Long or short silicic magma residence time beneath Hekla volcano, Iceland?

Timescales of magma transfer and differentiation processes can be estimated when the mechanism is known. When conventional major trace element analyses fail to distinguish between various differentiation, isotope compositions useful. Lower Th ratios in silicic relative basaltic magmas at a given volcano could result from storage over period several tens thousands years, or if process was fractional crystallization alone, crustal anatexis on much shorter timescale. Recently mapped bimodal tephra layers Mt. Hekla, Iceland, confirm lower (230Th/232Th) higher Th/U versus mafic magmas. Higher has been taken indicate either apatite fractionation partial melting. In situ analysis enveloping glass andesite, dacite rhyolite undertaken examine its capacity fractionate elements their ratios. Both U are compatible with partition coefficient ratio DU/DTh 1. Hence, separation melt negligible effect Hekla Partial melting hydrothermally altered crust remains preferred for producing beneath Hekla. Ten twenty percent metabasaltic 0.4–1.2 wt% H2O produces 4–6% water. Absence low ?18O values compared rift zones suggests mild hydration crust. Silicic formation, storage, eruption occurred timescale less than few centuries. Decreasing production during Holocene lifetime changes source readjustment system time.