In situ distributions of magnetic susceptibility in some igneous rocks

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Abstract

Measurements of in-situ magnetic susceptibility were compiled from mainly Precambrian crystalline basement rocks beneath the Colorado Plateau and ranges in Arizona, Colorado, and New Mexico. The susceptibility meter used samples about 33 cubic centimeters of rock and measures variations in the modal distribution of magnetic minerals that form a minor component volumetrically in these coarsely crystalline granitic to granodioritic rocks. Recent measurements include 50-150 measurements on each outcrop and show that the distribution of magnetic susceptibilities is highly variable, multimodal and strongly non-Gaussian so that a mean value has little significance. Rock bodies with the most multimodal distributions generally have complex tectonic histories including metamorphism and multiple tectonic events. Variations between outcrops within the same rock body are large; however, where distributions overlap, measurements appear to fill gaps within modal peaks. Histograms of measurements are a better representation of the magnetic susceptibility distribution for a given rock body than mean and standard deviation. The best effective magnetic susceptibility estimate for an outcrop can be obtained by computing themagnetic force of the measurements 3-5 m above the outcrop and finding the constant susceptibility that gives an equal integral of the force. The multifractal distribution of the minor minerals in the rocks explains the observed multimodal distributions of magnetic susceptibility at millimeter to meter scales.

Additional publication details

Publication type Book chapter
Publication Subtype Book Chapter
Title In situ distributions of magnetic susceptibility in some igneous rocks
Chapter 6
Year Published 2019
Language English
Publisher Nova Science Publishers, Inc.
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 16 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title Horizons in Earth Science Research. Volume 18
First page 183
Last page 208
Country United States
State Arizona, Colorado, New Mexico