Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long‐wavelength magnetic anomalies

Abstract

Rock magnetic and petrolopic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo-single-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400øC demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous reinanent magnetization along the present-day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present-day field of Central Australia. Susceptibility is nearly constant with temperature to within 30øC of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both reinanent and induced components, that are large enough to account for most long-wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high-grade rocks in the Canadian Shield. INTRODUCI•ON Background InformationThere has been an increasing interest in long-wavelength magnetic anomalies in recent years, in great part due to the magnetic data collected by the POGO and Mapsat satellites. These data sets have provided many insights into the different sources contributing to the earth's magnetic field, from core processes to external fields. Of direct interest to this paper is the component of the magnetic field due to the continental lithosphere. Attempts to identify and model the lithospheric components of magnetization (for reviews, see Mayhew et al. [1985]; and Mayhew and LaBrecque [1987]) have resulted in the recognition of widespread long-wavelength (>100 kin) magnetic anomalies over both the continents and the oceanic basins. Modeling the shape and amplitude of these anomalies suggests that their source resides in the lithosphere. The few magnetic studies of mantle material [Wasilewski et al., 1979; Wasilewski and Mayhew, 1982, 1992] have found that the continental mantle is relatively nonmagnetic, as is argued from thermodynamic considerations by Frost and Shire [1986]. Thus most modelers have considered the Moho to be the lower limit for the source of the observed anomalies. Inverse modeling of the anomalies typically shows that contrasts in mag...

Magnetic minerals in the Martian crust

“…In the Lofoten province (Norway), deep rocks had k 0 = 6 × 10 −2 SI on average, corresponding to M in = 2.4 A/m [ Schlinger , 1985]. In the Arunta block (central Australia), M r = 4.1 A/m and Q = 7.2 on average [ Kelso et al , 1993]. Serpentinized oceanic peridotites reach similar M in and M r levels, and commonly contain 2–4 weight% magnetite [ Dunlop and Prévot , 1982; Pozzi and Dubuisson , 1992].…”

A closer look at remanence‐dominated aeromagnetic anomalies: Rock magnetic properties and magnetic mineralogy of the Russell Belt microcline‐sillimanite gneiss, northwest Adirondack Mountains, New York

“…In a few cases, remanent magnetization is discussed for isolated short-wavelength anomalies [Rajagopalan et al, 1995;Kelso et al, 1993] but only rarely for long-wavelength observations [Kelso Although the ambiguities of potential field modeling are great, tight constraints on models can only be produced if detailed magnetic properties are known. The possibiliW that a strong negative remanence influences the resulting anomaly is often evident; strong positive remanence is often not so easily identified [McEnroe et al, 1996].…”

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

“…In addition to these primary components, a secondary viscous or thermoviscous overprint is possible and may even dominate present‐day remanence (e.g., Kelso et al, ). In Figure on the Day plot (Day et al, ) most samples plot close to the SD boundary, and therefore, we consider that thermoviscous processes are more likely than viscous.…”