P31A-2036
Vent 7504 of the San Francisco Volcanic Field (SFVF), Arizona: Sample Geochemistry and Implications for Cone Formation

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Debra Hurwitz Needham1, Dean B. Eppler2, Jacob E Bleacher3, James A Skinner4, Cynthia A Evans2, Wanda Feng5, John E Gruener2, Peggy A Whitson2, Barbara A Janoiko2 and Stanley A Mertzman6, (1)Oak Ridge Associated Universities Inc., Goddard Space Flight Center, Greenbelt, MD, United States, (2)NASA Johnson Space Center, Houston, TX, United States, (3)NASA GSFC, Greenbelt, MD, United States, (4)US Geological Survey, Flagstaff, AZ, United States, (5)Arizona State University, Tempe, AZ, United States, (6)Franklin and Marshall College, Department of Earth and Environment, Lancaster, PA, United States
Abstract:
Vent 7504 is a complex structure in the SFVF that has 3 unit classes: a central cone with exposed dikes and cinder-covered rheomorphic facies; a SE/NW-trending ridge north of the cone with cinder-covered rheomorphic facies; and three discrete lava flows that emanate to the N from the ridge and to the SW and NW from the cone. Field observations suggest the ridge was the northern crest of an initial, larger cone. The NW portion of this cone was most likely disrupted during a catastrophic breach of lava that had accumulated within the cone; this third of three lava flows carried rafted packages of the rheomorphic cone facies to the NW, forming the linear N ridge. The final phase of pyroclastic activity was concentrated in the SW portion of the original cone, covering the top of the cone with cinders and forming the more traditional conic-shaped construct observed today. This study describes the geochemistry of 9 samples collected from the mapped units (2 from the cone, 1 from the N ridge, 1 from the N lava flow, 2 from the SW lava flow, and 3 from the NW lava flow) to further constrain the formation of Vent 7504.

Geochemical analyses including back-scatter electron scanning electron microscopy and laboratory X-ray fluorescence spectroscopy were conducted on the 9 collected samples to measure bulk rock and olivine phenocryst compositions. Major element concentrations in the bulk rock and olivine compositions are strongly clustered in all samples, indicating they likely originated from a single magmatic source. Bulk rock SiO2 (~47.5 wt%) and alkali (Na2O + K2O, ~2.7 wt% + 0.71 wt%) concentrations are consistent with a basaltic classification for these samples. Trends in major elements relative to MgO are observed for the olivine phenocrysts: SiO2, Al2O3, Na2O, and TiO2 remain constant relative to MgO, but strong linear trends are observed in MnO, FeO, and NiO relative to MgO. These linear trends are expected given the potential for bivalent cation exchanges in the olivine crystal structure. The linear trends noted for the two SW lava flow samples show evidence for fractionation within the flow: samples with high MgO, NiO and low FeO, MnO contents occur close to the vent, whereas samples with low MgO, NiO and high FeO, MnO contents occur far from the vent. A similar trend is seen for the NW flow. Implications for cone formation will be discussed.

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