CATHODOLUMINESCENCE AND TRACE-ELEMENT ZONING IN QUARTZ PHENOCRYSTS AND XENOCRYSTS

Quartz phenocrysts and xenocrysts are a common component of volcanic r ocks but rarely contribute to our understanding of magmatic or eruptiv e processes due to an apparent internal chemical and structural homoge neity. Scanning electron microscopy cathodoluminescence imaging (SEM-C L) 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. Tw o contrasting crystal morphologies have been studied. Porphyritic euhe dral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey , UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1-40 m u m wide), planar, parallel to crystal faces, and overgrow more broadl y zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quies cent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing d uring convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, EI Salvador, northe rn Chile show broad internal SEM-CL zones with irregular boundaries, s ome of which are truncated by grain margins indicating dissolution. Cr ystal rounding may occur by dissolution or melting due to thermal or t hermal-chemical disequilibrium with surrounding melt, and embayments b y gas bubble enhanced convective dissolution are consistent with the d evelopment of incipient ocelli textures (strings of amphibole inclusio ns adjacent to quartz rims). Several episodes of rounding can be recog nised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz fro m both samples shows that Al contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that severa l factors control luminescence. Although Al substitution for Si in SiO 4 tetrahedra may be partly responsible for SEM-CL zoning recorded in v olcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role. Copyright (C) 1997 Elsevier Science Ltd.