Mineral diffusive and ^(226)Ra/^(230)Th timescales for the genesis of Icelandic basalts: Laki and the Grimsvötn magma system

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Bindeman, I., Sigmarsson, O., Eiler, J., Wang, Z.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2004
• Subjects: Laki; Iceland
• Online Access: https://authors.library.caltech.edu/39382/; https://resolver.caltech.edu/CaltechAUTHORS:20130716-082223687

Many Icelandic magmas are 1-3‰ lower in δ^(18)O than typical terrestrial basalts. We report oxygen isotope and trace element analyses in individual grains and bulk separates of olivine, plagioclase, and clinopyroxene phenocrysts and of basaltic glass from the1783-4 fissure eruption of Lakagigar (Laki)—the largest historic basaltic eruption—and from subsequent, smaller volume 20th century ashes from the same magma system (the subglacial Grimsvötn caldera). Previously and newly analyzed ash and lava samples of Laki basalts are homogeneous in δ^(18)O =3.1±0.1‰; 1996 and 1998 basaltic ashes are also homogeneous though slightly lower in δ^(18)O =2.9±0.1‰. In contrast, we find extreme heterogeneity in δ^(18)O in phenocrysts and disequilibrium and often reversed olivine-plagioclase and mineral-glass fractionations. Olivine phenocrysts (Fo_(89-75)) vary in δ^(18)O from 4.7‰ (typical of other low- δ^(18)O basalts from Iceland) to extremely low values of 2.5‰ (in equilibrium with the host glass). Plagioclase phenocrysts (An_(89-75)) are more uniformly low in δ^(18)O, varying from 3.28 to 2.85‰. Larger and Mg-rich olivines and Ca-rich plagioclase tend to have higher δ^(18)O values than the smaller, more Fe- and Na-rich ones, but these correlations are poor, perhaps because the phenocryst population is a mixture of grains that grew from their host magmas at different times as it varied in δ^(18)O, and/or of cumulates that precipitated from other magmas and were later entrained. Oxygen diffusion in plagioclase and olivine constrains their δ^(18)O zoning and reversed Δ^(18)O(Pl-Ol) as being transient exchange feature at ~1-2 kyr. In contrast to δ^(18)O values, fast diffusing trace element Ni, Mn, Ca in olivine, and Mg in plagioclase are consistent with equilibrium partitioning, and thus require >100 yrs. Laki lavas and plagioclase have 15% excess in (^(226)Ra/^(230)Th) that require shorter than 8kyr magma residence, but mineral diffusion age may pre- and post-date magma segregation depending on the time of their growth/entrapment. These overall fast timescales and the surprising whole-rock δ^(18)O homogeneity with low ^3He/^4He= 3.6, call for an effective magma mixing, storage, and homogenization for ~1 kyr. Mass balance requires that the initial normal δ^(18)O parental olivine tholeitic basaltic magma of the eastern rift zone, melted low- δ^(18)O Iceland crust 1000s yrs before eruption, followed by the addition of 15% of superliquidus, -10‰ silicic partial melt with 3-4wt% H_2O and 2-3wt% K_2O. Low- δ^(18)O Iceland’s anomaly is viewed as crustal in origin caused by glaciation.