Characterization and imaging magnetic minerals from ultramafic roots of a LIP: implication for deep crustal magnetic sources

Abstract

SUMMARY Satellite magnetic surveys have revealed features consistent with magnetization at depth in the lithosphere. Previous studies have reported magnetic minerals preserved in mantle nodules and in some eclogite facies rocks. Deep crustal rocks are another possible source for these deep lithospheric signals, but have not been extensively studied, in many cases due to the difficulty in obtaining samples unaffected by later near-surface alteration processes. Here, we used a combined approach involving petrophysical, rock magnetic and scanning magnetic microscopy (SMM) analyses on unaltered pristine ultramafic samples from the Reinfjord Ultramafic Complex in northern Norway. The focus was to identify the magnetic carriers using SMM and link the magnetic anomalies mapped in thin section to distinct rock magnetic measurements. The dominant magnetic carriers are Cr-magnetite exsolved from grains of Al-chromite, and magnetite exsolution lamellae from clinopyroxene. In addition, some samples have exsolved magnetite from Al-Cr-spinel and Fe-rich exsolution from Cr-spinel as carriers. Rock magnetic measurements suggest that these primary magnetic carriers, could retain magnetization to considerable crustal depths.