ZONING OF PHOSPHORUS AND OTHER MINOR ELEMENTS IN OLIVINE: AN EXPERIMENTAL STUDY

The presence of minor element zoning in olivine, most notably phosphorus, is ubiquitous in extrusive igneous materials as well as some intrusive samples (e.g., Milman-Barris et al. 2008). Zoning patterns include oscillatory and sector zoning and high-concentration olivine cores surrounded by low concentration rims. The origins of P zonation in olivine were investigated through a series of 1 atm dynamic crystallization experiments performed on a variety of melt compositions over a range of cooling rates (1-30°C/hr). Coupled P substitutions involving other minor elements, i.e., Sc and Cr, were also explored as strong correlations with trivalent elements are observed in natural and experimental olivines.

Scandium-bearing experiments suggest that phosphorus substitution into the olivine structure in the tetrahedral site through a coupled reaction charge balanced with trivalent element substitution and octahedral vacancies is the predominant mechanism by which these trace elements are incorporated into olivine. Oxygen fugacity affects substitution behavior when multivalent Cr is part of the equation as Cr²⁺ does not participate in P-Cr coupled substitution.

The results of these experiments support the suggestion that the production of P zonation is related to disequilibrium growth processes and is governed by crystal growth rate, not cooling rate. The development of oscillatory zoning in experimental samples that were subject only to constant rate cooling processes indicates that such zoning in natural samples cannot be ascribed to changing magmatic conditions without additional information. Intrinsic processes in the boundary layer of the olivine lattice can result in oscillatory zoning in an otherwise static magmatic system. Additionally, variations in apparent K_DPolv/melt result in highly variable P uptake behavior over the crystallization interval. This has implications for the modeling of P distribution in magmatic systems.