DEEP WEATHERING PROFILE OF THE PINE HILL SERPENTINITE

The processes that control chemical weathering in serpentinites are not well understood. Although previous studies have examined serpentine soil chemistry, none have focused on the transition from serpentine bedrock to soils. Furthermore, questions remain regarding the release of heavy trace metals into solution and the mechanisms controlling this process. In this study, we aim to characterize the zone between the soil-rock interface and the weathering front. We hypothesize that 1) deep weathering of serpentinite extends beyond the soil-bedrock interface and 2) the deep weathering of primary serpentinite minerals contributes to a measurable release of Cr.

The Pine Hill Serpentinite is ideal for exploring deep weathering because a tall, vertical quarry face allows us to sample easily without the need for extensive drilling. We (1) determined the bulk chemistry (using XRF) and mineralogy (using a mix of XRD, EPMA and EDS modal mapping) of the serpentinite from the weathered surface to the unweathered parent rock, (2) quantified changes in bulk chemistry and mineralogy through normalized elemental concentration profiles (τ-plots) and changes in modal percentages with respect to depth, and (3) will use these data to model the weathering advance front and estimate the amount of Cr leaving the solid phase.

Bulk mineralogy shows no significant changes with depth. Modal abundances reveal a composition of ~79% serpentine (primarily antigorite), 18% augite, 2% magnetite and 1% chromite. All of these phases contain Cr in solid solution. Minor sulfides and other accessory minerals account for <1% modal abundance. Mn-oxides precipitated along fractures in the shallowest samples are Cr- and Ni-rich. Elemental concentrations, normalized to Zr concentrations, show depletion profiles for Ca (~30-40%) down to a depth of 1 m, which is most likely weathering from the primary Ti-rich augite. Trace metals Cr, Ni, Cu and Zn show possible enrichment in the top 1 m of the profile, likely due to scavenging by Mn-(oxy)hydroxides observed in the shallow profiles. Weathering of primary silicates releases Cr, while Ni, Cu and Zn are most likely associated with weathering sulfides.