Magnetic Mineralogy and Petrophysical Properties of Ultramafic Rocks: Consequences for Crustal Magnetism

Abstract

Magnetic properties from the Reinfjord Ultramafic Complex, in northern Norway, which formed as part of a deep magmatic conduit system, have been investigated to determine the magnetic signature of ultramafic rocks now exposed at the surface and deeper in the lower crust. The dominant carriers in these ultramafic rocks are a chrome‐spinel with Fe‐rich exsolution blebs and exsolution lamellae of magnetite in clinopyroxene. Except locally, in a fault zone and in discrete small fractures, these rocks show only minor to no alteration. We infer that the magnetic oxides characterized here are representative of pristine magnetic carriers in similar rocks deeper in the crust. These oxides can be stable in lower crustal, possibly upper mantle, depths when temperatures are below the Curie temperature of magnetite, taking into account pressure effects. These ultramafic rocks are candidates for potential sources of long‐wavelength anomalies.

Origin of Long‐Wavelength Magnetic Anomalies at Subduction Zones

“…A Curie temperature for magnetite of 580 • C (Clark, 1997) gives a limit of the extent of any magnetization at depth, whether in the slab or wedge above it. The influence of pressure may raise the Curie temperature by ∼20 • C at lower crustal/upper mantle depths (ter Maat et al, 2019). The thermal models of Syracuse et al (2010) suggest that within subduction zones, the deepest extent of the Curie isotherm at or below the slab Moho varies significantly between different trenches and may lie anywhere between 50 and 250 km in depth.…”

Mapping and Modeling Sources of Natural Remanent Magnetization in the Microcline–Sillimanite Gneiss, Northwest Adirondack Mountains: Implications for Crustal Magnetism

“…This information is critical to the study of the past history of the Earth's magnetic field and is also a key tool to interpret magnetic anomalies on Earth and other rocky planetary bodies. The mineral sources and nature of remanent magnetization reflected in crustal magnetic anomalies are ongoing topics of research (Clark, 1999;Brown & McEnroe 2008;Ferre et al, 2020;McEnroe et al 2001aMcEnroe et al , 2002McEnroe et al , 2009aMcEnroe et al ,b, 2018ter Maat et al, 2019ter Maat et al, , 2020Michels et al, 2018Michels et al, , 2020Purucker & Whaler, 2007).…”

The Architecture of a Root Zone of a Large Magmatic Conduit System From High Resolution Magnetic, Gravity and Petrophysical Data: The Reinfjord Ultramafic Complex

“…Despite the visual evidence of serpentinization along the core, local occurrences of sulphides and/or small pyroxenite or gabbroic veins within the CS can increase or decrease the density of the rock without measurably influencing the magnetic susceptibility. High densities and magnetic susceptibilities are observed in samples from the marginal zone at the contact between gabbro and ultramafic rocks (ter Maat et al., 2019). Site averages of susceptibilities at these localities range between 0.007 SI and 0.02 SI.…”

“…Extensive petrophysical data (ter Maat et al., 2019) from oriented surface samples and unoriented samples from two deep drill cores (R3 and R4) were used here to investigate the subsurface geometry of the RUC by combining 3D gravity and magnetic modeling with petrology and structural information. The gravity method is well suited for detecting the density contrast between the ultramafic rocks and the gneissic and gabbroic country rocks, and for investigating the distribution of these rocks at depth.…”