The Henkel plot (logarithm of magnetic susceptibility versus density of rock samples) reveals that most rocks fall on either a “magnetite trend” or a “paramagnetic trend”. Interpretation of gravity and magnetic surveys is improved when the mineralogical and lithological basis of these trends is understood. We present a quantitative mineralogical mixing model, involving the components QFC (quartz-feldspar-calcite), FM (ferromagnesian silicates), and M (magnetite), and discuss the geological processes which produce or modify these mixtures. Igneous rocks mostly plot on the magnetite trend, where the FM/M ratio is about 10. The density-susceptibility mineralogical mixing model is compatible with the CIPW mineral calculation for igneous classification from chemical analyses. Sedimentary and metamorphic processes usually involve oxidation, reduction, and/or iron loss, all which are magnetite-destructive and lead to petrophysical measurements along the paramagnetic trend where FM/M >1000. Mineralization, with the introduction of sulfides and oxides leads to dense rocks which do not plot along the magnetite nor paramagnetic trends. This quantitative analysis provides a method to integrate geological processes in the interpretation of geophysical surveys.

Serpentinization of ultramafic rocks is fundamental to modern plate tectonics and for volatile (re-)cycling into the mantle and magmatic arcs. Serpentinites are also highly reactive with CO2 such that they are prime targets for carbon sequestration. Serpentinization and carbonation of ultramafic rocks results in changes in their physical properties such that they should be detectable using geophysical surveys; this could provide constraint on the reactivity of rocks without extensive sample characterization. We constrain the physio-chemical relationships in altered ophiolitic ultramafic rocks using petrographic observations, major-element chemistry, quantitative X-ray diffraction, and physical properties on a suite of >400 samples from the Canadian Cordillera. Serpentinization results in a systematic decrease in density that reflects the increase in serpentine abundance and carbonation results in an increase in density, mostly reflecting the formation of magnesite; based on these data we present two formulations for determining extent of serpentinization: one based on major-element chemistry and the other on density. Magnetic susceptibility is variable during serpentinization; most harzburgitic samples show a 100-fold increase in magnetic susceptibility, whereas most dunitic samples and a minor proportion of harzburgitic samples show very little change in magnetic susceptibility. We use quantitative mineralogy and physical properties of the samples to constrain a model for using density and magnetic susceptibility to approximate the mineralogy of ultramafics rock. Although further work is required to understand the role of remanence in applying these models to geophysical data, this presents an advancement and opportunity to prospect for the most reactive ultramafic rocks for carbon sequestration.