Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..560p&link_type=abstract
Meteoritics, vol. 30, no. 5, page 560
Physics
1
Iron Meteorites, Re-Os Ages
Scientific paper
For age determinations of specific events in the evolution of iron meteorites it is essential to obtain a reasonable range of physical-chemical fractionation of Re/Os. The possible stages of iron meteorite evolution which can, in principle, result in Re-Os fractionation include: a) condensation of PGE and of FeNi in the solar nebula; b) subsequent oxidation of part of the Fe by reaction with S; c) segregation and partial removal of massive FeS during melting on parent planetesimals; d) melting followed by fractional crystallization of the metal phase and crystallization of the remaining FeS; e) element redistribution during subsolidus phase transformations to produce schreibersite sheaves and finally precipitation or exsolution of minor micro-phases, e.g., rhabdite, at a later stage. Using negative ion, thermal ionization, we have determined the Re and Os concentrations and Os isotopic compositions in metal samples of iron meteorites. We have also analyzed metal-FeS pairs from two IA iron meteorites and a metal-phosphide pair from one IIIA iron. The analytical results for the paired analyses are shown in Table 1. The Re, Os partition coefficients between these coexisting phases are shown in Table 2. The whole-rock, metal samples for group IA, IIA, IIIA, IVA and IVB irons are consistent with a single, well-defined best fit line which yields a slope of 0.07863+/-0.00031 (corresponding to an age of 4.62+/-0.02 AE;lambda=1.64x10^(-11) a^(-1)) and an initial ^(187)Os/^(188)Os = 0.09560+/-0.00018 (all uncertainties are 2 sigma). There is also a suggestion from the data for the IIA and IVA groups of a difference in ages of 60+/-45 m.y. We find that, in the presence of the metal phase, neither Re nor Os enter into the sulphide. Therefore, the production or segregation of sulphide [e.g., under steps (b) and (c), above] act only as diluents for the Re and Os concentrations and can have no influence on the Re-Os isotope systematics and age determinations. Given the refractory nature of Re and the PGE, under the reducing conditions in the solar nebula, we consider that the condensation of Re and the PGE and their subsequent incorporation into the FeNi did not result in significant Re/Os fractionation. We conclude that the data for the non-magmatic IA and IVA groups show a limited Re/Os fractionation and that the major dispersion in Re/Os for FeNi is obtained for members of the magmatic iron groups and is dominated by the melting and fractional crystallization of the metal phase. This Re/Os fractionation process then enables the determination of essentially whole-rock isochrons for iron meteorites, which date the melting and fractional crystallization of the metal phase on the parent planetesimals. The data on the Tres Castillos (IIIA group) FeNi show relatively low Re and Os concentrations, consistent with the Ni concentration. The results on the Tres Castillos FeNi are consistent with the well-defined, whole-rock Re-Os isochron determined by all irons analyzed. The data on the massive schreibersite lamella (3 cm x 5 mm) show low but significant levels of Re and Os, with metal to schreibersite partition coefficients of about 11 and 29 for Re and Os, respectively. The data, furthermore, demonstrate that the schreibersite has Re/Os which is significantly fractionated, by a factor of 2.7, relative to the coexisting FeNi. The schreibersite data yield a model age of 4.30+/-0.09 AE, indicating a disturbance at a time younger than 4.30 AE. The FeNi-schreibersite pair yields a two point line with a slope corresponding to an age of 4.11+/-0.11 AE. We consider this relatively young age as an indication of 1) a relatively late disturbance of the phosphide; 2) an indication of a relatively slow cooling rate at 1-2 degrees C/my; or 3) an indication that even the massive schreibersite may continue to act as an open system at reasonably low temperatures. We also consider that the Re-Os data on the schreibersite permit for the first time the application of the Re-Os system to the determination of a possible internal isochron and possibly a direct measurement of cooling history. This work was supported by NASA Grant NAGW-3337. Division Contribution 5562(905).
Papanastassiou Dimitri Anastassios
Shen Janice
Wasserburg Gerald J.
No associations
LandOfFree
Re-Os in FeNi, Sulfide, and Phosphide: The Possible Determination of Internal Isochrons for Iron Meteorites does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Re-Os in FeNi, Sulfide, and Phosphide: The Possible Determination of Internal Isochrons for Iron Meteorites, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Re-Os in FeNi, Sulfide, and Phosphide: The Possible Determination of Internal Isochrons for Iron Meteorites will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-829759