Fractionation of yttrium and holmium during basaltic soil weathering

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The anomalously low affinity of yttrium (Y) for iron (Fe) (oxyhydr)oxides relative to lanthanides with similar ionic radius (e.g., Ho) has been demonstrated in experiments with isolated Fe minerals and in a variety of marine systems that contain high concentrations of solid phase Fe. However, it has not previously been demonstrated to occur during soil genesis, despite the common observation that many soils become enriched in Fe over time. We hypothesized that Y would become progressively depleted in soils relative to Ho with increased weathering. Since, trivalent Y has an anomalously low Misono softness relative to other trivalent ions included in the rare earth element and yttrium group (REY3+), we also investigated whether soil REY fractionation reflects variation in Misono softness. To test this, we measured trends in total REY concentrations for Hawaiian soils derived from basaltic parent materials aged 0.3–4100ky, and measured REYs released from the same samples during short-time (3h) dissolution experiments conducted as part of a previous investigation linking dissolution with surface charge properties (Chorover et al., 2004). The chondrite-normalized Y/Ho ratios in the parent Hawaiian basalt (Chond[Y/Ho]=0.998) and continental dust (Chond[Y/Ho]=0.994) inputs are remarkably similar, and thus we can interpret deviations from Chond[Y/Ho]∼1.0 to result from soil biogeochemical processes and not source mixing. Between 0.3 and 20ky, the Chond[Y/Ho] ratio of the subsurface soils decreased from 0.96±0.07(2σ) to 0.71±0.05, and then remained unchanged across the rest of the weathering sequence. In contrast, the Chond[Y/Ho] ratio of the surface soils decreased from 0.99±0.07 to 0.76±0.05 at 150ky and then, most likely due to continued dust inputs, increased to 1.04±0.07 in the oldest soils. Analysis of the short-time dissolution experiments revealed preferential release of Y relative to Ho (and also La relative Pr) at intermediate pH where aqueous REY concentrations are governed by proton competition for adsorption sites. Proton-competition-control over REY release is bounded at high pH by the onset of colloidal dispersion—represented by the point of minimum dissolution (p.m.d.) of Al—and at low pH by the soil’s point of zero net charge (p.z.n.c.) and/or when proton-promoted dissolution of REY-containing solids, including Fe-(oxyhydr)oxides, control REY release. Results of our dissolution experiments suggest that complexation of REYs by dissolved organic matter (DOM) does not drive Y–Ho fractionation during pedogenesis, but rather may suppress it. Synthesis of these field and laboratory experiments suggests the Y/Ho ratio decreases early in soil development (<20ky) when weathering rates are high and competitive proton adsorption affects REY fractionation. Given that Fe-(oxyhydr)oxide sorbents exhibit greater affinity for Ho relative to Y, their prevalent neo-formation during incipient pedogenesis likely plays a central role in Y–Ho fractionation in these soils. Persistence of low Chond[Y/Ho] ratios in the subsurface soils even at 4100ky suggests Y–Ho fractionation continues, albeit at a slower rate, as weathering proceeds.