The Journal of Petrology, Volume 38, Issue 11: November 1997.

The origin of highly silicic glass in mantle xenoliths from the Canary Islands

E-R Neumann^* and E Wulff-Pedersen

Mineralogisk-Geologisk Museum, University of Oslo, Sarsgt. 1, N-0562 Oslo, Norway, ^*Corresponding author

Spinel harzburgite, lherzolite, dunite and wehrlite mantle xenoliths from the Canary Islands (La Palma, Hierro, Tenerife and Lanzarote) contain a spectrum of silicate glasses as inclusions in minerals, along grain boundaries, and in interstitial glass pockets. These glasses show a range in composition form basaltic ( 44 wt % SiO₂), to highly silicic, TiO₂-FeO-MgO-CaO-P₂O₅-poor types (up to 71 wt % SiO₂). Glasses in spinel harzburgites and lherzolites are generally silica oversaturated, whereas those in spinel dunites and wehrlites have somewhat lower SiO₂ contents and are generally silica undersaturated. Glasses in xenoliths from La Palma and Tenerife are rich in K₂O compared with those from Hierrro and Lanzarote. Daughter minerals coexisting with highly silicic glass in polyphase inclusions are similar in composition to the main phases in the host xenoliths (Of>90, Cr-diopside, chromite), whereas those in less silicic glasses are richer in Al₂O₃, TiO₂ and FeO, and poorer in MgO. The systematic relations found to exist between glass composition, mineralogy of the host xenolith and locality (island) cannot reflect random variations in the geochemistry of 'exotic' melts infiltrating the mantle lithosphere, but instead suggest a cogenetic relationship between the melts and their mantle host xenoliths. The silicic glasses are interpreted as the products of reactions at 8-12 kbar between infiltrating alkali basaltic magmas and peridotitic wall-rocks which, in orthopyroxene-bearing rock-types, involves formation of silicic melt+olivine at the expense of orthopyroxene. In xenoliths from La Palma and Tenerife, where interstitial phlogopite is commonly present, phlogopite has been partly or totally consumed by the reactions between relatively mafic melts and peridotite, giving rise to silicic glasses with high K₂O contents and K₂O/Na₂O ratios. The low K₂O concentrations and K₂O/Na₂O ratios in glasses in anhydrous xenoliths suites from Hierro and Lanzarote are believed to result for reactions between infiltrating melts and anhydrous and/or amphibole-bearing mantle wall-rocks. The silicic melts appear to have been mobile over distances exceeding the diameter of a xenolith, that is, at least 20-30 cm.

Key words: Canary Islands; silicic glass inclusions; mantle xenoliths; melt-wall-rock reactions

Pages 1513-1539

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