WEATHERING OF PLAGIOCLASE UNDER DIFFERING CLIMATIC REGIMES: A PRELIMINARY MICROPROBE ANALYSIS

Climate plays a major role in bedrock weathering and subsequent sediment production. Consequently, sediment composition reflects, to some, degree, climatic regime. In this research, we focus on the weathering of plagioclase in granitoid bedrock exposed in various climate systems as follows: Anza Borrego, California (hot-dry), Yabucoa, Puerto Rico (hot-wet), Veitastrond, Norway (cold-wet), and Wright Valley, Antarctica (cold-dry).

The electron microprobe was used to measure elemental compositions of several (6-13) spots along transects (~500 - 800µm long) from grain cores to grain edges of 10 plagioclase grains of both fresh and relatively weathered bedrock samples. The table summarizes averages of key major-oxide ratios, calculated using 44 spots from 5 plagioclase grains in the fresh sample and 34 spots from 5 plagioclase grains from the weathered sample.

	CaO/Na2O	K2O/Na2O	Al2O3/SiO2	CaO/Al2O3
Fresh	1.11	0.02	0.45	0.30
Weathered	0.99	0.02	0.43	0.28

The largest difference between fresh and weathered grains is the change in CaO/Na2O, signaling Ca loss in the weathered sample. Additionally, when plotted with distance from grain core to edge, CaO/Na2O typically decreases. We are conducting additional analyses to assess whether this pattern reflects a robust weathering effect.

Future work will include similar analyses of the plagioclase on granitoid bedrock from the other climatic regimes to assess similarities and difference in the plagioclase weathering, as it relates to climate.