Hot, shallow mantle melting under the Cascades volcanic arc

Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic are in northern California, as d emonstrated by compositional variations observed in high-alumina olivine th oleiites. The lavas studied erupted from seven vents defining a 75-km-long, east-west transect across the are, from near Mount Shasta to east of Medic ine Lake volcano. The increase in melting depth across the are parallels mo deled isotherms in the mantle wedge and does not parallel the inferred dip of the slab, The depth of mantle melting at which the high-alumina olivine tholeiites were created is similar to 36 km at the western end of the trans ect and 66 km at the eastern end. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.