A LEAKY TRANSFORM IN SOUTHERN ICELAND? OIB-LIKE MAGMA ON THE REYKJANES PENINSULA

Iceland is a remarkably active volcanic province straddling the Mid-Atlantic Ridge. The origin of Icelandic magma is complicated by interaction between a mantle plume and the ridge itself. The Brennisteinsfjöll volcanic system is the easternmost and least studied volcanic system on the Reykjanes Peninsula (RP). To the west, the transtensional RP goes offshore transitioning to the oceanic Reykjanes Ridge. To the east is the South Iceland Seismic Zone (SISZ), a series of bookshelf strike-slip faults that extend eastward to the Eastern Volcanic Zone (EVZ). This study investigates Thríhnúkar, a small fissure system in the Brennisteinsfjöll volcanic system, to address magma dynamics at this complex geodynamic junction. The youngest eruption constructed the Holocene basaltic cone of Thríhnúkagígur with a 120 m deep cave below the summit, exposing the magmatic plumbing system in the shallow crust and an older unconsolidated tephra cone at the base of the cave from a previous interglacial period.

Whole rock major and trace elements and Sr, Nd, and Pb isotope ratios are presented for a suite of vertically co-located lava and tephra samples that span at least two interglacial periods. The data span most of the range of reported mafic compositions in Iceland, ranging from more primitive compositions in parts of the Holocene plumbing system to slightly more evolved compositions in the buried tephras from the previous interglacial period (47.3-49.5 wt% SiO₂ and 1.22-2.36 wt% TiO₂, Mg# of 47-61). Major elements plotted against Mg# display linear trends, as do most trace element ratios. The linear relationships of these volcanics suggest that these magmas have a common, long-lived source that evolves by similar processes through time.

Icelandic magmas can be broadly characterized by the relative contributions of OIB-like and MORB-like components. Trace element ratios including Sr/Nd, Zr/Y and Nb/Y show that the Thríhnúkar volcanics have more of an OIB-like component relative to other lavas to the west on the RP and a greater affinity to lavas in the EVZ, closer to the Icelandic plume. This is also reflected in the ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios. These results suggest that the RP acts as a 'leaky transform' erupting magmas derived largely from the Icelandic plume rather than a depleted MORB-like mantle.