RATES OF MAGMA ASCENT AND DEPTHS OF MAGMA RESERVOIRS BENEATH LA PALMA(CANARY-ISLANDS)

Peridotitic mantle xenoliths from historic and prehistoric eruptions o n La Palma show many similarities. Prolonged reactions of the xenolith s with their host magmas have been used to place constraints on the ma gma transport system beneath the island. All xenoliths show crystallin e selvages and 0.9-2.6 mm wide diffusion zones in olivine along most: of their surface. Diffusion kinetics in olivine, combined with fluid i nclusion barometry, document that selvages and diffusion zones formed at crustal levels within 8 to about 100 years. Some xenolith fractures lack selvages and were in contact with the host magma for less than 4 days. A multistage magma ascent is proposed: (i) peridotite wall rock was fragmented and became incorporated into the ascending magma years to decades prior to the eruption; (ii) the xenoliths were rapidly tra nsported to, and deposited in, crustal magma reservoirs, forming selva ges and diffusion zones at the xenolith rims; (iii) renewed fragmentat ion of the xenoliths occurred days to hours prior to eruption, possibl y by decompressive strain fracturing during rapid ascent.