REACTIONS BETWEEN MANTLE XENOLITHS AND HOST MAGMA BENEATH LA PALMA (CANARY-ISLANDS) - CONSTRAINTS ON MAGMA ASCENT RATES AND CRUSTAL RESERVOIRS

Spinel-bearing peridotitic from the 1949 eruption on La Palma were mod ified mineralogically and chemically during prolonged reaction with th eir host magma. The magmatism that brought the peridotites to the surf ace caused two distinct generations of xenolith fractures: (1) Old fra ctures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompani ed by 0.9-2 mm wide diffusion zones where peridotite olivine became le ss forsteritic through diffusive exchange with the host magma. Old fra ctures represent most of each xenolith's surface. (3) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Ca lculations based on a model of Fe-Mg interdiffusion give an age of 6 t o 83 years and <4 days for old and young fractures, respectively. A co mbination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than repres enting wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, c onstraining a multistage magma ascent: Years to decades prior to erupt ion, ascending magma ruptured peridotitic wall-rock possibly through h ydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exc eeding 0.2 ms(-1). The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7-11 km depth. T he xenoliths became deposited and subsequently embedded in a mush of s ettled phenocrysts, while selvages and veins crystallized until the er uption commenced. At the end of the eruption, the xenoliths were final ly transported to the surface within hours to days. Decompression duri ng the rapid ascent induced internal stresses and caused renewed fragm entation of the xenoliths, producing the young fractures.