Soil Genesis and Mineralogy Across a Volcanic Lithosequence.

The interaction of subduction, volcanism and silicate mineral weathering drives global carbon cycles. A volcanic lithosequence of varying silica compositions, existing under the same late-Holocene climatic conditions, was used to study the effects of lithology on the neogenesis of silicate minerals. The Clear Lake Volcanic Field is an active volcanic center in the northern California Coast Range where regional deformation associated with the San Andreas Fault system activates mafic to silicic eruptions associated with slab window melting.  This late-Pleistocene lithosequence, set in a mesic/xeric climate, provides a unique environment for studying mineralogical transformations and pedogenesis across a range of silica compositions experiencing a similar weathering environment. We hypothesized that clay mineralogy would trend from 2:1 to 1:1 layer silicates with the increasing silica content of parent material. Pedons formed in different lithologies, including tephra, rhyolitic obsidian, dacite, andesite and basaltic andesite were investigated. Soil samples were collected by genetic horizon, size fractionated, and the clay fraction analyzed via x-ray diffraction.  X-ray diffraction data showed similar trends across the lithosequence, with clay fractions dominated by kaolin minerals, including kaolinite and halloysite, with lesser amounts of hydroxy-aluminum interlayer clays and gibbsite. The upper portion of the profiles were predominantly kaolinite (0.7 nm), and clay mineralogy tended to shift from kaolinite to halloysite (1.0 nm) as the degree of kaolin hydration increased with increasing depth.  Thus, in contrast to our hypothesis, parent materials of divergent silica compositions converged to a state dominated by kaolin minerals. Therefore, despite the divergent silica compositions of the lithosequence, a common weathering environment results in similar clay mineralogy, demonstrating climatic control on the neogenesis of clay minerals in the xeric climate of California.