6. TRACE AND RARE EARTH ELEMENT CHEMISTRY OF VOLCANIC ASHES FROM SITES 918 AND 919: IMPLICATIONS FOR ICELANDIC VOLCANISM1Peter D. Clift2 and J. Godfrey Fitton3 |
ABSTRACTMiocene to Holocene sediments cored at Sites 918 and 919 in the Irminger Basin contain windblown ash layers derived from Icelandic volcanoes. They represent the most explosive volcanism of the rift system. In contrast, only those eruptions that produced lava flowing far from the rift zone are now exposed on Iceland due to subsidence close to the rift axis. Sediments older than 3.2 Ma contain little fresh glass, due to the hydrating effect of diagenetic fluids, although one basaltic layer dated at 10.5 Ma and three rhyolitic layers dated at 12.1–12.7 Ma were found to contain fresh glass. Major, trace, and rare earth element data have been collected from aphyric glass shards from these ash layers, using a combination of electron and ion microprobe technology. The ashes are dominantly basaltic and tholeiitic in character and are principally derived from the rift zone, with minor input from off-axis sources, probably the Snaefellsnes Peninsula. Their trace element characteristics are consistent with derivation from a mantle source similar to that below Iceland today, although the degree of this enrichment, as modeled by Zr/ Nb, can be seen to be variable over short periods of time and to have greater range than that found in exposed Icelandic lavas of the same age. The variations are not attributable to fractional crystallization. The ashes indicate that either different volcanoes were erupting compositionally variable lavas at any one time or that the composition varied rapidly with time, or both. These variations are homogenized by magma mixing prior to eruption in the large volume flows preserved in the Tertiary lava flows exposed in Iceland. Chemical variation seen in the ash layers may be due to variation in the degree of mantle melting, mantle heterogeneity, or derivation of melt from different depths within the melting column. Melting is thought to have commenced within a garnet-bearing mantle and continued in a spinel-bearing zone. |
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