Al-rich chondrules from the Ybbsitz H4-chondrite: evidence for formation by collision and splashing

Abstract

Three Al-rich chondrules (Al 2O 3 > 10 wt.%) from the H4-chondrite Ybbsitz were chemically and mineralogically studied. All three clearly crystallized from molten droplets as individual objects. They consist predominantly of olivine, spinel, and a feldspar-rich mesostasis. The three Al-rich chondrules have high concentrations of incompatible elements. The rare earth element patterns are fractionated with enrichment of light rare earths and a strong positive Eu anomaly. This type of fractionated refractory element pattern is indicative of igneous processes. Ca,Al-rich inclusions from carbonaceous chondrites also show high enrichments in refractory elements. Their patterns are, however, either flat or fractionated according to variable volatilities of refractory elements in the solar nebula (e.g. Yb anomaly). The high level of refractory siderophiles observed in Ca,Al-rich inclusions from carbonaceous chondrites is not found in the Al-rich chondrules. We therefore conclude that the Al-rich Ybbsitz chondrules formed by different processes than the Ca,Al-rich inclusions from carbonaceous chondrites. Melting of inhomogeneous precursors, generally assumed for the formation of ferromagnesian chondrules, is also unlikely to produce such extreme compositions. The possibility that metamorphism is the cause of the high content of incompatible elements can be excluded. We therefore propose a new model for the formation of the Al-rich chondrules. They may have formed by collisions of partly molten ferromagnesian chondrules with other objects (chondrules). During this process Al-rich melt fractions (residual liquids) may be ejected and form independent Al-rich chondrules. This model explains the high enrichment and the fractionated patterns of incompatible elements in Al-rich chondrules as well as their generally much smaller sizes compared with ferromagnesian chondrules. The Eu anomaly is probably due to diffusive exchange of Eu between matrix and the Al-rich chondrules. Eu is decoupled from the other REE in many ferromagnesian chondrules of equilibrated ordinary chondrites, including three ferromagnesian chondrules from Ybbsitz analyzed in this study.