A New Window into Magmatic Processes: Zoning of Phosphorus and Other Elements in Olivine

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Intellectual merit. This project explores the significance and utility of recent discovery of phosphorus zoning in olivines from terrestrial komatiites, basalts, andesites, and dacites and from lunar rocks and a Martian meteorite. P2O5 contents of these olivines vary from below detection (<0.01 wt. %) to 0.2-0.4 wt. % over a few microns, with no correlated variations in Fo content. P zoning can delineate P-rich crystal cores with skeletal, hopper, or euhedral shapes; various types of oscillatory zoning; structures suggesting solution-precipitation; and sector zoning. Crystallization experiments on high-Mg, model Hawaiian basalts at constant cooling rates (15-30 C/hr) reproduce many of these features. It is inferred that P-rich zones in synthetic and natural olivines reflect incorporation of P during rapid olivine growth and that the zoning patterns primarily record crystal-growth-rate variations. In experimental and some natural olivines (e.g., microphenocrysts), Al contents are linearly and positively correlated with both Cr and P, suggesting coupled substitutions. However, in phenocrysts, Cr zoning is often less intense than P zoning, and Al zoning is weak to absent. The premise of this proposal is that elemental maps of P and associated elements in olivines represent a potential archive of information regarding magmatic processes that is largely unknown and therefore untapped. The experimental projects described in this proposal are designed to provide a quantitative understanding of factors controlling development of the zoning and the use of diffusive relaxation to infer time scales of magmatic processes. Specific tasks include: (1) Determining the effects of cooling rate on phosphorus uptake in olivine and constraining the olivine-liquid partition coefficient for phosphorus. (2) Determining the effects of bulk Cr and Al concentrations and fO2 on kinetically driven P partitioning between olivine and silicate melt. (3) Constraining Al, Cr, and P diffusivities in olivine. (4) Further characterization of P, Cr, and Al zoning in natural olivines, with particular emphasis on microphenocrysts from Hawaiian lavas, MORBs, komatiites, high-Al basalts, etc. Broader impacts. Projects described in this proposal meet several NSF criteria for broader impacts: (1) The primary researcher funded by this proposal is a woman, strengthening the NSF mandate to "establish research collaborations with members of underrepresented groups". (2) A graduate student will undertake part of the work described in this proposal thus advancing graduate student education. (3) Results of these projects will be promptly published in the geologic literature and presented at national conferences in order to disseminate the results to a broad audience. (4) Undergraduates from Caltech and other academic institutions continue to do summer research projects in the experimental petrology laboratory - an outreach that exposes students to research in a discipline that is not part of the standard earth science curriculum. These activities are in accordance with the NSF goal of integrating research and education.