MANTLE COMPOSITION OF THE ZUNI BANDERA VOLCANIC FIELD DETERMINED BY ZN/FE RATIOS OF IN-SITU OLIVINE

The mechanism driving the formation of isolated, generally mafic volcanic fields on the margins of the Colorado Plateau is a topic of extensive debate. Most models that deal with the subject assume a homogenous peridotite mantle source composition, but evidence from mantle xenoliths suggests a heterogeneous mantle composition underlying some areas. This has significant impacts on the physical properties assumed in models of Colorado Plateau uplift.

Recently, ratios of Zn, Mn, and Fe in mafic volcanics have been developed as proxies for tracing major element heterogeneities in the mantle. Zn/Fe ratios are especially promising proxies; Zn and Fe do not fractionate differently between olivine, orthopyroxene, and melt but fractionate strongly when garnet or clinopyroxene are the dominant minerals of the source. Sources containing garnet or clinopyroxene are characterized by high Zn/Fe ratios. Using this proxy on mafic volcanic fields of the Colorado Plateau offers a better understanding of mantle heterogeneities in these locations.

The Zuni Bandera Volcanic Field (ZBVF) is a mafic field located on the eastern margin of the Colorado Plateau, near the Rio Grande Rift and resting directly on the Jemez Lineament. Basalts from this field are both alkaline and tholeiitic, most likely originating from garnet- (aesthenospheric) and spinel-facies (lithospheric) depths respectively. The ZBVF is an ideal location to test the Zn/Fe proxy because of its proximity to Mt. Taylor, a location just north of the field where mantle xenoliths can be found. Samples were collected based off a GIS-produced map of the geology of the ZBVF that incorporates previously published geochemical data, showing Mg content of specific flows as a proxy for olivine content. Zn/Fe ratios will be determined by ongoing analyses of in-situ olivine crystals from highly mafic flows in the ZBVF using laser-ablation-ICP-MS. Zn/Fe ratios of olivine from Mt. Taylor mantle xenoliths will be used to confirm mantle compositions determined by ZBVF Zn/Fe ratios of olivine. Results from this ongoing analysis will not only test the accuracy of the relatively new Zn/Fe proxy for mantle composition, but will confirm the composition of the mantle beneath the Colorado Plateau in this location.