The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

Abstract

We investigate the relative influence of mantle upwelling velocity and source heterogeneity on the melting rates recorded by 230 230 Th excesses and distance from the inferred plume centre is consistent with a model of decreasing mantle upwelling velocity with increasing distance from the plume axis. However, the model is not substantiated by the ( 231 Pa/ 235 U) data as the correlation with distance from the plume centre is weak. On the scale of individual eruption centres, the observed U-series are influenced by variations in melt transport time, source porosity, and local variations in mantle upwelling velocity. Broad correlations between ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) and highly incompatible trace element ratios for samples from the Western Volcanic Zone provide, however, evidence for a significant underlying effect of source heterogeneity on the U-series data. Low 230 Th and 231 Pa excesses in enriched samples from the Western Volcanic Zone with high U/Th, Nb/U and Nb/La indicate that partial melts from an enriched source component, characterised by high melt productivity but low bulk D U /D Th , influence the U-series systematics of the erupted melts. These results re-affirm the presence of comparatively larger abundances of enriched material in the mantle source beneath the South Western Rift of Iceland, which has been suggested based on relationships between highly incompatible element and Pb isotope ratios in Icelandic basalts. Overall, our results highlight the importance of lithological heterogeneity on the melting behaviour of the upper mantle and the composition of oceanic basalts.

Timescales of storage and recycling of crystal mush at Krafla Volcano, Iceland

“…Concentrations of U, Th, Ra and Ba for whole rock and groundmass are within the range of data from previous U-series studies of Icelandic basalts (Condomines, et al 1981;Hemond, et al 1988a;Kokfelt, et al 2003;Kokfelt, et al 2009;Koornneef, et al 2012a;Sigmarsson 1996;Sigmarsson, et al 1992a;Sigmarsson, et al 1992b;Sigmarsson, et al 1991;Zellmer, et al 2008 (Zellmer, et al 2008). All plagioclase separates analyzed in this study had ( 226 Ra)/( 230 Th) ratios for whole rock samples in this study were both 1.05 (Table 2; Figure 6b), again similar to previous results for Icelandic basalts (Bindeman, et al 2006;Kokfelt, et al 2003;Kokfelt, et al 2009;Sigmarsson 1996;Zellmer, et al 2008 (Blundy and Wood 2003a;Fabbrizio, et al 2009).…”

“…Note that for Population 03-2 laser data, Sm concentrations were below detection, so the value shown for Sm (the minimum value measured by laser-ablation in any of these samples) is an estimate of the maximum concentration. Data in the GeoRoc compilation for panel (a) are from the following references: (Arnorsson and Oskarsson 2007;Condomines, et al 1983;Condomines, et al 1981;David, et al 2000;Fitton, et al 2003;Grönvold and Mäkipää 1978;Hemond, et al 1993;Kempton, et al 2000;Kokfelt, et al 2003;Kokfelt, et al 2009;Koornneef, et al 2012a;Koornneef, et al 2012b;Kurz, et al 1985;Maclennan, et al 2001;Martin and Sigmarsson 2010;Nichols, et al 2002;Nicholson, et al 1991;Nicholson and Latin 1992;Sigvaldason and Oskarsson 1986;Stracke, et al 2003a;Stracke, et al 2003b;Sveinbjornsdottir, et al 1986;Tera, et al 1986;Thirlwall, et al 2004;Torssander 1988;White and Hochella 1992;Wood, et al 1979) Figure 4: Barium concentrations (in ppm) vs. anorthite concentrations (in mol % An) in plagioclase populations for Krafla samples 03KRA01 and 03KRA03. Concentrations of Ba were measured by laser-ablation ICP-MS. Anorthite content for the same spots was measured by electron microprobe for spots where Ba analyses were conducted at UCD and by laser-ablation ICP-MS where Ba analyses were conducted at OSU (see Methods for details).…”

“…A novel aspect of this study, however, is that we can 1.19, with higher activity ratios associated with basaltic compositions and lower activity ratios associated with dacites and rhyolites (Condomines, et al 1981;Hemond, et al 1988a;Kokfelt, et al 2003;Kokfelt, et al 2009;Koornneef, et al 2012a;Sigmarsson 1996;Sigmarsson, et al 1992a;Sigmarsson, et al 1992b;Sigmarsson, et al 1991;Zellmer, et al 2008 Th disequilibria will decay to secular equilibrium within ~8-10 kyr (depending on the magnitude of the initial disequilibrium), at least some part of the crystals in the measured bulk separates must have crystallized recently. Thus, the U-series results taken together indicate a maximum (average) age of ~8-9 kyr for the plagioclase crystals scavenged from the mush and therefore a relatively short time frame for generation and storage of the crystal mush beneath Krafla.…”

Simplified mantle architecture and distribution of radiogenic power

“…In order to consider the impact of a pyroxenetic source lithology, we can derive a bulk partition coefficient of D sol/liq (Th) = 0.005 ± 0.002 (2 σ m ) based on melting experiments of MORB‐like pyroxenite and eclogite at 2.9–3.1 GPa [ Klemme et al ., ; Pertermann et al ., ] (see supporting information). This exercise yields a minor deviation from our baseline that trivially affects our inverse source model and corroborates other studies that have shown that the bulk partitioning behavior of Th (and U) does not vary significantly between peridotitic and pyroxenitic sources [e.g., Stracke et al ., ; Prytulak and Elliott , ; Koornneef et al ., ]. However, pyroxentic source components have been shown to melt with a much higher production rate and at greater pressures/depths than ambient mantle peridotite [ Yaxley , ; Hirschmann et al ., ; Pertermann and Hirschmann , ].…”

Exploring the role of mantle eclogite at mid-ocean ridges and hotspots: U-series constraints on Jan Mayen Island and the Kolbeinsey Ridge

“…Iceland (Koornneef et al, 2012)), age-constrained 230 Th-226 Ra disequilibria are relatively low (closer to secular equilibrium) for the Jan Mayen region, and are notably lower at Jan Mayen Island than for the NKR. If they record the primary melt generation process, these ( 226 Ra/ 230 Th) values nearer secular equilibrium could suggest relatively high melting rates, possibly due to the presence of a more fusible lithology like eclogite.…”