Paper No. 12
Presentation Time: 9:00 AM-6:30 PM
NO REQUIREMENT FOR NON-PERIDOTITE SOURCES FOR SNAEFELLSNES OFF-AXIS LAVAS, ICELAND
Compositional heterogeneity in the Icelandic mantle can be broadly described as a mixture of a 'depleted' component (high 143Nd/144Nd; low 206Pb/204Pb), sampled by high-degree melts at the rift zones, and more fusible 'enriched' components (low 143Nd/144Nd; high 206Pb/204Pb), best expressed in low-degree melts in the off-axis zones. The HIMU-like trace element and isotopic features of the dominant 'enriched' component is generally attributed to recycled oceanic crust, but the lithology is less well-constrained. Two recent studies of rift zone lavas led to divergent opinions: a positive correlation between 187Os/188Os and Ni-enrichment in olivines suggested a pyroxenite component, while modelling of major element compositions could be explained just through melting of variably fertile peridotite. This study focuses on post-glacial lavas from the Snaefellsness off-axis zone that represent the 'enriched' extreme of Icelandic compositions, dominated by a mantle component with 206Pb/204Pb > 19 and 143Nd/144Nd < 0.5129. The relative abundances of first-row transitional metals in mafic lavas can preserve a signal of lithological heterogeneities in mantle sources, and element ratios can be chosen (e.g. Fe/Mn, Zn/Fe, Co, Fe) that show limited fractionation during peridotite melting but are sensitive to melting of non-peridotite lithologies. Preliminary whole rock Fe/Mn data on representative samples from the Snaefellsnes off-axis zone and the main rift zones have been acquired using a collision-cell ICP-MS method. Variations of Fe/Mn and 187Os/188Os with 143Nd/144Nd are similar, with the Snaefellsnes off-axis lavas extending to slightly lower Fe/Mn and 187Os/188Os than the 'depleted' lavas in the main rift zones. The low Fe/Mn (54-59) of Snaefellsnes lavas indicate a peridotite source (Fe/Mn < 64). PRIMELT-2 modelling of major element compositions is also consistent with a peridotite mantle source. Data on other transition metal ratios (Zn/Fe, Co/Fe) will be determined to confirm this conclusion. In contrast, lavas with the highest Fe/Mn and 187Os/188Os values, indicative of non-peridotite sources, are volumetrically minor and only found in lavas with intermediate 143Nd/144Nd. These data are revealing how the isotopic diversity of the Icelandic mantle is linked to its lithological diversity.