Magnetic properties and potential field modeling of the Peculiar Knob metamorphosed iron formation, South Australia: An analog for the source of the intense Martian magnetic anomalies?

Abstract

[1] Magnetic property measurements show that the strongly metamorphosed Peculiar Knob iron formation (IF), South Australia, is coarse-grained, high-grade hematite with variable amounts of magnetite and maghemite. This body exhibits a relatively low magnetic susceptibility (<0.3 SI) that cannot explain the associated intense magnetic anomaly, 30,000 nT, in terms of induced magnetization alone. Peculiar Knob IF possesses an extremely intense ($120 A m À1 ) remanence, directed steeply upward. This ancient remanence reinforces the local Earth's field (inclination À63°). A simple geological model, constrained by drilling and physical property measurements, explains both the observed magnetic and gravity anomalies, consistent with the Poisson theorem. Koenigsberger ratios (Qs) of 10 and greater, as found here, are rare in nature. We postulate that acquisition of a thermoremanent magnetization (TRM) by the ore during postmetamorphic cooling from above the Curie/Néel temperature accounts for the intense remanence and high Qs. Although the hematite is in the multidomain size range, the coercivity is higher than expected. Also, the natural remanent magnetization (NRM) values are less than 10% of the expected value for a saturated TRM of hematite. On the basis of reflected light, scanning electron microscope observations, and rock magnetism, we propose that the common fine intergrowths of a very small amount of magnetite and/or maghemite within the hematite host are responsible for the relatively high coercivity and contribute to the NRM. These intergrowths are not normal exsolution lamellae and were likely present at high temperature. This study suggests that coarse-grained hematiterich bodies that carry TRM and have been subjected to high-grade (>680°C) metamorphism may be possible sources for some of the prominent Martian anomalies.

Rich, attractive and extremely dense: A geophysical review of Australian IOCGs

“…At Acropolis the coincident magnetic anomaly is due to the presence of magnetite lenses within the ore system (Cross, 1993), which are not common in most IOCGs in the Gawler. In some extreme cases (e.g., Peculiar Knob) it is possible for hematite dominated deposits with almost no gravity anomaly to have extremely high (30,000 nT) magnetic anomalies, due to extreme upward directed remanence (Schmidt et al, 2007).…”

Magnetism at Depth: A View from an Ancient Continental Subduction and Collision Zone

“…Understanding the nature of lithospheric magnetization is challenging, because thermal gradients are a key factor and they are not well known, hence the magnetic crust can vary in thickness from less than 15 to more than 70 km. Over the last three decades there have been limited studies that relate magnetic anomalies to properties of deep-crustal rocks exposed at the surface (Brown et al, 2014;Dunlop et al, 2010;Ferr e et al, 2014;Kelso et al, 1993;Liu et al, 2012;McEnroe et al, 1998McEnroe et al, , 2001aMcEnroe et al, , 2001bMcEnroe et al, , 2002McEnroe et al, , 2004McEnroe et al, , 2006McEnroe et al, , 2007McEnroe et al, , 2009aMcEnroe et al, , 2009bPilkington & Percival, 1999Reynolds et al, 1990;Schlinger, 1985;Schmidt et al, 2007;Shive et al, 1992). Many exposures have been subjected to later processes such as retrograde metamorphism (Strada et al, 2006), or serpentinization that can alter the magnetic mineralogy.…”

Effects of Magnetic Anisotropy on Total Magnetic Field Anomalies

“…Magnetic anisotropy in jotunitic dikes in the Rogaland Anorthosite Province, SW Norway, possesses P j up to~1.4, and the magnetic foliation is believed to be parallel to the mean dike planes (Bolle et al, 2010). Schmidt et al (2007) give an overview of susceptibilities parallel and normal to bedding in banded iron formations (BIFs): the susceptibility is smallest normal to the bedding plane, and P values are between 1.4 and 2.5 for eight locations and ≤1.1 in four formations. Thus, most BIFs exhibit larger mean P values than what was used for the synthetic models presented here, and the effect of the anisotropy will be even stronger, leading e.g., to a larger error in interpreted dip of the structure when anisotropy is neglected.…”