Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.325b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 325
Computer Science
2
Ages, 39Ar-40Ar, Basaltic Achondrites, Dating, Eucrites, Impact Heating, Happy Canyon, Ilafegh 009, Millbillillie
Scientific paper
Eucrite and howardite meteorites are impact breccias from the HED parent body, which may have been the 520 km diameter asteroid 4 Vesta [1]. Using textural and compositional information on pyroxenes from individual clasts, [2] proposed a classification scheme based on the degree of thermal metamorphism exhibited by nearly all eucrites. In the highest grade, chemical zoning of pyroxenes has been erased and areas of impact melt sometimes have been produced. Thus, HED meteorites appear to be products of an early bombardment history of the inner portion of the main asteroid belt, just as most lunar highland rocks are products of early impact bombardment of the Moon. To determine the time period for major impacts on the HED parent body, we have measured ^39Ar-^40Ar ages for a large number of eucritic clasts from eucrites and howardites. All of these samples indicate partial to complete K-Ar chronometer resetting by several different impact events much more recent than eucrite crystallization times of ~4.45-4.55 Ga. For example, the ^39Ar- ^40Ar ages of paired eucrites Y-791186 and Y-792510 were essentially entirely reset 3.45 +- 0.05 Ga ago. A clast from Millbillillie gave a slightly older ^39Ar-^40Ar resetting age of 3.55 +- 0.02 Ga. (A similar degassing age of 3.5 +- 0.1 Ga was recently reported for Stannern [3]). Clast EET87531,21 gave a degassing age of 3.83 +- 0.05 Ga; clast EET82600 suggests an age of 3.89 +- 0.07 Ga; a clast from LEW85300 suggests a similar age to these. Several eucritic clasts gave ^39Ar-^40Ar degassing ages near 4.0 Ga. These include two clasts from EET87509 (,71 at 4.00 +- 0.05 Ga and ,74 at 3.93 +- 0.06 Ga), EET87509,24 (4.07 +- 0.02 Ga), Y-792769,68 (3.99 +- 0.04 Ga), and Y-790020,5 (4.03 +- 0.03 Ga). Clast and matrix samples from Y-75011 gave slightly different ages of 3.98 +- 0.03 Ga and 3.94 +- 0.04 Ga. Analyses of several additional eucritic clasts gave less well-defined ^39Ar-^40Ar release spectra that are consistent with this range in degassing ages. High temperature extractions of some (but not all) of these samples suggest small retention of ^40Ar during the degassing events. Five high temperature extractions of Clast EET87503,23 (releasing 35% of the ^39Ar) define an age of 4.41 +- 0.01 Ga. With a few exceptions, ^39Ar-^40Ar analyses of individual clasts from eucrites and howardites show reset ages across a wide range which is at least as broad as 3.4-4.2 Ga ago. These samples represent the full range of thermal metamorphism [2], but it is not yet clear whether these ages actually date the time of homogenization and exsolution observed in pyroxenes. The narrower range of lunar metamorphic ages, typically ~3.8-4.0 Ga ago, probably is due to the major influence of the large Imbrium and Serenitatis events. Early bombardment probably extended to the entire inner solar system, and it may have lasted much longer than suggested by lunar highland ages. Large, lunar-basin impacts with their associated large ejecta deposits and significant quantities of melt cannot occur on smaller bodies like 4 Vesta [4]. Because the extent of isotopic resetting decreases with decreasing crater size and amount of melt generated, meteorite parent bodies significantly smaller than Vesta might not exhibit impact resetting during this time period. Thus, it is interesting to contrast ^39Ar-^40Ar ages of eucrites with those of two meteorites, Ilafegh 009 and Happy Canyon, which represent impact melting on the EL (enstatite chondrite) parent body [5]. Higher temperature ^39Ar-^40Ar ages we recently obtained for both of these meteorites are 4.4-4.5 Ga. References: [1] Binzel R. P. and Shui X. (1993) Science, 260, 186. [2] Takeda H. and Graham A. L. (1991) Meteoritics, 26, 129. [3] Kunz J. et al. (1992) Meteoritics, 27, 245. [4] Cintala M. J. and Grieve R. A. (1993) Proc. Internat. Conf. Large Meteorite Impacts and Planetary Evolution. [5] McCoy T. J. et al. (1992) LPS, XXIII 869.
Bogard Donald D.
Garrison Daniel H.
No associations
LandOfFree
39Ar-40Ar Dating of Eucrites and Howardites and the Early Bombardment of the HED Parent Body 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 39Ar-40Ar Dating of Eucrites and Howardites and the Early Bombardment of the HED Parent Body, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and 39Ar-40Ar Dating of Eucrites and Howardites and the Early Bombardment of the HED Parent Body will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1072472