OLIVINE HOSTED MELT INCLUSIONS FROM MAFIC SMALL ERUPTIVE CENTERS, CENTRAL SOUTHERN VOLCANIC ZONE, CHILE

In the central Southern Volcanic Zone (CSVZ) two chemically distinct basalt types have been found and research in one geographical area suggests that the difference arises from distinct melting processes in the mantle wedge with flux-melting for large stratovolcanos and decompression melting of “aged, subduction-modified” wedge for small eruptive centers (SEC, Hickey-Vargas et al., 2002; Sun 2001).

The predominant basaltic magma type erupted is usually, but not always, associated with composite volcanic centers, whereas the second is found exclusively at small eruptive centers, especially those located along the north-south trending Liquine-Ofqui Fault Zone (Lopez-Escobar et al., 1994). The latter is characterized by higher LREE/HREE, lower abundances of fluid-mobile elements and lower Sr / Sr ratios compared with the dominant basalt type (Hickey-Vargas et al., 1989; Lopez Escobar et al., 1995; Sun, 20014; Hickey-Vargas et al., 2002).

In a subduction setting, the overall geochemical profile suggests a process where varying amounts of slab derived hydrous fluids are added to the mantle wedge, which then partially melts to an extent proportional to the amount of fluid added. However, models for continuum of fluid addition and flux-melting are not successful. Hypotheses arising from the study of the Villarrica region will be tested by examining basalts from a field of young SEC located along the Liquine-Ofqui Fault Zone to the east of Osorno and Calbuco volcanos to answer the following questions.

1) Are the LREE-enriched, fluid-mobile element poor SEC basalts formed by decompression melting of aged, subduction modified mantle wedge? 2) What are the conditions of melting to form the contrasting stratovolcano and SEC basalts, specifically the possible ranges of temperature, pressure and volatile contents? 3) Does the pattern of fluid-mobile element abundances in the SEC support an origin by fluid exsolution from their sources?