8 Tectonics/Structure/Precambrian/Geochemistry/Petrology/Volcanology

Friday, 12 April 2002
8:00 AM - 12:30 PM

LARGE AREA X-RAY MAPS AND COMPLEX ZONATION IN LOWER-CRUSTAL PYROXENE MEGACRYST, MAGNET COVE, ARKANSAS

Paper number: 8-11
Program: Technical Sessions
Time: 11:20 AM – 11:40 AM

Authors

Abstract

High-quality elemental maps of large crystals are difficult to obtain because of their size. Using a JEOL 8600 microprobe and Geller MicroAnalytical software, we are mapping slices of a 7x5x4 cm pyroxene megacryst from a lamprophyre dike near Magnet Cove High School, Arkansas. Images of large areas are made by mosaic mapping, automated stage - beam rastering that produces "tiles" that are then mosaicked to form crystal-scale images. Low magnification (<100X) back-scattered (BE) images are obtained in a single pass. X-ray maps require longer pixel dwell times and >1000X scan magnification in order for wavelength dispersive spectrometers to remain in focus during rastering. 1-cm2 sections require about 16 hours to map 10,000 "tiles". To obtain an X-ray map of a megacryst slice, individual mosaic section maps were mosaicked to produce complete elemental maps (up to 30 cm2). Although time intensive, results are fairly spectacular.

The pyroxene megacryst shows faceted faces, but large-area BE and X-ray maps reveal a glomerocryst. A large euhedral megacryst (ca. 7x3 cm) has overgrown numerous smaller euhedral crystals oriented perpendicular to one prism face. Elemental X-ray maps on four megacryst slices reveal a number of different compositional zonations: (1) marble-cake intergrowths of Mg-rich, Fe-, Ti-, Al-poor and Mg-poor, Fe-, Ti-, Al-rich zones in the smaller crystals, possibly representing initial crustiform growth along a conduit wall; (2) concentric zoning of the megacryst involving increasing Ca, Fe, Ti and Al with decreasing Mg toward its rim during growth; and (3) differences in absolute elemental abundances in different crystallographic sectors of the megacryst, probably reflecting rapid crystal growth. In addition, a more diffuse Ti-Al comb-like transition exists where the megacryst has overgrown the smaller crystals. Thermobarometry using mineral compositions from the enclosing lamprophyre matrix indicate that the glomerocryst may have formed under mid- to lower-crustal conditions. Compositional zonations and asymmetric nature of crystal distribution in the glomerocryst strongly suggest the crystal aggregate originated through sidewall crystallization along a deep-seated fissure, and was subsequently ripped from the conduit wall during ascent of the volatile-rich spessartite magma.