Petrology and mineralogy of Type II, FeO-rich chondrules in Semarkona (LL3.0): Origin by closed-system fractional crystallization, with evidence for supercooling

Abstract

The petrology of type II porphyritic olivine chondrules in Semarkona (LL3.0) has been studied in detail. Olivines in these chondrules are euhedral, Fe-richand are strongly zoned from cores to rims of grains in FeO (Fa 10–30), Cr 2O 3 (0.2–0.6 wt%), MnO (0.2–0.7 wt%) and CaO (0.1–0.4 wt%). Interstitial mesostasis is rich in Si, Aland Ca and is glassy with abundant microcrystallites. Minor minerals include troilite, Fe,Ni metaland chromite. Some olivine grains contain euhedral, fayalite-rich cores that are probably produced during initial supercooling of the chondrule melt. Rare relict grains of forsteritic olivine have compositions very similar to olivines in type IA chondrules in Semarkona and may result from disaggregation of such chondrules. Apart from these relics, all properties of type II chondrules can be described by closed-system fractional crystallization of droplets which were essentially entirely molten. The chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks. Cooling rates of the order of 1000°C/h are indicated, from peak temperatures of around 1600°C. The chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization. There is no evidence for chondrule-nebula exchange either during or after crystallization, or any secondary processes such as aqueous alteration or thermal metamorphism. Bulk composition data suggest that precursor material included a refractory (Ca, Al, Ti-rich) component, similar to one of the precursor components of type IA chondrulesand a non-refractory (Fe and Si-rich) component. Type IA chondrules may have formed from type II chondrules by loss of Fe and volatiles. Alternatively, the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different Fe Mg ratios.