The use of mineral interfaces in sand-sized rock fragments to infer ancient climate

Abstract

The kinds of mineral interfaces preserved in rock fragments of modern plutoniclastic sand are closely correlated with. climate and topography in the source area. Stepwise linear-regression analysis of the relationship among environmental and petrographic parameters for sand collected from 16 drainages of varying morphology developed on five granodiorite plutons demonstrates that sand composition varies systematically with temperature and precipitation, and drainage steepness and area, for small drainages in temperate climates. The durability of each kind of mineral interface in sand-sized rock fragments derived from the plutons depends on mechanical, chemical, and textural factors. Interfaces are mechanically stressed by differential expansion of minerals during unroofing; interfaces between minerals of similar expansibility/compressibility are more likely to remain uncracked and thus survive weathering. The rate and extent of chemical dissolution of interfaces depends on the crystallography and chemistry of adjoining minerals; interfaces between minerals of similar structure that have short atomicbond lengths are more likely to survive. Mineral inclusions are texturally isolated from rock-disaggregating processes and thus more likely to survive. Interfaces of opaque with tectosilicate minerals are enriched relative to other interfaces by progressive disaggregation, the rate and extent of which increase with increasing temperature. Interfaces of biotite with tectosilicates are depleted relative to other interfaces with increasing precipitation. Regression analysis yields statistically plausible, testable models of the dependence of modem sand composition on modern climate. Such models can be used to quantitatively estimate ancient mean annual air temperature and precipitation based on ancient sand composition.