The Stardalur magnetic anomaly revisited—New insights into a complex cooling and alteration history

Abstract

This study provides new rock magnetic and magneto-mineralogical data including Mössbauer spectroscopy of basaltic drill cores from the Stardalur volcanic complex, Iceland, in order to better understand the strong magnetic anomaly, which is caused by an extraordinary high natural remanent magnetization (NRM). NRM and magnetic susceptibility ( χ) display a positive linear correlation ( R 2 = 0.81) and reach very high values up to 121 A/m and 148 × 10 −3 SI. Although a Curie temperature of 580 °C and a Verwey transition at about −160 °C is indicative of magnetite, χ– T heating experiments in argon and air atmosphere and thermal demagnetization measurements of NRM revealed a slight cation-deficiency. According to induced remanent magnetization experiments the remanence is carried solely by this low coercive phase. Minor titanomaghemite with a T C at about 340 °C only occurs in samples with larger oxide grains (20–80 μm). High vesicle abundances and the exsolution texture of Fe–Ti oxides suggest subaerial extrusion of the lava. A high oxygen fugacity (probably above the NNO buffer) and a low Ti/(Ti + Fe) ratio of the basaltic melt are suggested as a precondition for high concentration of magnetic minerals and therefore high primary TRM. During high temperature oxidation, ilmenite exsolution-lamellae, developed in titanomagnetite, and symplectic magnetite (+ pyroxene) formed by the breakdown of olivine. This secondary magnetite, grown at temperatures above the Curie temperature, increases the primary TRM. Early stage hydrothermal alteration (below about 375 °C) led to maghemitization of (titano)magnetite, clearly indicated by shrinkage cracks and irreversible χ– T curves. During later stage hydrothermal alteration, NRM intensity increased slightly due to the growth of secondary magnetite at lower temperatures (about 250–300 °C). This hydrothermally formed magnetite acquired only a low CRM but increased magnetic susceptibility significantly. According to our results it is suggested, that hydrothermal alteration does not necessarily lower remanent magnetization, but contributes to an increase in magnetization. The interplay of the three factors melt composition, small grain sizes of secondary magnetite due to decomposition of silicates and new formation under hydrothermal conditions caused the strong magnetic anomaly at the surface.