Density and magnetic susceptibility relationships in non-magnetic granites; a "wildcard" for modeling potential fields geophysical data

Abstract

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂ 18 O or [SiO 2 ] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called "non-magnetic" granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90's and early 2000's, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal. We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra).

Full text

EGU2020-8736
https://doi.org/10.5194/egusphere-egu2020-8736
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
Density and magnetic susceptibility relationships in non-magnetic
granites; a “wildcard” for modeling potential fields geophysical
data.
Emilio L. Pueyo1, Mª Teresa Román-Berdiel2, Conxi Ayala1, Francesca Loi2, Ruth Soto1, Elisabeth
Beamud3, Elena Fernandez de Arévalo1, Ana Gimeno1, Luis Galán1, Stefanía Schamuells4, Nuria
Bach-Oller4, Pilar Clariana1, Félix M. Rubio1, Antonio M. Casas-Sainz1, Belén Oliva-Urcia5, José Luis
García-Lobón1, Carmen Rey1, and Joan Martí4
1Instituto Geológico y Minero de España, Spain
2Geotransfer. Earth Science Dpt. Universidad de Zaragoza, Spain
3Paleomagnetic Laboratory, CCiTUB - ICTJA CSIC, Barcelona, Spain
4Instituto de Ciencias de la Tierra "Jaume Almera" CSIC, Barcelona, Spain
5Universidad Autónoma de Madrid, Spain
Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic
bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r],
magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during
the modeling of rock volumes. Several works have already demonstrated that ∂18O or [SiO2]
display a negative correlation to density and to magnetic susceptibility. These relationships are
particularly stable (and linear) in the so-called “non-magnetic” granites (susceptibilities falling
within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-
alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust
correlations between density and magnetic susceptibility at different scales in CA granites from
the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal
is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS)
data, performed during the 90’s and early 2000’s, together with new data acquired in the last few
years, as an indirect measurement of density in order to carry out the 3D modelling of the
gravimetric signal.
We sampled some sections covering the main range of variability of magnetic susceptibility in the
Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized
by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We
performed new density and susceptibility measurements along two main cross-sections (Maladeta
and Mont Louis-Andorra). In these outcrops, numerous measurements (usually more than 50)
were taken in the field with portable susceptometers (SM20 and KT20 devices). Density data were
derived from the Arquimedes principle applied on large hand samples cut in regular cubes

weighting between 0.3 and 0.6 kg (whenever possible). These samples were subsampled and
measured later on with a KLY-3 susceptibility bridge in the laboratory. Additionally, some density
data were derived from the geometry and weighting of AMS samples.
After the calibration of portable and laboratory susceptometers, density and magnetic
susceptibility were plotted together. Regressions were derived for every granite body and they
usually followed a linear function similar to: r = 2600 kg/m3+ (0.5 * k [10-6 S.I.]). As previously
stated, this relationship is only valid in CA and paramagnetic granites, 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). These relationships allow transforming magnetic
susceptibility data into density data helping in the 3D modelling of the gravimetric signal when
density data from rock samples 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.
Powered by TCPDF (www.tcpdf.org)