Sources of magnetization in lower crustal ultramafic rocks
Abstract
Magnetic anomalies, at all length scales, provide indirect information on the occurence of and variation in the concentration of magnetic minerals in the Earth's crust. Long-wavelength anomalies can reflect regional scale sources at shallow-to-intermediate depths, or smaller sources at greater depth. The extent to which lower crustal rocks are sufficiently magnetic to contribute to long-wavelength anomalies is a longstanding debate. To study the potential magnetization of deep crustal rocks, we are limited to exposures of unaltered uplifted rocks which originate in the deep crust. Studying the petrophysical and rock magnetic properties on these uplifted rock bodies aids in predicting similar magnetic behavior in correlative rock types in situ deeper in the crust.
Here, we present the results of a petrophysical and rock magnetic study on the ultramafic rocks of the Reinfjord Ultramafic Complex (RUC). These rocks are part of the Seiland Igneous Province, a magmatic plumbing system that formed in the deep crust (25-35 km depth). Such deep crustal magmatic plumbing systems are rarely exposed at the surface, and therefore the RUC provides a unique opportunity to study the magnetic behavior of oxides from deeper parts of the lithosphere. The ultramafic rocks show minor to no alteration, except in local fault areas which are heavily serpentinized. The limited serpentinization in the dunites and wehrlites implies that their magnetic properties may be representative of the same magnetic oxides at depth. To identify which of the samples were pristine and therefore representative for lower crustal rocks we used a criterium based on density and optical microscopy. This resulted in the selection of samples with a density above 3.2 g/cm 3 . The primary magnetic carriers in these rocks were characterized using conventional and novel rock magnetic measurements. The primary magnetic carriers of the RUC are Cr-magnetite blebs exsolved from Al-chromite, and oxy-exsolved magnetite lamellae within clinopyroxene. The magnetic carriers have a range of domain states, from SD to MD size. Many samples contain SD carriers and are therefore potential stable carriers at lower crustal conditions. We used scanning magnetic microscopy to directly correlate measured magnetic signal on the thin section scale to the magnetic carriers.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP13A..01T
- Keywords:
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- 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1594 Instruments and techniques;
- GEOMAGNETISM AND PALEOMAGNETISM