Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts

Quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity. Scanning electron microscopy cathodoluminescence imaging (SEM-CL) and secondary ion mass spectroscopy (ion microprobe) analysis (SIMS) are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning. Two contrasting crystal morphologies have been studied. Porphyritic euhedral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey, UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1-40 mu m wide), planar, parallel to crystal faces, and overgrow more broadly zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quiescent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing during convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, EI Salvador, northern Chile show broad internal SEM-CL zones with irregular boundaries, some of which are truncated by grain margins indicating dissolution. Crystal rounding may occur by dissolution or melting due to thermal or thermal-chemical disequilibrium with surrounding melt, and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures (strings of amphibole inclusions adjacent to quartz rims). Several episodes of rounding can be recognised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz from both samples shows that Al contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that several factors control luminescence. Although Al substitution for Si in SiO4 tetrahedra may be partly responsible for SEM-CL zoning recorded in volcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role. Copyright (C) 1997 Elsevier Science Ltd.