Abstract
We have studied glass-bearing leucocratic (granitic to Qz-monzonitic) crustal xenoliths from the Tindfjoll Pleistocene volcanic complex, SW Iceland. The xenoliths consist of strongly resorbed relicts of anorthitic plagioclase, K-rich feldspar and rounded quartz in color-less through pale to dark-brown interstitial glass. Spongy clinopyroxene and/or rounded or elongated crystals of orthopyroxene are in subordinate amount. Magnetite, ilmenite, zircon, apatite, allanite and/or chevkinite are accessory minerals. The xenoliths more likely are relicts of earlier-formed, partially melted Si-rich rocks or quartz-feldspar-rich crystal segregations, which suffered latter interaction with hotter and more primitive magma(s). Icelandic lavas are typically low in delta O-18 compared to mantle-derived, "MORB"-like rocks (similar to 5.6 +/- 0.2 %), likely due to their interaction with, or contamination by, the upper-crustal rocks affected by rain and glacial melt waters. Surprisingly, many quartz and feldspar crystals and associated colorless to light-colored interstitial glasses of the studied xenoliths are not low but high in delta O-18 (5.1-7.2 %, excluding three dark-brown glasses of 4-5 %). The xenoliths contain abundant, low-to high-delta O-18 (2.4-6.3 %) young zircons (U-Pb age 0.2-0.27 +/- 0.03 Ma; U-Th age 0.16 +/- 0.07 Ma), most of them in oxygen isotope equilibrium with interstitial glasses. The delta O-18 values >5.6 % recorded in the coexisting zircon, quartz, feldspar and colorless interstitial glass suggest crystallization from melts produced by fusion of crustal rocks altered by sea-water, also reflecting multiple melting and crystallization events. This suggests that "normal"-delta O-18 silicic magmas may not be ultimately produced by crystallization of mafic, basaltic magmas. Instead, our new single-crystal laser fluorination and ion microprobe O-isotope data suggest addition of diverse partial crustal melts, probably originated from variously altered and preconditioned crust.
