Journal of Petrology | Volume 38 | Number 11 | Pages 1513-1539 | 1997
© Oxford University Press 1997
The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands
Mineralogisk-Geologisk Museum, University of Oslo, Sarsgt. 1, N-0562 Oslo, Norway
Received April 5, 1997; Revised typescript accepted July 1, 1997
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Abstract |
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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 (Fo>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
* Corresponding author
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