MAGNESIUM ISOTOPE SYSTEMATICS IN ARID HAWAIIAN SOILS

Magnesium accumulates in carbonate in the dry volcanic soils of leeward Hawaii Island. This mineralogical fractionation is unusual because it is more common for Mg to end up in secondary clays. In an effort to understand more about the fate of Mg, we investigated the Mg isotopic values and Mg mass distribution in a soil developed on the 350 ka Pololu lava flow of the Kohala Volcano of Hawaii. Soil horizons from this site were subjected to bulk dissolutions and sequential extractions to isolate mineralogic fractions. The bulk Mg isotopic value for this soil was slightly heavier (δ²⁶Mg =-0.12‰±0.58) than the parent material from which it developed (δ²⁶Mg =-0.30‰±0.22). The sequential extraction fractions show that this overarching enrichment in the heavier isotopes of Mg in the soil is a small fractionation compared to the range of isotopic values expressed in different soil mineral pools. Adsorbed Mg (weighted average δ²⁶Mg =-1.02‰±0.38), and Mg in carbonates (weighted average δ²⁶Mg =-1.69‰±0.31) are isotopically lighter than simple mixing between Mg derived from parent material (δ²⁶Mg =-0.30‰±0.22) and rainfall (δ²⁶Mg =-0.89‰±0.14) would allow. Mg associated with organic matter (weighted average δ²⁶Mg =-0.07‰±0.38) is isotopically heavier than parent material. Taking all three of these soil reservoirs of Mg together, 29% of the Mg present in the soil is accounted for, with an overall isotopic composition of δ²⁶Mg =-1.29‰±0.70. We have yet to analyze the remaining soil sequential extraction fractions for isotopic compositions, but these must be enriched in isotopically heavy magnesium to satisfy mass and isotopic balance. The remaining soil fractions sampled by our sequential extraction consist of noncrystalline aluminosilicate phases, iron sesquioxides, kaolin minerals, and refractory phases. Of the remaining extractions, 45% of the Mg resides in the noncrystalline phases, and 10% resides in the iron sesquioxides. As soils develop on leeward Hawaii the relative proportions of these soil mineral fractions evolves, thereby suggesting that the overall Mg isotopic composition of the soil should evolve with time. Further insight into the development of Mg isotope fractionations in this soil are provided by investigating the amount of Mg remaining in the soil compared to a relatively immobile index element (Zr).