Evolution of mare basalts - The complexity of the U-Th-Pb systemAn attempt has been made to gain more insight into mare-basalt evolution by performing a very detailed leaching and mineral-separation U-Th-Pb systematics study on mare basalt 15085. It is found that about 20-50% of the U, Th, and Pb reside on the grain boundaries or in the mesostasis and that the Pb-207/Pb-206 ratios of the grain boundaries and crystal interiors are distinctly different. These distinct trends appear to represent either continuous or episodic postcrystallizational disturbances to the U-Th-Pb system of this rock. Using U and Pb partition coefficients, it is concluded that existing two- and three-stage U-Pb evolution models do not accurately describe mare-basalt genesis. An alternative two-stage + KREEP mixing model is proposed as a simple approximation to U-Pb evolution in lunar rocks. Most Rb-Sr and Sm-Nd data are compatible with this model.
Document ID
19780057747
Acquisition Source
Legacy CDMS
Document Type
Conference Proceedings
Authors
Unruh, D. M. (Geological Survey Denver, CO, United States)
Tatsumoto, M. (U.S. Geological Survey Denver, Colo., United States)