Relaciones de densidad y susceptibilidad magnética de rocas graníticas en el dominio paramagnético Density and magnetic susceptibility relationships of granitic rocks in the paramagnetic domain

Abstract

The so-called Henkel's plot (1976) allows relating density and magnetic susceptibility of rocks. In this work we focused on three paramagnetic Pyrenean granites from the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons. In total, 128 sites with 310 density measurements and > 2600 susceptibility readings cover the main range of variability of magnetic susceptibility and density of these bodies. Linear fittings were estimated for every granitic body as well as a global estimation for the entire dataset: r (kg/m 3) = 2566 (kg/m 3) + 0.541 k (10-6 S.I.) (R:0.97). This fitting is only valid in the paramagnetic domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows transforming magnetic susceptibility data into density data (or the other way around) helping to constraint gravity and magnetic modelling when density data or magnetic susceptibility are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

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10ª Asamblea Hispano Portuguesa de Geodesia y Geofísica - Toledo, noviembre de 2022
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Relaciones de densidad y susceptibilidad magnética de rocas graníticas en el dominio
paramagnético
Density and magnetic susceptibility relationships of granitic rocks in the paramagnetic domain
PUEYO, Emilio L (1) *; ROMÁN-BERDIEL, T. (2); CALVÍN, P. (1); BOUCHEZ, J.L. (3); BEAMUD, E. (4); AYALA, C. (5,6);
SOTO, R. (1); CLARIANA, P. (1); MARGALEF, A. (7); (6); RUBIO, F.M. (5); REY- MORAL, C. (5); GARCÍA, A. (5); MARTÍ, J. (4); CASAS,
A.M. (2); GARCÍA- LOBÓN, J.L. (5)
(1) CN IGME (CSIC), Unidad de Zaragoza, C/ Manuel Lasala 44, 9ºB, 50006 Zaragoza, Spain
(2) Departamento de Ciencias de la Tierra, Geotransfer Research Group, Instituto de Investigación en Ciencias
Ambientales (IUCA), Universidad de Zaragoza, Spain
(3) Federal University of Toulouse, Géosciences Environnement Laboratory, Avenue E. Belin, 31400 Toulouse
(4) Paleomagnetic Laboratory CCiTUB-Geo3Bcn, Geosciences Barcelona – CSIC, C/ Lluis Solé i Sabarís s/n, 08028
Barcelona, Spain
(5) CN IGME (CSIC), C/ La Calera 1, 28760 Tres Cantos, Madrid, Spain
(6) Currently visiting researcher at Geosciences Barcelona – CSIC, C/Lluis Solé i Sabarís s/n, 08028 Barcelona, Spain
(7) Andorra Research + Innovation (AR+I), Av Rocafort 21-23, 3, AD600 Sant Julià de Lòria, Andorra
RESUMEN
El denominado diagrama de Henkel (1976) relaciona la densidad y la susceptibilidad magnética de las rocas. En este
trabajo nos hemos centrado en tres cuerpos graníticos pirenaicos paramagnéticos: Mont Louis-Andorra, Maladeta y
Marimanha. En total, 128 sitios con 310 mediciones de densidad y > 2600 lecturas de susceptibilidad cubren todo el rango
de variabilidad de susceptibilidad magnética y densidad de estos cuerpos. Se calcularon las regresiones lineales para cada
cuerpo granítico por separado así como una estimación global para todo el conjunto de datos: r (kg/m3) = 2566 (kg/m3)
+ 0,541 k (10-6 S.I.) (R:0,97). Este ajuste es sólo válido en el dominio paramagnético, donde el hierro se fracciona
principalmente en filosilicatos que contienen hierro y la presencia de magnetita es despreciable (o al menos su
contribución a la susceptibilidad general). Esta relación, que puede ser ligeramente diferente en otros cuerpos, permite
transformar los datos de susceptibilidad magnética en datos de densidad (o al revés) ayudando a restringir la modelación
magnética y gravimétrica cuando los datos de densidad o susceptibilidad magnética son escasos. Dada la gran cantidad
de estudios de ASM en todo el mundo, junto con la rapidez y la rentabilidad de las mediciones de susceptibilidad con
dispositivos portátiles, esta metodología permite densificar y homogeneizar los datos petrofísicos.
SUMMARY
The so-called Henkel’s plot (1976) allows relating density and magnetic susceptibility of rocks. In this work we focused on
three paramagnetic Pyrenean granites from the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons. In total,
128 sites with 310 density measurements and > 2600 susceptibility readings cover the main range of variability of magnetic
susceptibility and density of these bodies. Linear fittings were estimated for every granitic body as well as a global estimation
for the entire dataset:
r
(kg/m3) = 2566 (kg/m3) + 0.541
k
(10-6 S.I.) (R:0.97). This fitting is only valid in the paramagnetic
domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at
least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows transforming magnetic
susceptibility data into density data (or the other way around) helping to constraint gravity and magnetic modelling when
density data or magnetic susceptibility are scarce. Given the large amount of AMS studies worldwide, together with the
quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying
and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric
signal.
Henkel, H. (1976). Studies of density and magnetic properties of rocks from northern Sweden. Pure and applied geophysics,
114(2), 235-249.

10ª Asamblea Hispano Portuguesa de Geodesia y Geofísica - Toledo, noviembre de 2022
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