Geochemical variability in a single flow from northern Iceland

Abstract

[1] Compositional variability is present for almost every element analyzed in 70 whole rock samples and 40 olivine-hosted melt inclusions from the Borgarhraun lava flow in NE Iceland. The variation of incompatible element concentrations can be produced by incomplete mixing of fractional melts, while the compatible element variations in the whole rock samples can be explained by addition/removal of crystals found in the flow. The melt inclusion incompatible element compositions are more variable than the whole rock compositions, and the magnitude of variability of the whole rock samples can be matched if each hand specimen is made of magma that was formed by mixing of 20-30 batches of melt with the composition of the inclusions. Clinopyroxene barometry and the major element composition of the whole rock samples suggest that mixing took place at pressures of $0.9 GPa in sub-Moho magma chambers. The spatial distribution of the concentrations of incompatible elements is not random in Borgarhraun and samples with a separation of <4 km have concentrations that are more similar than expected from a random distribution of geochemistry. This separation distance corresponds to a lava volume of 0.014-0.14 km 3 . This volume may be controlled by magma mixing and episodic eruption of melts from a sub-Moho chamber. The concentration variations of compatible elements with position in the flow are little different to those expected for random variation. It is likely that the crystal removal and addition processes that control major element compositions generate variability on short length scales.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

“…[39] Furthermore, recent 3-D seismic and tomographic imaging of the 9°30′N-9°50′N region show that AMC segmentation is evident at even smaller (∼5 to 10 km) spatial scales [Toomey et al, 1990;Carbotte et al, 2008Carbotte et al, , 2009 and ≤1 km-scale geochemical variability has been reported for extensive lava fields along other segments of the EPR [Perfit et al, 1994a;Hall and Sinton, 1996;Smith et al, 2001;Castillo et al, 2002;Sinton et al, 2002;Bergmanis et al, 2007;Geshi et al, 2007], the Juan de Fuca Ridge [Embley et al, 2000;Stakes et al, 2006], the Mid-Atlantic Ridge [Machado et al, 1982], and on Iceland [Maclennan et al, 2003]. These studies challenge the view that individual ocean ridge magma chambers effectively homogenize magma injected from compositionally variable parts of the mantle [e.g., Fornari et al, 1988;Sinton and Detrick, 1992;Niu and Batiza, 1997] and indicate that this heterogeneity can be petrologically evident in a suite of lavas from one eruption, particularly if sampled with sufficient density [e.g., Rubin et al, 2009].…”

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

“…The difference in residence time between the parent and daughter nuclides causes in-growth of the daughter nuclide when the parent nuclide is retained preferentially in the solid residue during partial melting. Since the Iceland lavas show evidence for compositional variation created within melt channels (Maclennan et al, 2007;Stracke et al, 2003b;Maclennan, 2008b;Stracke and Bourdon, 2009;Koornneef et al, 2012), a dynamic melting model where melts do not re-equilibrate with their surrounding matrix appears most appropriate for interpreting the U-series data in Icelandic rocks Kokfelt et al, 2003;Bourdon et al, 2005Bourdon et al, , 2006Prytulak and Elliott, 2009;Stracke et al, 2003aStracke et al, , 2006Stracke and Bourdon, 2009). Moreover, recent experiments show that melts form interconnected melt networks at small degrees of melting (Zhu et al, 2011), suggesting that channelized melt transport may already be established during the initial stages of partial melt-ing.…”

“…As a result, local variability in 230 Th and 231 Pa excesses in erupted lavas integrating over variable depth ranges of the melting region would be expected. Large variability in trace element ratios that are sensitive to the degree of melting, e.g., La/Sm and La/Yb, in Icelandic lavas indicate that melts are indeed extracted from, and integrate over, different depths of the melting region (Wood, 1981;Elliott et al, 1991;Maclennan et al, 2003Maclennan et al, , 2007Stracke et al, 2003b;Kokfelt et al, 2006;Koornneef et al, 2012). The lack of correlation between La/Sm and ( 230 Th/ 238 U) or ( 231 Pa/ 235 U), however, confirms that the U-series disequilibria, due to the incompatibility of the U-series nuclides, are little influenced by progressive melting, and are primarily established during the initial stages of melting.…”

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

“…have signal-to-noise ratios (σ t /σ r , where σ t is the true variability within a sample set and σ r is an estimate of analytical error (Maclennan et al 2003)) in excess of 1.71 (Fig. 6).…”