LEAD-ISOTOPE SYSTEMATICS OF GRANITOID WEATHERING

The isotopic composition of lead released by chemical weathering of gr anitoids was investigated in order to evaluate the sensitivity of this natural tracer as a tool for monitoring mineral weathering and soil d evelopment. The isotopic composition of lead was found to change syste matically with the relative degree (or maturity) of weathering in both Cretaceous granitoids from the Sierra Nevada batholith (SN) and Preca mbrian granitoids from the Wind River Range, Wyoming (WRR). In the SN, lead released from crushed bedrock by dilute acid leaching (to simula te the initial stages of weathering) has a composition more radiogenic than observed in soils (average Pb-206/Pb-207 = 1.335). Lead released by leaching from soils developed on a relatively young terrain (appro ximately 10 kyr old) is less radiogenic (average Pb-206/Pb-207 = 1.270 ) than crushed bedrock, and lead released by leaching from soil develo ped on older surfaces (approximately 100 kyr) is even less radiogenic (average Pb-206/Pb-207 = 1.255). Lead released from the total digestio n of the rock has the least radiogenic composition (average Pb-206/Pb- 207 = 1.240). In the WRR, systematic changes in the isotopic compositi on of lead released by leaching were observed in soils developed on gl acial moraines of variable ages. Lead released from soil developed on a 21 kyr old moraine was the most radiogenic (Pb-208/Pb-207 = 3.257). Lead released from soil developed on a 130 kyr old moraine was less ra diogenic (Pb-208/Pb-207 = 2.780), and lead released from soil develope d on greater-than-or-equal-to 350 kyr old moraine was the least radiog enic (Pb-208/Pb-207 = 2.532). The observed systematic changes in the i sotopic composition of lead with time in both field areas demonstrates the potential of lead isotopes to ascertain the degree (or maturity) of chemical weathering of granitoids, and may have applications in det erminations of the relative ages of some glacial deposits.