CONSTRAINING THE FORMATION CONDITIONS OF TYPE II CHONDRULES FROM ORDINARY CHONDRITES THROUGH EXPERIMENTAL ANALOGS

Chondrules are millimeter-sized beads of rock that melted and cooled as free-floating objects in the solar nebula. Since they are the dominant component of chondritic meteorites, it is important to understand the conditions under which they formed. Type I chondrules contain reduced olivine and pyroxene, and type II chondrules are more oxidized. Most type I chondrules have an interstitial glassy mesostasis, but some contain primary (igneous) plagioclase. A recent experimental study showed that the presence of plagioclase requires very slow cooling rates (down to 1 ˚C/hr), which puts an upper limit on cooling rates for plagioclase-bearing type I chondrules. In contrast, type II chondrules do not contain plagioclase. Plagioclase has not been reproduced in previous type II analog experiments, but this may be because runs are typically quenched from temperatures above which plagioclase may nucleate. We are carrying out 1-atm experiments that attempt to duplicate textures and mineral compositions observed in type IIA (porphyritic, olivine-rich) chondrules. Since plagioclase does not occur in these chondrules, defining the conditions that produce plagioclase will determine a lower limit on cooling rates.

Our experiments successfully reproduced type IIA chondrule textures, as well as mineral compositions and zoning. The cooling rates we investigated (a maximum of 30 ˚C/hr) are at the low end of the 10 to 1000 ˚C/hr range for porphyritic chondrules that has been proposed previously. The experiments do not necessarily exclude faster cooling rates for type IIA chondrules, but they show that slow cooling rates are plausible.

In our experiments, plagioclase crystallized under very slow cooling conditions, with a final cooling stage of 1 ºC/hr, while run products cooled at 30 ºC/hr did not contain any plagioclase. Hence, these conditions bracket the cooling rate at which plagioclase crystallization occurs, and indicate that a lower limit to the cooling rate of type II chondrules lies between the two sets of conditions. Since natural type IIA chondrules do not contain plagioclase, we can infer that natural type II chondrules did not experience very slow cooling rates, unlike some type I chondrules. We plan to conduct further experiments to better constrain the conditions under which plagioclase forms in type II chondrules.