The oscillatory trace element zoning of augite phenocrysts

Abstract

Oscillatory trace element zoning is described in a sector-zoned augite phenocryst in an alkaline basalt from Lihir Island, Papua New Guinea, using an ion-microprobe. The oscillations of Sc, Ti, V, Cr, Sr and Zr concentrations show large ranges: Cr varies from 91 to 3995 ppm (a factor of 44), Zr from 3.9 to 106 ppm (a factor of 27), V from 81.9 to 623 ppm (a factor of 7.6), Ti from 1229 to 7824 ppm (a factor of 6) and Sr from 88.4 to 288 ppm (a factor of 3.3). These oscillatory variations, observed within 350 microns in a phenocryst, may have occurred in as little as 5 days, and are difficult to reconcile with surface equilibrium-based crystal growth mechanisms. Regular antithetic variations between compatible and incompatible elements suggest that the observed oscillations resemble plagioclase oscillatory zoning and could be interpreted using a reaction-transport kinetics modeling approach.