Sigurdur Steinthorsson

The heterogeneous Iceland plume: Nd‐Sr‐O isotopes and trace element constraints

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neovolcanic zones of Iceland (picrites, olivine and quartz tholeiites, transitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usually associated with central volcanoes, has low δ18O values (+5 to +1‰) resulting from interaction with the hydrothermally altered Icelandic crust. The second group, which contains picrites, olivine tholeiites, and alkali basalts, has normal mantle oxygen isotopic compositions (δ18O = +5 to +6‰) which are thought to represent those of the mantle source. Nd isotopic compositions vary greatly, from 143Nd/144Nd = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the chemical compositions of Icelandic volcanics (147Sm/144Nd = 0.12=0.28) requires >200 m.y., a period that greatly exceeds the maximum age of Icelandic crust. Previous models, in which the Sr isotopic variations were explained in terms of evolution of crustal reservoirs, are invalidated, and mantle reservoirs with different Nd and Sr isotopic compositions are indicated. The Iceland data define a linear array in the Sr-Nd isotope diagram which overlaps both mid-ocean ridge basalt and oceanic island basalt fields and indicates mixing between depleted and enriched end-members. Alkali basalts come preferentially from an isotopically and chemically enriched component of the Iceland plume, and picrites come from a more refractory, more depleted portion. Positive Sr, Rb, and Ba anomalies are present in picrites and other lavas with low trace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crustal material.

Calcium-rich melt inclusions in Cr-spinels from Borgarhraun, northern Iceland

Abstract Two groups of primitive melts are observed as inclusions in Cr-spinels in a picrite (Fo88–92.4) from northern Iceland. Group I inclusions have 12.2–13.8 wt% MgO, high CaO (14.6–15.8 wt%), low Al2O3 (10.1–11.8 wt%) and very high CaO/Al2O3 (1.3–1.5); they occur in spinels with a cr# between 0.63 and 0.70. Group II inclusions occur in spinels with slightly lower cr# (0.55–0.60) and they have 13.3–15.8 wt% MgO, 12.0–13.8 wt% CaO, 11.2–13.4 wt% Al2O3 and CaO/Al2O3 0.9–1.1. Both groups have low concentrations of the incompatible minor elements (TiO2, K2O and P2O5) and relatively high SiO2. The two groups converge at MgO approximately 10 wt%. A single inclusion in a low cr# spinel could represent a more evolved member (9.00 wt% MgO) of either group. Group I inclusions are on a clinopyroxene control line while Group II inclusions are close to being on an olivine control line. Groups I and II cannot be derived one from the other. They require high degree of melting from a refractory mantle source which probably is pyroxenitic to account for the high CaO. Melts with such high CaO/Al2O3 cannot be generated by melting commonly assumed lherzolites, so that pyroxenite or wehrlite mantle component is the suggested source.

Opaque minerals in geothermal well no. 7, Krafla, Northern Iceland

Abstract Study of the opaque minerals from well No. 7, Krafla, indicates two mineral assemblages: (1) hydrothermally altered igneous minerals and (2) secondary minerals that have precipitated from the geothermal fluid at depths down to 2140 m, and at temperatures up to more than 340°C. Chief amongst the chemically precipitated minerals are pyrite, pyrrhotite and goethite, which is described here for the first time in an Icelandic geothermal drill hole. The geothermal system at Krafla has been periodically disturbed by the influx of volcanic emanations; this article attempts to interpret, by use of thermochemical calculations, the processes affecting the precipitated mineral assemblage.

The ferric/ferrous ratio in basalt melts at different oxygen pressures

Samples of two types of natural basalt were equilibrated over a range of oxygen pressures at four different temperatures, and then quenched to room temperature. Except at the lowest temperature, where magnetic crystals formed under the most oxidizing conditions, totally amorphous samples were obtained. The Mössbauer spectra of 45 samples of the quenched basaltic melts were measured at room temperature. The relationship obtained between the oxidation state and oxygen fugacity differs to some extent from those relations previously described in the literature, in not yielding a linear relationship between log(farric/ferrous) and log(fo2). This might indicate a more involved redox process than that described by a simple reaction between oxides and/or the influence of the cation composition in the process of glass formation. An investigation was made of the kinetics of the redox process. For the experimental setup used, redox equilibrium would be reached within three hours. Finally, three naturally quenched basalt glasses were analyzed for comparison; two showed lower oxidation states than previously found in Icelandic rocks.

Natural concentrations of mercury in Iceland

The results are presented of a survey of mercury concentrations in various parts of the Icelandic environment. Values for air and gas samples include: <0.03 μg/m3 for Reykjavik, 15–20 km away from a hydrothermal area; 1–3 μg/m3 for air in a hydrothermal area near Lake Mývatn; 12–30 μg/m3 for air in Heimaey during the 1973 eruption; and 16 μg/m3 for a sample of fumarole gas. Values for fresh igneous rocks, of various compositions, extrusive (subaerial), subaqueous (up to 3000 m depth), and intrusive, range between 2 and 9 ppb. Highest values obtained for uncontaminated samples are 37 ppb for a pyrite-bearing zone in a hydrothermal drill hole, and 125 ppb for a volcanic sublimate. The sources of the mercury levels observed are briefly discussed.