TY - JOUR
T1 - Differentiation history of the mesosiderite parent body
T2 - Constraints from trace elements and manganese-chromium isotope systematics in Vaca Muerta silicate clasts
AU - Wadhwa, M.
AU - Shukolyukov, A.
AU - Davis, A. M.
AU - Lugmair, G. W.
AU - Mittlefehldt, D. W.
N1 - Funding Information:
We thank those who generously provided us with the samples studied here: thin sections and bulk samples of Vaca Muerta clasts 4659, 4670, 4671, 4677, 4679, and 4695 were made available by the late Dr. Martin Prinz (AMNH); thin section and bulk sample for Vaca Muerta pebble 16 was provided by Dr. Alan Rubin (UCLA). We are extremely grateful to Chris MacIsaac for his invaluable assistance in the clean lab, for measuring the Mn and Cr concentrations and for many helpful suggestions during the course of this work. We thank Mark Robinson and the Center for Planetary Sciences at Northwestern University for access to computing facilities and programming help during preparation of this manuscript. This paper benefited from thoughtful reviews by Rick Carlson, Carsten Münker, and Dimitri Papanastassiou and the editorial efforts of Rich Walker. This work has been supported by NASA grants to MW, AMD and GWL, and an NSF grant to MW.
PY - 2003/12/15
Y1 - 2003/12/15
N2 - We report here the results of a study of trace element microdistributions and 53Mn-53Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal 53Mn-53Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, 53Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 ε in pebble 16; ∼1.07 ε in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 ε in 4659; ∼0.53 ε in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when 53Mn was still extant. However, the slope of the 53Mn-53Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a 53Mn/55Mn ratio of 3.3 ± 0.6 × 10-6) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a 53Mn/55Mn ratio of 4.7 ± 0.5 × 10-6), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.
AB - We report here the results of a study of trace element microdistributions and 53Mn-53Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal 53Mn-53Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, 53Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 ε in pebble 16; ∼1.07 ε in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 ε in 4659; ∼0.53 ε in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when 53Mn was still extant. However, the slope of the 53Mn-53Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a 53Mn/55Mn ratio of 3.3 ± 0.6 × 10-6) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a 53Mn/55Mn ratio of 4.7 ± 0.5 × 10-6), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.
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U2 - 10.1016/j.gca.2003.08.005
DO - 10.1016/j.gca.2003.08.005
M3 - Article
AN - SCOPUS:0346335721
SN - 0016-7037
VL - 67
SP - 5047
EP - 5069
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 24
ER -