES and 57Fe Mössbauer spectroscopy. The average values of Fe3+/ΣFe of serpentine increase from 0.57 in LEW 85331 CM 2.1 to 0.71 in Sayama CM 2.6. However, similarly these variations may be statistically insignificant because the average values overlap by considering 1σ of these values. Ratios between integrated and area intensities of (Fe3+/ΣFe) in each meteorite do not show remarkable differences by considering 1σ. These results indicate that the redox state of aqueous alteration in CM chondrite parent body did not change considerably. Because standard deviation of Fe3+/ΣFe ratios within a meteorite is large, it is suggested that redox states may have been locally heterogeneous. Comparison between Fe*/(Mg + Fe*) and (Fe3+ + Al) / [(Fe3+ + Al) + (Si + Mg + Fe2+)] ratios revealed that Fe3+ and Al3+ cations in serpentine that replaced mesostasis of chondrules decreased as aqueous alteration proceeded. In contrast, Fe2+ and Mg2+ in the serpentine increased. This result supports the reaction of replacing elements proposed by [8]. Fe2+ and Mg2+ were probably supplied from phenocrysts in chondrules that replaced by serpentine. Fe3+ and Al3+ cations expelled from serpentine that replaced mesostasis were probably incorporated in abundant serpentine that were formed by replacement of phenocrysts as aqueous alteration proceeded.
References: [1] Brearley A. D., Jones R. (1998) RM 38, Chap. 3. [2] Keil K. (2000) PSS 48, 887-903. [3] Zega T. J. (2003) AM 88, 1169-1172. [4] Beck P. et al. (2012) GCA 99, 305-316. [5] Mikouchi T. et al. (2012) LPSC 43 #1496. [6] Rubin A. et al. (2008) GCA 71, 2361-2382. [7] Zolensky M. E. et al. (1989) Icarus 78, 411-425. [8] Browning L. B. et al. (1996) GCA 60, 2621-2633.