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Title: The Valence of Titanium in Refractory Forsterite

Abstract

Models of melts in equilibrium with refractory forsterite (RF) suggest that RF contains trivalent Ti (Pack et al., 2005), with major implications for the origin of RF. We tested this prediction, measuring Ti valence by XANES in RF from Tagish Lake. One of the more intriguing components of unequilibrated chondrites is the population of relatively Ca-, Al-rich, Fe-poor isolated olivine grains known as 'refractory forsterite'. Some workers have concluded that these grains formed in and were released from chondrules. Other workers, noting that these grains occur in all major types of unequilibrated chondrites yet have many features in common, favor a single, pre-chondrule source from which the grains were distributed into their present hosts. Thought by some to be condensates, refractory forsterite (RF) grains have been found in chondrules, but their high CaO contents and trends of decreasing CaO contents from core to rim are inconsistent with closed-system fractional crystallization, so those grains must be relict. Previous workers calculated the compositions of melts parental to RF from the minor and trace element contents of RF grains and known olivine/liquid partition coefficients. Results indicated that melts in equilibrium with RF would have been uniformly enriched in refractory lithophile elements by {approx}20more » x CI, as is observed in many refractory inclusions. Two exceptions are V, for which enrichments <20 x CI were derived, and Ti, for which enrichments of 50-65 x CI were obtained. For V, the depletion was attributed to its enhanced volatility under reducing conditions. The inferred Ti content is higher than observed in refractory inclusions. This rather unreasonable result was obtained using the olivine/liquid partition coefficient for Ti{sup 4+} (D{sub Ti{sup 4+}}). A higher D for Ti would allow lower Ti contents in parental melts, and noted that the larger ionic radius of Ti{sup 3+} relative to that of Ti{sup 4+} could yield a D{sub Ti{sup 3+}} that is a factor of {approx}50 greater than D{sub Ti{sup 4+}}. The presence of significant proportions of Ti{sup 3+} in RF would provide important evidence of reducing conditions where the grains formed, support the model of [8] and rule out formation in typical, FeO-bearing chondrules. To test the prediction of [8], we have used X-ray absorption near-edge structure (XANES) analysis to determine the valence of Ti in two RF grains from the Tagish Lake carbonaceous chondrite.« less

Authors:
; ; ;  [1]
  1. (UofC)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
USDOE
OSTI Identifier:
1009015
Resource Type:
Conference
Resource Relation:
Conference: Lunar and Planetary Science XXXVIII;March 12-16, 2007;Houston, Texas
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 58 GEOSCIENCES; REFRACTORIES; CHONDRITES; OLIVINE; ORIGIN; TITANIUM; VALENCE

Citation Formats

Simon, S.B., Sutton, S., Newville, M., and Grossman, L.. The Valence of Titanium in Refractory Forsterite. United States: N. p., 2007. Web.
Simon, S.B., Sutton, S., Newville, M., & Grossman, L.. The Valence of Titanium in Refractory Forsterite. United States.
Simon, S.B., Sutton, S., Newville, M., and Grossman, L.. Tue . "The Valence of Titanium in Refractory Forsterite". United States. doi:.
@article{osti_1009015,
title = {The Valence of Titanium in Refractory Forsterite},
author = {Simon, S.B. and Sutton, S. and Newville, M. and Grossman, L.},
abstractNote = {Models of melts in equilibrium with refractory forsterite (RF) suggest that RF contains trivalent Ti (Pack et al., 2005), with major implications for the origin of RF. We tested this prediction, measuring Ti valence by XANES in RF from Tagish Lake. One of the more intriguing components of unequilibrated chondrites is the population of relatively Ca-, Al-rich, Fe-poor isolated olivine grains known as 'refractory forsterite'. Some workers have concluded that these grains formed in and were released from chondrules. Other workers, noting that these grains occur in all major types of unequilibrated chondrites yet have many features in common, favor a single, pre-chondrule source from which the grains were distributed into their present hosts. Thought by some to be condensates, refractory forsterite (RF) grains have been found in chondrules, but their high CaO contents and trends of decreasing CaO contents from core to rim are inconsistent with closed-system fractional crystallization, so those grains must be relict. Previous workers calculated the compositions of melts parental to RF from the minor and trace element contents of RF grains and known olivine/liquid partition coefficients. Results indicated that melts in equilibrium with RF would have been uniformly enriched in refractory lithophile elements by {approx}20 x CI, as is observed in many refractory inclusions. Two exceptions are V, for which enrichments <20 x CI were derived, and Ti, for which enrichments of 50-65 x CI were obtained. For V, the depletion was attributed to its enhanced volatility under reducing conditions. The inferred Ti content is higher than observed in refractory inclusions. This rather unreasonable result was obtained using the olivine/liquid partition coefficient for Ti{sup 4+} (D{sub Ti{sup 4+}}). A higher D for Ti would allow lower Ti contents in parental melts, and noted that the larger ionic radius of Ti{sup 3+} relative to that of Ti{sup 4+} could yield a D{sub Ti{sup 3+}} that is a factor of {approx}50 greater than D{sub Ti{sup 4+}}. The presence of significant proportions of Ti{sup 3+} in RF would provide important evidence of reducing conditions where the grains formed, support the model of [8] and rule out formation in typical, FeO-bearing chondrules. To test the prediction of [8], we have used X-ray absorption near-edge structure (XANES) analysis to determine the valence of Ti in two RF grains from the Tagish Lake carbonaceous chondrite.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Mar 06 00:00:00 EST 2007},
month = {Tue Mar 06 00:00:00 EST 2007}
}

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