OLIVINE: UNEARTHING MAGMA EVOLUTION AT SPRINGERVILLE VOLCANIC FIELD, ARIZONA

Springerville volcanic field is the third largest monogenetic field in the United States, located in eastern Arizona at the southern margin of the Colorado Plateau. It consists of about 400 vents in a 1,200 sq mi radius. Volcanic activity occurred approximately 3.08-0.75 Ma and during this time Springerville’s locus of active volcanism moved from west to east, coincident with lava composition shifting from tholeiitic to alkali-olivine basalt.

Our goal is to combine whole rock and varied trace element geochemistry to interpret magma evolution and potential variations in mantle source for Springerville volcanism. We analyzed olivine from nine samples on a JEOL 8500F field emission electron microprobe for major and trace (Mn, Co, Ni, Ca, Al) elements. These samples include six different basalt types (based on variations in mineralogy) found within the Springerville volcanic field. Group 1 (olivine-rich) has a range of forsterite (Fo) from 85.5-49.5 mol % and trace elements varying from 8418-1378 ppm Mn, 5987-1335 ppm Ca, and 1843-72 ppm Ni. Trace element abundances are generally lower in Group 2 (picritic) although Fo contents are higher (87.6-64.2 Fo mol %). Group 3 (plagioclase-olivine) and Group 4 (hornblende-olivine) are generally more evolved with lower Ni and Fo values (less than Fo 82). Groups 5 (pyroxene-olivine) and 6 (diktytaxitic) have similar trace element compositions to the other groups but generally less variability in forsterite (Fo 83-69).

Olivine phenocrysts are significant probes of parental melt composition for two reasons. First, olivine is often the first phase to crystallize at low pressures in most mantle delivered magmas. Second, olivine crystallizes over a range of temperatures. Therefore olivine compositions may be used to track magma evolution. Sobolev et al. 2007 previously estimated the mixing proportions of melts delivered from different geodynamic settings and found that trace element abundances in olivine can be utilized to quantify variations in mantle composition (peridotite versus pyroxenite). Thus variations in olivine composition in conjunction with whole rock data may indicate changes in mantle sources and different magma evolution prior to eruption.