Skovgaard Anna C.

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Skovgaard
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Anna C.
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  • Preprint
    Compositional characteristics and spatial distribution of enriched Icelandic mantle components
    ( 2010-03-16) Peate, David W. ; Breddam, Kreston ; Baker, Joel A. ; Kurz, Mark D. ; Barker, Abigail K. ; Prestvik, Tore ; Grassineau, Nathalie ; Skovgaard, Anna C.
    We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank zone regions in Iceland (Vestmannaeyjar in the south, Snæfell in the east, and Snæfellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr-Nd-Pb-Hf), stable isotope data (δ18O), and trace element data to characterise the compositional features of the trace-elementenriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones (including Öræfajökull in the south-east) and the Northern and Eastern rift zones. Most Icelandic lavas have negative Δ207Pb (–4 to –1), with higher values (–1 to +4) found only in samples from Öræfajökull, Snæfell, and parts of the Reykjanes Peninsula. At Snæfell, this EM1-type component is characterised by a low δ18Oolivine signature (+4.1‰ to +4.6‰), moderate 206Pb/204Pb values (18.4-18.6) and MORB-like 3He/4He (6.9-7.5 R/RA). Samples from Vestmannaeyjar and Snæfellsnes have mantle-like δ18Oolivine (+4.9‰ to +5.0‰), and radiogenic 206Pb/204Pb values (18.9-19.3) that fall on the NHRL for 208Pb/204Pb (Δ208Pb –5 to +5). Compared to the Vestmannaeyjar lavas, Snæfellsnes lavas have higher La/YbN (5-11 vs. 3-5), lower εNd (5.5-6.5 vs. 6.8-7.6) and lower 3He/4He (6.3-8.6 R/RA vs. 11.4-13.5 R/RA). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3He/4He values (> 15 R/RA) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high Δ207Pb samples, high-precision Pb isotope data produce a kinked array on an 206Pb/204Pb vs. 208Pb/204Pb plot, which is not consistent with simple binary mixing between two end-members. This requires significant lateral heterogeneity within the Icelandic mantle and the presence of more than just two compositionally-distinct local mixing end-member components. Samples from each of the main axial rift zones define different trends. Despite the tectonic continuity between the Northern Volcanic Zone and the Eastern Volcanic Zone, lavas from these two rift zones define separate sub-parallel linear arrays. Lavas from the adjacent Western Volcanic Zone and the Eastern Volcanic Zone define oblique linear arrays that converge on a common local end-member that is not involved in the magmatism of the Northern Volcanic Zone. Therefore, there is a distinct NE-SW compositional heterogeneity within the Icelandic mantle.
  • Article
    Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high 3He/4He
    (American Geophysical Union, 2008-04-17) Jackson, Matthew G. ; Hart, Stanley R. ; Saal, Alberto E. ; Shimizu, Nobumichi ; Kurz, Mark D. ; Blusztajn, Jerzy S. ; Skovgaard, Anna C.
    We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high 3He/4He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with 3He/4He in a data set of global OIBs, demonstrating that a major element signature is associated with the high 3He/4He mantle. Additionally, we find that OIBs with high 3He/4He ratios have moderately radiogenic 187Os/188Os (>0.135). The TITAN enrichment and radiogenic 187Os/188Os in high 3He/4He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high 3He/4He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high 3He/4He mantle domain.