Philipp Ruprecht - Magma Chamber Dynamics

MFIX

RESEARCH

...Magma Mixing Recorded in Phenocryst Phases at Volcán Quizapu, Chile

...Phenocryst Zonation Patterns at El Misti Volcano, Peru, and the Andahua Monogenetic Cones, Peru

Phenocryst Zonation Patterns at El Misti Volcano, Peru, and the Andahua Monogenetic Cones, Peru

El Misti Volcano (5822 m) is located only 20 km away from Arequipa (2414 m), the second largest city in Peru. El Misti’s volcanic activity is characterized by alternations of effusive and explosive eruptions as well as avalanche deposits, wheras debris flows have been channelized mainly towards the city of Arequipa. Recent and historic eruptions as well as fumarolic activity indicate that the magma system is active and new volcanic events may occur in the future. The volcano is mainly comprised of porphyritic andesite lavas. Tephra is only occasionally exposed in valley section. Within the crater a small plug with fumarolic activity has grown.

The Andahua Volcanic Field (no foto) is a Holocene to historic scoria and lava field extending over an area of about 50 km x 30 km ENE of Nevados de Coropuna complex of stratovolcanoes to the northwest of El Misti volcano. The field is situated inside the Andahua Valley. Cinder cones and lava flows are produced by strombolian eruptions.

Detailed microprobe work was performed on phenocrysts of 20 samples from El Misti volcano. I focussed on plagioclase zonation pattern recorded in major elements (Anorthite content) and trace elements (e.g. Fe and Mg). Fe-An data was also obtained for three Andahua samples.

Three typical plagioclase phenocrysts can be seen to the right (click images to enlarge). MIS-02-05_Plg12 is a backscattered-electron (BSE) image of a phenocryst from a normal lava erupted at El Misti. The other two microprobe BSE images show an El Misti plug phenocryst (MIS-02-109_Plg1) and a phenocryst from an Andahua cinder cone (AND01-I-02).

These phenocrysts can be divided into two groups. All plagioclase phenocrysts from the normal El Misti lavas show intricate zonation features indicating a complex growth history (i). On the other hand plagioclase phenocrysts from small volume eruptions (El Misti plug and Andahua Cinder Cones) have simple growth histories with albite-rich cores overgrown by anorthite-rich rims.

Combining this textural information based on An variations during crystal growth with trace element data can reveal valuable information within a magmatic system to explore the dominance of closed- vs. open-system processes. The plot below shows the relation between Fe as well as Mg with varying An content in the investigated plagioclase phenocrysts.

Large and correlated Anorthite - Fe (or An-Mg) contrasts at resorption zones identify compositional mixing related to recharge. Subhorizontal An-Fe variations record thermal or water-induced effects. El Misti volcano, a large and mature stratovolcano, is dominated by closed-system effects during upper crustal magma evolution. The small volume erupted at El Misti plug, however, is affected by strong open-system effects.