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

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 20; no. 4; pp. 1794 - 1817
• Main Authors: Maat, Geertje W., McEnroe, Suzanne A., Church, Nathan S., Larsen, Rune B.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.04.2019
• Subjects: Anomalies; chrome spinel; Chromium; crustal magnetism; Earth; Earth crust; Fault zones; Fractures; Geomagnetic field; High temperature; Iron; Magnetic anomalies; Magnetic field; Magnetic fields; Magnetic properties; Magnetism; Magnetite; Mantle; Mineralogy; Minerals; Oxides; Pressure effects; rock magnetism; Rock properties; Rocks; Temperature; Ultramafic rocks; Upper mantle; Wavelength; Norway
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC008132

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. Plain Language Summary Satellites are used to map the Earth's magnetic field and explore magnetic anomalies created by this field in crustal rocks. To understand the magnetic signal from deep‐seated rocks, we need to explore the sources and nature of the magnetic mineralogy. This paper reviews the magnetic properties of rocks that formed deep in the Earth's crust (at 25‐ to 35‐km depth), 570–560 million years ago, now well exposed in Northern Norway. Despite a history of being exhumed from deep in the crust, these rocks only show minimal alteration and present a rare opportunity to study their pristine magnetic properties. The primary magnetic minerals in these ultramafic rocks are an exsolved chrome‐spinel, and magnetite lamellae in clinopyroxene. These minerals formed at high temperatures and could be representative of minerals residing deeper in the lower or upper mantle. These phases are magnetically stable up to the Curie temperature of magnetite. These ultramafic rocks are a potential source of magnetization in the lower crust and the upper mantle. Key Points Primary magnetic oxides are chrome‐spinel with Fe‐rich exsolution lamellae and magnetite lamellae in pyroxene These carriers are stable at lower crust to uppermost mantle conditions Ultramafic rocks are potential sources of long‐wavelength‐anomalies