Lead isotope systematics of granitoid weathering

The isotopic composition of lead released by chemical weathering of granitoids was investigated in order to evaluate the sensitivity of this natural tracer as a tool for monitoring mineral weathering and soil development. The isotopic composition of lead was found to change systematically with the relative degree (or maturity) of weathering in both Cretaceous granitoids from the Sierra Nevada batholith (SN) and Precambrian granitoids from the Wind River Range, Wyoming (WRR). In the SN, lead released from crushed bedrock by dilute acid leaching (to simulate the initial stages of weathering) has a composition more radiogenic than observed in soils (average ²⁰⁶Pb ²⁰⁷Pb = 1.335). Lead released by leaching from soils developed on a relatively young terrain (~10 kyr old) is less radiogenic (average ²⁰⁶Pb ²⁰⁷Pb = 1.270) than crushed bedrock and lead released by leaching from soil developed on older surfaces (~ 100 kyr) is even less radiogenic (average ²⁰⁶Pb ²⁰⁷Pb = 1.255). Lead released from the total digestion of the rock has the least radiogenic composition (average ²⁰⁶Pb ²⁰⁷Pb = 1.240). In the WRR, systematic changes in the isotopic composition of lead released by leaching were observed in soils developed on glacial moraines of variable ages. Lead released from soil developed on a 21 kyr old moraine was the most radiogenic ( ²⁰⁸Pb ²⁰⁷Pb = 3.257). Lead released from soil developed on a 130 kyr old moraine was less radiogenic ( ²⁰⁸Pb ²⁰⁷Pb = 2.780) and lead released from soil developed on ≥350 kyr old moraine was the least radiogenic ( ²⁰⁸Pb ²⁰⁷Pb = 2.532). The observed systematic changes in the isotopic 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 determinations of the relative ages of some glacial deposits.