Paper No. 51-2
Presentation Time: 1:30 PM-5:30 PM
OPEN SYSTEM PROCESSES RECORDED BY PLAGIOCLASE DISEQUILIBRIUM, LASCAR VOLCANO, CENTRAL ANDES VOLCANIC ZONE, CHILE
VAN HORN, Bennett1, MICHELFELDER, Gary2, BUCHELI-OLAYA, Cesar1 and SHAULIS, Barry3, (1)Department of Geography, Geology, and Planning, Missouri State University, 901 S. National Ave, Springfield, MO 65897, (2)Department of Geography, Geology, and Planning, Missouri State University, 901 S. National Ave., Springfield, MO 65897, (3)University of Arkansas Stable Isotope Laboratory, University of Arkansas, Fayetteville, AR 72701
Lascar Volcano (Volcán Láscar) (5592 m, 23°22’S, 67°44’W) is located in the Central Andes Volcanic Zone (CVZ) of northern Chile and is considered the most active volcano in the CVZ. Historic activity dates to 1848 and has involved mostly dome growth and vulcanian eruptions with one large plinian eruption in April 1993. Here, we ascertain the evolution of the magma plumbing system through focused geochemical study to determine the level of crustal contamination, storage and crystal residence times, and magma mixing. We use plagioclase hosted in pumice from the April 1993 plinian eruption and compare these data to the Soncor Ignimbrite (26.5 ka), the Tumbres eruption (9.3 ka), the Tumbres- Talabre Lava (7.1 ka), and the Piedras Grandes unit (>26.5 ka). These data will allow for the analysis of trace element contents of modern and historical eruptions and for the identification of changes within the magma bodies underlying Lascar by using eruptive units as a proxy.
Spot analyses and line transects from plagioclase and pyroxene crystals were collected using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Within plagioclase crystals from the 1993 eruption, Sr content ranges from 806 to 1775 ppm and Ba ranges from 158 to 688 ppm. Contents vary significantly between individual crystals and are not isolated to crystal cores or rims but appear to fluctuate across each crystal.
Plagioclase and pyroxene trace element data suggests the maturity of Lascar Volcano has evolved through time. Modeling of these data from the plagioclase crystals has revealed that there is either mixing between crystal populations or evolution of the chemical compositions of crystals occurring within the system. The Tumbres- Talabre lava and the April 1993 plinian eruption appear as distinct endmembers with the Tumbres eruption representing a mix between these two populations. Plagioclase crystals of the Soncor ignimbrite is suggested to be unrelated to the process affecting the other eruptions or present as a distinct population.
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