ZONING OF PHOSPHORUS IN MAGMATIC OLIVINE

EMP X-ray maps show that although Hawaiian olivine (ol) phenocrysts are generally unzoned in Fe/Mg they are prominently zoned in P (bdl to ~0.2 wt. % P₂O₅). Al and Cr can also be zoned, and their zoning (when present) is correlated with that of P. P-rich zones are texturally variable and can be present from core to rim. P zoning is often oscillatory, and oscillatory zones typically parallel outer crystal edges. One set of euhedral crystals (unzoned in Fo content and exhibiting sinuesis) contains P-rich cores outlining skeletal, hopper shapes similar to those grown in high-cooling-rate experiments. P zoning is present both in kink-banded and non-kink-banded ol; in one kink-banded crystal, P-rich zones are discontinuous or patchy. Normally zoned (in Fo) ol microphenocrysts (i.e., in groundmass) display prominent oscillatory and sector zoning in P, Al, and Cr; the Al and Cr zoning is more pronounced in microphenocrysts than in phenocrysts. Although Hawaiian ol are our largest studied population, ol from Gorgona Island and Zimbabwe komatiites, Reunion basalt, a Martian meteorite, and andesite from Volcan San Pedro, Chile all display P zoning as in Hawaiian basalts, suggesting this is a widespread feature of magmatic ol.

1-atm cooling experiments (15-30°/h; QFM) on an Mg-rich Hawaiian basalt composition produced ol with strong zoning in P that is highly coupled to zoning in Al, Cr, and Ti. Oscillatory and sector-zoning are observed in some samples. The presence of oscillatory P, Al, and Cr zoning in ol from these experiments demonstrates such zoning can form by crystal growth during simple cooling histories.

Our interpretation is that high-P, -Al, and -Cr zones form during rapid ol growth, due to enriched melt boundary layers and/or effects of rapid growth on partitioning and that variations in growth rate alone can lead to oscillatory zones. These oscillations are superimposed on normal Fo zoning in the growing ol, but whereas rapidly diffusing 2+ cations homogenize during residence of the ol in hot magma, sluggish diffusion of P⁵⁺ results in preservation of a unique record of early ol growth. Al and Cr diffuse more rapidly than P, and their partial homogenization in ol phenocrysts could allow thermal histories to be constrained. Al and Cr are less affected by diffusion in microphenocrysts, which have shorter high-T histories.