Chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the Franciscan complex of California

Abstract

Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of California produces serpentine soils containing high concentrations of Cr as well as other potentially toxic elements including Ni, Co, and Mn. Chromium concentrations in serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as high as 4,760 mg kg⁻¹, nearly three times greater than the serpentinite protolith (∼1800 mg kg⁻¹). Chromium-containing minerals within the bedrock include chlorite (∼0.3 Cr wt. %), enstatite (∼0.4 Cr wt. %), augite (∼0.7 Cr wt. %), chromite (∼10.8 Cr wt. %), magnetite (8.2-10.3 Cr wt. %), and an ultra fine-grained mixture of spinel and a silicate phase containing ∼13.3 Cr weight percent. Chromium concentrations in serpentine soil profiles fluctuate between 1,725 to 4,760 mg kg⁻¹ and do not correspond to variations in soil pH, organic carbon, or electrical conductivity. The enrichment and variability of soil Cr is directly related to the modal abundance and weathering of chromite, Cr-magnetite, and the spinel-silicate mixture. By comparison, Cr silicates account for < 10 percent of the total soil Cr. Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-enriched end-member (FeCr₂O₄). Chromium occurs in the trivalent state in both the rock and soil samples. The apparent resistance of Cr-spinels to weathering, evident from extraction experiments, suggests that these minerals are not large inputs for Cr in soil solutions and vegetation associated with serpentine soils. Chromium-bearing igneous and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are more likely sources of chemically mobile and bioavailable Cr.