Tungsten isotopic constraints on the age and origin of chondrules

Gerrit Budde, Thorsten Kleine, Thomas S. Kruijer, Christoph Burkhardt, and Knut Metzler

PNAS2016 ; published ahead of print February 29, 2016, doi:10.1073/pnas.1524980113



The origin of chondrules—millimeter-sized silicate-rich spherules that dominate the most primitive meteorites, the chondrites—is a long-standing puzzle in cosmochemistry. Here, we present isotopic evidence that chondrules and matrix from the Allende chondrite contain different and complementary proportions of presolar matter, indicating that chondrules and matrix formed together from a single reservoir of solar nebula dust. This finding rules out formation of chondrules in protoplanetary impacts and demonstrates that chondrules are the product of localized melting events in the solar protoplanetary disk. Moreover, the isotopic complementarity of chondrules and matrix requires that chondrules formed in a narrow time interval and were rapidly accreted to a parent body, implying that chondrule formation was a critical step toward forming planetesimals.”

Chondrules may have played a critical role in the earliest stages of planet formation by mediating the accumulation of dust into planetesimals. However, the origin of chondrules and their significance for planetesimal accretion remain enigmatic. Here, we show that chondrules and matrix in the carbonaceous chondrite Allende have complementary 183W anomalies resulting from the uneven distribution of presolar, stellar-derived dust. These data refute an origin of chondrules in protoplanetary collisions and, instead, indicate that chondrules and matrix formed together from a common reservoir of solar nebula dust. Because bulk Allende exhibits no 183W anomaly, chondrules and matrix must have accreted rapidly to their parent body, implying that the majority of chondrules from a given chondrite group formed in a narrow time interval. Based on Hf-W chronometry on Allende chondrules and matrix, this event occurred ∼2 million years after formation of the first solids, about coeval to chondrule formation in ordinary chondrites. ”