Patrick Launeau

Fabric analysis using the intercept method

Abstract The intercept method is a convenient, yet powerful method for the numerical analysis of fabrics. For geological applications it can be used when a set of objects (e.g. pores, grains or aggregates of a specific mineral, lineaments, etc.) with unique characteristics can be identified in an image. The intercept method analyses boundaries of objects as a population of lines. How intercepts are definined is therefore important in order to establish the significance of the intercept counts. When the method is applied to digital images, the subdivision of the information into square pixels must also be considered. We apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts. Analysis of the resulting rose of intercept counts by a Fourier series considerably facilitates the interpretation of the data. A rose of directions can readily be derived for the phase boundaries. If a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively, the Fourier components of the rose of intercepts also give the sectional strain. The Fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section. Two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement, strain analysis and lineament analysis. We also illustrate the importance of Fourier series decomposition of rose of intercept counts for fabric interpretations. Intercept results are also shown to be compatible with those derived by other methods (eigenvector, SURFOR, inertia tensor, autocorrelation methods).

Magnetic fabric evidence for conduit‐fed emplacement of a tabular intrusion: Dinkey Creek Pluton, central Sierra Nevada batholith, California

Anisotropy of magnetic susceptibility measurements in the Cretaceous Dinkey Creek Pluton (DCP), central Sierra Nevada batholith, reveal a well-defined structural pattern and zonation in bulk susceptibility (K). Outward decrease in K within the pluton is due to a decrease in ferrimagnetic contribution, corresponding to a compositional zoning from tonalite to granite. Magnetic fabrics in the pluton formed during supersolidus flow of a crystal-rich magma, except in the eastern domain where subvertical magnetic lineations and NW striking magnetic foliations are attributed to deformation associated with a regional shear zone. Magnetic foliations in the central domain of the DCP define a NNW trending synformal structure, passing structurally upward into a dome beneath a small roof pendant, interpreted to represent different structural levels in a zone of magma upwelling. Arcuate foliation trajectories in the plutons SW lobe resemble deformed passive markers in two-dimensional, NW to SW, horizontal, channel flow of non-Newtonian magma. Analysis of the fabric pattern in cross-section estimates the preerosion thickness of the lobe to between 915 and 3660 m. The DCP was emplaced as a tabular body with vertical sides and a gently inclined roof. Magma with increasingly more mafic compositions entered the chamber continuously or episodically via a NNW trending conduit. Space for the pluton was created by vertical inflation, probably accommodated by depression of its floor. These results suggest that the mid crust to upper crust of magmatic arcs may be constructed by vertical stacking of tabular granitic plutons with high width/thickness ratios like the DCP.

Analogue and numerical modeling of shape fabrics: application to strain and flow determination in magmas

We summarise numerical and analogue models of shape fabrics, and discuss their applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplacement of partially crystallised magmas often employ the analytical and numerical solutions of the Jefferys model, which describe the movement of non-interacting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas however are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mechanical interactions between crystals whose shape distribution is not necessarily homogeneous and constant during emplacement deformation. Experiments carried out in both monoparticle and multiparticle systems show that shape fabrics begin to develop early in the deformation history and evolve according to the theoretical models for low-strain regimes. At large strains and increasing crystal content, the heterogeneous size distribution of natural crystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This effect has been observed in all 3D experiments with particles of similar size and for strain regimes of high vorticity. On the other hand, studies of feldspar megacrysts sub-fabrics in porphyritic granites suggest that these ones record a significant part of the strain history. Thus, fabric ellipsoid for megacrysts evolves closer to the strain ellipsoid than for smaller markers. This behaviour results from the fact that the matrix forms of the melt and smaller crystals behave like a continuous medium relative to the megacrysts. Consequently, in absence these markers, and because the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminate. In that case, strain quantification and geometry of the flow requires the addition of external constrains based on other structural approaches.

