Compound Chondrules fused Cold

TL;DR

This paper proposes that compound chondrules formed from slow solid collisions followed by brief heating in a specific temperature range, challenging the traditional molten collision hypothesis and linking to MRI activity.

Contribution

It introduces a new formation scenario for compound chondrules involving solid collisions and subsequent heating, explaining observed frequencies and linking to MRI activity.

Findings

Coalescence times at 900-1025K are hours to decades.

Solid collisions can produce compound chondrules.

Temperature window aligns with MRI activity regime.

Abstract

About 4-5% of chondrules are compound: two separate chondrules stuck together. This is commonly believed to be the result of the two component chondrules having collided shortly after forming, while still molten. This allows high velocity impacts to result in sticking. However, at T ~ 1100K, the temperature below which chondrules collide as solids (and hence usually bounce), coalescence times for droplets of appropriate composition are measured in tens of seconds. Even at 1025K, at which temperature theory predicts that the chondrules must have collided extremely slowly to have stuck together, the coalescence time scale is still less than an hour. These coalescence time scales are too short for the collision of molten chondrules to explain the observed frequency of compound chondrules. We suggest instead a scenario where chondrules stuck together in slow collisions while fully solid; and the resulting chondrule pair was subsequently briefly heated to a temperature in the range of 900-1025K. In that temperature window the coalescence time is finite but long, covering a span of hours to a decade. This is particularly interesting because those temperatures are precisely the critical window for thermally ionized MRI activity, so compound chondrules provide a possible probe into that vital regime.