Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.508g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 508
Mathematics
Logic
1
Age, Argon-Argon Ages, Fractionation, Metal-Silicate, Inclusions, Silicate, Meteorites, Iie Iron
Scientific paper
Several IIE iron meteorites contain small silicate inclusions, dispersed within metal, which suggest formation by a common process involving different degrees of heating and silicate fractionation from a chondrite-like parent (see discussion and references in McCoy [1]). The isotope chronology of IIE meteorites addresses two major questions concerning their origin. How many formation events are required, and do the isotopic ages also represent the times of silicate differentiation in some meteorites, or do they represent later impact heating events? We have determined ^39Ar-^40Ar ages of whole silicate samples of Watson, Techado, and Miles [1]. Although each meteorite gives a complex Ar age spectrum, each spectrum gives a well-defined age plateau over a significant (55-65%) portion of the total ^39Ar release. The ^39Ar-^40Ar degassing ages derived are 3.656 +/-0.005 Ga for Watson, 4.482 +/-0.025 Ga for Techado, and 4.408 +/-0.011 Ga for Miles (one-sigma errors). Absolute ages have an additional ^-0.5% uncertainty arising from the hornblende age monitor used. None of our Ar-Ar spectra show any significant evidence for an age older than those given, and only Miles shows modest evidence for recent diffusive loss of ^40Ar (affecting ^-10% of the ^39Ar release). Previous studies of Kodaikanal gave these ages: Rb-Sr = 3.7 +/-0.1 Ga [2], Pb-Pb = 3.676 +/-0.003 Ga [3], and K-^40Ar = 3.5 Ga [4]. Netschaevo gave a ^39Ar-^40Ar age of 3.74 Ga +/-0.03 Ga [5], and Watson gave a K-^40Ar age of 3.5 Ga [6]. (Some ages have been adjusted for changes in decay and irradiation constants.) All three meteorites suggest a common formation age of ^-3.70 +/-0.05 Ga. The ^39Ar-^40Ar age for Techado is identical to a ^39Ar-^40Ar age of 4.49 +/-0.03 Ga reported for Weekeroo Station [5] and to a Rb-Sr age of 4.51 Ga for Colomera [7]. These ages resemble ^39Ar-^40Ar ages of unshocked ordinary chondrites, and suggest that metal-silicate mixing and cooling to closure for Ar diffusion occurred early in parent body history. The ^39Ar-^40Ar age for Miles, however, appears slightly younger and is similar to Rb-Sr ages for Weekeroo Station of ^-4.28-4.39 Ga [8, 9]. Young isotopic ages do not obviously correlate with the degree of melting and silicate fractionation, except that three of four dated IIEs showing significant fractionation give older ages. Totally unrelated events may have melted and fractionated similar silicates to produce comparable mixtures with IIE metal at both ^-4.5 and ^-3.7 Ga ago. This requires at least the younger event to have been an impact, possibly related to impact chronometer resetting observed in lunar highland rocks and HED meteorites near this time. However, petrologic data suggest that IIE meteorites may also represent a suite of samples that responded in different degrees to a single, early mixing event [1]. This explanation suggests that isotopic ages of Watson, Netschaevo, and Kodaikanal were reset by strong impact heating, possibly involving melting of individual silicate clasts, in one or more events long after their initial formation. Impacts may also explain the apparent younger ages observed for Miles and Weekeroo Station. A problem for the origin of IIEs in a single, early event is the apparent requirement from initial ^87Sr/^86Sr of Kodaikanal that the Rb/Sr ratio was increased significantly at a time near 3.7 Ga [2]. Whether such fractionation could occur within or across shock-melted silicate inclusions [10] deserves further consideration. References: [1] McCoy, this volume. [2] Burnett and Wasserburg (1967) EPSL, 2, 397. [3] Gopel et al. (1985) Nature, 317, 341. [4] Bogard et al. (1969) EPSL, 5, 273. [5] Niemeyer (1980) GCA, 44, 33. [6] Olsen et al. (1994) Meteoritics, 29, 200. [7] Sanz et al. (1970) GCA, 34, 1227. [8] Burnett and Wasserburg (1967) EPSL, 2, 397. [9] Evensen et al. (1979) LPS X, 376. [10] Bence and Burnett (1969) GCA, 33, 387.
Bogard Donald D.
Garrison Daniel H.
No associations
LandOfFree
30Ar-40Ar Ages of Silicates from IIE 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 30Ar-40Ar Ages of Silicates from IIE Iron Meteorites, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and 30Ar-40Ar Ages of Silicates from IIE Iron Meteorites will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-829962