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Fluids on differentiated asteroids: Evidence from phosphates in differentiated meteorites GRA 06128 and GRA 06129
Author(s) -
SHEARER Charles K.,
BURGER Paul V.,
PAPIKE James J.,
SHARP Zachary D.,
McKEEGAN Kevin D.
Publication year - 2011
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2011.01233.x
Subject(s) - apatite , geology , geochemistry , plagioclase , chemistry , mineralogy , paleontology , quartz
– Paired meteorites Graves Nunatak 06128 and 06129 (GRA) represent an ancient cumulate lithology (4565.9 Ma ± 0.3) containing high abundances of sodic plagioclase. Textures and stable isotope compositions of GRA indicate that superimposed on the igneous lithology is a complex history of subsolidus reequilibration and low‐temperature alteration that may have extended over a period of 150 Myr. In GRA, apatite is halogen‐rich with Cl between 4.5 and 5.5 wt% and F between 0.3 and 0.9 wt%. The Cl/(Cl+F+OH) ratio of the apatite is between 0.65 and 0.82. The Cl and F are negatively correlated and are heterogeneously distributed in the apatite. Merrillite is low in halogens with substantial Na in the 6‐fold coordinated Na‐site (≈2.5%) and Mg in the smaller octahedral site. The merrillite has a negative Eu anomaly, whereas the apatite has a positive Eu anomaly. The chlorine isotope composition of the bulk GRA leachate is +1.2‰ relative to standard mean ocean chloride (SMOC). Ion microprobe chlorine isotope analyses of the apatite range between −0.5 and +1.2‰. Textural relationships between the merrillite and apatite, and the high‐Cl content of the apatite, suggest that the merrillite is magmatic in origin, whereas the apatite is a product of the interaction between merrillite and a Cl‐rich fluid. If the replacement of merrillite by apatite occurred at approximately 800 °C, the fluid composition is f (HCl)/ f (H 2 O) = 0.0383 and a HCl molality of 2.13 and f (HCl)/ f (HF) = 50–100. It is anticipated that the calculated f (HCl)/ f (H 2 O) and a HCl molality are minimum values due to assumptions made on the OH component in apatite and basing the calculations on the apatite with the lowest X Cl . The bulk δ 37 Cl of GRA is a >2σ outlier from chondritic meteorites and suggests that parent body processes resulted in fractionation of the Cl isotopes.