High precision Mg and Cr isotope measurements by MC-ICP-MS and their applications to date the first stage of planet formation in the early solar nebula.

Details High precision Mg and Cr isotope measurements by MC-ICP-MS and their applications to date the first stage of planet formation in the early solar nebula. High precision Mg and Cr isotope measurements by MC-ICP-MS and their applications to date the first stage of planet formation in the early solar nebula.

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.v41a..06y&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #V41A-06

Other

1000 Geochemistry, 1027 Composition Of The Planets, 1094 Instruments And Techniques, 1100 Geochronology, 1115 Radioisotope Geochronology

Scientific paper

We developed high precision Mg and Cr isotope measurement techniques by MC-ICP-MS at UC Davis to probe physicochemical conditions in the early solar nebula and establish high-resolution chronology from dust to the earliest planet building processes. (1) We obtained stable isotopic composition of Mg (δ25Mg) together with δ26Mg*, the radiogenic in growth from the decay of the short-lived, now extinct 26Al (T1/2 ~0.7 Myr) in whole rock fragments and mineral separates from high temperature refractory Ca-Al-rich inclusions (CAIs) in primitive meteorite Allende, the oldest known (first) solid objects in our solar system. Enrichment of heavy Mg isotopes (up to +6 permil) in d25Mg suggest that the igneous Type B CAIs have lost 14-47 percent of original Mg by evaporation in the early solar nebular during high temperature. Compact and fluffy Type A CAIs, on the other hand, shows light isotope enrichment with d25Mg as low as -4 permil), suggesting condensation from a nebular gas already depleted in heavy Mg isotopes. All of these objects plot on a well-defined 26Al-26Mg isochron with 26Al/27Al=(5.17+/-0.10)x10-5, suggesting the total duration of CAI formation event is less than 20,000 years. U-Pb dating in the same material anchors the event at 4567.4+/-0.3 Myr ago [1]. (2) We have applied 53Mn-53Cr chronometer to date chondrule obtained from primitive chondrite, and establish that chondrules in ordinary chondrite postdate the CAI by ~2 Myr [2]. (3) We further apply 53Mn-53Cr chronometer to a suite of carbonaceous chondrites whole rock samples, which are "cosmic sediments" made of CAIs and chondrules that are "cemented" together by fine grained matrices rich in organics and presolar grains. All carbonaceous chondrites exhibit 53Cr* anomalies that are correlated with 55Mn/52Cr ratio, which in turn is governed by proportion of matrix/refractory components. Thus 53Mn-53Cr chronometer dates the accretion timescale ("sedimentation" or "compaction" time) of undifferentiated primitive meteorites from the early solar nebula. This represents the first stage of planet formation in the standard model, and must have occurred between +0.91Myr to -1.17Myr at 4568 Myr ago [3]. [1] Jacobsen B. et al. (2008) EPSL (in review). [2] Yin Q.-Z. et al. (2007) ApJL, 662, L43-L46. [3] Moynier F. et al., (2007) ApJL 671, L181-L183.

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