INVESTIGATION OF ADAKITES FROM MOUNT ST. HELENS BY GEOCHEMICAL ANALYSES OF MELT INCLUSIONS

Adakites are unusual volcanic rocks whose origin is not entirely clear. Senso stricto, adakites represent slab melts that have interacted to varying degrees with the mantle wedge, but the use has often come to unfortunately refer to any volcanic rock with geochemical characteristics such as Sr/Y≥20, Yb≤1.9 ppm, Sr≥400 ppm, SiO₂≥56wt%, etc. (Richards and Kerrick, 2007). Adakites are often considered analogues of ancient tonalite-trondhjemite-granodiorite (TTG) suite plutonic igneous rocks that make up a significant portion of the Archean continental crust from 2.5-4.0 Ga because of their similar geochemistry. Recent evidence suggests that adakites may represent complex mixtures that mayhave a continental crust component as well. This poses a problem to the understanding of TTG-suite formation if adakites do not represent slab melts. This study focuses on investigating adakites that have been defined from the Kalama and Smith Creek periods (Spirit Lake Stage; 3.9-0 ka) of Mount St. Helens (Defant and Drummond, 1993; Smith and Leeman, 1987). The use of melt inclusions and mineral chemistry allow for a better understanding of the igneous processes that took place compared with bulk rock data. Melt inclusions were identified from crystals within these eruptive units and their geochemistry was determined by electron microprobe and laser ablation-inductively coupled plasma mass spectrometry. Melt inclusions were often found within crystal growth zones in the form of melt inclusion assemblages (MIA), which allowed for monitoring of minute changes within the magma chamber as the crystals grew.