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Hui, Hejiu; Hess, Kai-Uwe; Zhang, Youxue; Nichols, Alexander R. L.; Peslier, Anne H.; Lange, Rebecca A.; Dingwell, Donald B.; Neal, Clive R. (2018): Cooling rates of lunar orange glass beads. In: Earth and Planetary Science Letters, Vol. 503: pp. 88-94
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It is widely accepted that the Apollo 17 orange glass beads are of volcanic origin. The degree of degassing of the glass beads depends on their cooling rates, so the estimation of volatile contents in the parental magmas of these lunar pyroclastic glasses also depends on the cooling rates. The cooling rate can be estimated using the calorimetric properties of the glass across the glass transition. In this study, a series of heat capacity measurements were carried out on hand-picked lunar volcanic orange glass beads during several cycles of heating to temperatures above their glass transition using a differential scanning calorimeter. The cooling rate of orange glass beads (sample 74220,867) was calculated to be 101 K/min using the correlation between glass cooling rates and fictive temperatures estimated from their heat capacity-temperature paths. This cooling rate is close to the lower end of the range that best fits the diffusion profiles of several volatile species in the glass beads, and at the upper end of the cooling rate range recorded in glasses quenched subaerially on Earth. The cooling rate is likely to be controlled by the cooling environment (cooling medium) such that the lunar volcanic glass beads could have been cooled in a gaseous medium released from volcanic eruptions on the Moon, but not during "free flight" in vacuum. The existence of a gas medium suggests, in turn, that there may have been at least a short-lived or episodic atmosphere on the early Moon at around 3.5 Ga.