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

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.