Oxygen isotope heterogeneity and disequilibria of olivine crystals in large volume Holocene basalts from Iceland: Evidence for magmatic digestion and erosion of Pleistocene hyaloclastites

Abstract

This work considers petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland, which are also the largest in the world: Laki (1783–84 AD, 15 km 3), Eldgjá (934 AD, 18 km 3), Veidivötn (900, 1480 AD, multiple eruptions, >2 km 3), Núpahraun (ca. 4000 BP, >1 km 3) and Thjórsárhraun (ca 8000 BP, >20 km 3). We present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals, coexisting individual and bulk plagioclase phenocrysts, and their host basaltic glasses with average precision of better than 0.1‰ (1SD). We also report O isotope analyses of cores and rims of 61 olivine crystals by SIMS with average precision on single spots of 0.24‰ (1SD) in 13 samples coupled with electron microprobe data for major and trace elements in these olivines. Within each individual sample, we have found that basaltic glass is relatively homogeneous with respect to oxygen isotopes, plagioclase phenocrysts exhibit crystal to crystal variability, while individual olivines span from the values in equilibrium with the low-δ 18O matrix glass to those being three permil higher in δ 18O than the equilibrium. Olivine cores with maximum value of 5.2‰ are found in many of these basalts and suggest that the initial magma was equilibrated with normal-δ 18O mantle. No olivines or their intracrystalline domains are found with bulk or spot value higher than those found in MORB olivines. The δ 18O variability of 0.3–3‰ exists for olivine grains from different lavas, and variable core-to-rim oxygen isotopic zoning is present in selected olivine grains. Many olivines in the same sample are not zoned, while a few grains are zoned with respect to oxygen isotopes and exhibit small core-to-core variations in Fe–Mg, Ni, Mn, Ca. Grains that are zoned in both Mg# and δ 18O exhibit positive correlation of these two parameters. Electron microprobe analysis shows that most olivines equilibrated with the transporting melt, and thin Fe-richer rim is present around many grains, regardless of the degree of olivine-melt oxygen isotope disequilibrium. The preservation of isotopic and compositional zoning in selected grains, and subtle to severe Δ 18O (melt-olivine) and Δ 18O (plagioclase-olivine) disequilibria suggests rather short crystal residence times of years to centuries. Synglacially-altered upper crustal, tufaceous hyaloclastites of Pleistocene age serve as a viable source for low-δ 18O values in Holocene basalts through assimilation, mechanical and thermal erosion, and devolatilization of stoped blocks. Cumulates formed in response to cooling during assimilation, and xenocrysts derived from hyaloclastites, contribute to the diverse δ 18O crystalline cargo. The magma plumbing systems under each fissure are likely to include a network of interconnected dikes and sills with high magma flow rates that contribute to the efficacy of magmatic erosion of large quantities (10–60% mass) of hyaloclastites required by isotopic mass balance. Olivine diversity and the pervasive lack of phenocryst-melt oxygen isotopic equilibrium suggest that a common approach of analyzing bulk olivine for oxygen isotopes, as a proxy for the basaltic melt or to infer mantle δ 18O value, needs to proceed with caution. The best approach is to analyze olivine crystals individually and demonstrate their equilibrium with matrix.