Mapping of an ophiolite complex by high‐resolution visible‐infrared spectrometry

The Sumail massif of the Oman peridotite has been surveyed by the high spectral resolution imager HyMap. The field measurements have been taken simultaneously to the HyMap campaign with a GER 3700 spectrometer. To compare the two data sets, further calibration and atmospheric correction are made through empirical line corrections. A continuum removal by a modified Gaussian model on reflectance is then performed for each pixel to minimize atmospheric scattering and shadowing effects. Classification from spectral distance is established using field observations and comparisons at full spectral resolution to identify peridotite subunits (harzburgites and dunites) and to distinguish plagioclase-wehrlites from gabbros. Detections of hydrothermal transformations of olivine in serpentines and their alteration in carbonates along fracture networks, characterized by narrow spectral signatures, validate the classification. This demonstrates the utility of the high spectral resolution and the development of appropriate processing methods for geological identifications.

Evidence of magmatic flow by 2-D image analysis of 3-D shape preferred orientation distributions

The 3-D Shape Preferred Orientation (SPO) ellipsoid can be obtained by image analysis on a minimum of three perpendicular sections, when the 2-D measurements can be assimilated to ellipses. As numerous phenomenons can modify the SPO in magma (boundary condition effects, crystal interactions, joint migrations, etc.), the ellipsoid calculation is first tested on a set of digital models of simple shear flow. Those models, made of scattered shape ratio distribution, show that a suspension of crystals in a simple shear flow of the magma produces SPO parallel to the shear direction with an intensity given by the average shape ratio of the crystals, without any link with the amount of shear flow. This steady state SPO along the flow direction is particularly useful to study magma emplacement even if it is also shown that a critical shear rate ? between 4 and 8, for crystal shape ratio between 2 and 5 respectively, is sufficient to completely reorient a SPO. Therefore the SPO does not record magmatic strain as may do an enclave, which is an interface between two magmas with low viscosity contrast, that can record the whole strain of the magma by its passive deformation along the flow. An infinite strain is necessary here to parallelize the enclave on the shear flow direction. The application to a natural case (gabbronorite of the Bushveld, South Africa) shows that we must take care of the mineral chosen to describe a flow in a magma and that a careful classical study of the structures observed in thin sections is always required. To allow anyone to test the quality of the 2-D/3-D conversions, a web site is associated to this publication with a free access to all the image analysis and ellipsoid programs presented below.

Rheology and microstructure of experimentally deformed plagioclase suspensions

We present the result of the first deformation experiments at high-temperatures and high-pressures on synthetic magmatic suspensions of strongly anisometric particles. The results highlight the interplay between the rheological response and the development of microstructures and they demonstrate the critical importance of the shape of crystals on the mechanical behaviour of magmas. Plagioclase suspensions with two crystal fractions (0.38 and 0.52) were deformed both in compression and in torsion in a Paterson apparatus. With increasing crystal fraction, the rheological behaviour of the magmatic suspension evolves from nearly steady-state flow to shear weakening, this change being correlated with a microstructural evolution from a pervasive strain to a strain partitioning fabric. Magmatic suspensions of plagioclase have viscosities approximately five orders of magnitude higher than suspensions of equivalent crystallinities made of isometric particles such as quartz.

The Tellnes ilmenite deposit (Rogaland, South Norway): magnetic and petrofabric evidence for emplacement of a Ti-enriched noritic crystal mush in a fracture zone

Abstract The Tellnes ilmenite deposit, a world class titanium deposit, occurs in the Ana–Sira anorthosite (Rogaland anorthosite province, South Norway). It is mainly made up of an ilmenite-rich norite that has been previously interpreted as injected in a crystal mush state, in a weakness zone of the enclosing anorthosite. This emplacement mechanism has produced a faint orientation in the ore due to the flow of the mush. The internal flow structure of the orebody is studied here using the low-field anisotropy of magnetic susceptibility (AMS) method. Partial anhysteretic remanent magnetization (pAARM) indicates that coarse magnetite is the main mineral responsible for the magnetic fabric. Parallelism of the magnetic fabric with the shape-preferred orientation of the ore-forming minerals is checked using image analysis (IA) from oriented sections (intercept method). Interpretation of the AMS data verified by pAARM and IA, provides information on the magmatic foliation and lineation. Emplacement flow of the ilmenite norite crystal mush occurred in the direction of the orebody, parallel to its walls, and with an average SE 18° plunge. The feeder zone was likely situated below a network of veins on the SE end of the orebody. The sickle-shaped outcrop of the deposit suggests a transcurrent, dextral opening of a WNW–ESE-striking weakness zone across the anorthosite pluton.

Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite

Structural and petrological study of the Oman ophiolite shows that normal faults at oceanic spreading centers can root in incompletely crystallized cumulates and form at the same time as melt is being supplied to the magma chamber. In the vicinity (≤200 m) of a ridge-facing normal fault located a couple of kilometers off a former axial mantle diapir, blocks of layered cumulates were tilted at high angles relative to the Moho. Block rotation was accommodated by a swarm of anastomosed fault planes connected to the main fault. These planes are underlain by screens of gabbroic micropegmatites (former melt layers) and by ptygmatic folds, pointing to viscous deformation of a compacting crystal mush. Flat-lying, undeformed cumulate layers from the same parent melts as the deformed ones settled directly over the tilted blocks. Water introduction into the melt is evidenced by the crystallization of anomalously high anorthite (An 90%–95%) cumulus plagioclase, regardless of the degree of evolution of the magma. Shearing continued at subsolidus temperatures along some, but not all, fault planes, attested to by the local development of plastic deformation structures. The subsolidus development of amphibole-spinel coronas around olivine grains points to pervasive percolation of high-temperature hydrothermal fluids through the cumulates. As the crust cooled further, water penetration progressively focused within the main fault zone, where the crust was fractured and eventually altered in greenschist facies conditions.

Magma flow inferred from preferred orientations of plagioclase of the Rio Ceará-Mirim dyke swarm (NE Brazil) and its AMS significance

Abstract Low-field magnetic and plagioclase fabrics were compared in Mesozoic mafic dykes of the Rio Ceará-Mirim swarm. Coarse titanomagnetites with pervasive ilmenite lamellae constitute the main carrier of the magnetic anisotropy. The hysteresis parameters of the mafic dykes fall in the pseudo-single domain field. The resulting AMS ellipsoid is usually oblate and has a very low anisotropy (<3%). Textures indicate that the oxi-exolution processes and size reduction of the ferrimagnetic domains occurred at subsolidus temperatures on cooling of the dykes. The magmatic fabric was determined by the shape preferred orientation of plagioclase laths. It rarely matches the magnetic fabric. Besides their contrasting shape ellipsoids, prolate and oblate respectively, their corresponding principal directions diverge from each other or exchange their positions depending on the symmetry of the ellipsoids. These discrepancies are attributed principally to small differences in the net shape of Ti-poor magnetite after exsolution of ilmenite and in the inherently oblique fabric of grains with different shapes. These results draw attention to the need to use independent methods to confirm the conclusions about flow fabrics of weakly anisotropic mafic dykes based only in AMS.

Comparison between field measurements and airborne visible and infrared mapping spectrometry (AVIRIS and HyMap) of the Ronda peridotite massif (south-west Spain)

The high spectral resolution of Airborne Visible Infrared Imaging Spectrometer (AVIRIS; 224 channels from 400 to 2455 nm) and HyMap (127 channels between 437 and 2485 nm) images is necessary to conduct geological analysis with remote petrological determinations of rock types or soils, or to determine vegetation groups. When airborne images and field spectra are well adjusted between each other, and when the vegetation does not interfere in the analysis, the spectral shape analysis (SSA) method represents an easy treatment to reveal a large amount of geological information. The method presented in the paper takes into account both wells and peaks of spectra resulting from a combination of absorption features and continuum shapes. It was conducted on the Ronda peridotite, in the south-west of Spain, which was imaged by AVIRIS in 1991 and by HyMap in 2000, and which was partially sampled in the field using a GER 3700 spectrometer in 1997, 2000 and 2001. In this study, the AVIRIS and HyMap images are processed to infer geological features using first photo-interpretation of colour composite images and then using the SSA method. This allows us to distinguish easily the peridotite massif from its surrounding rocks, to identify petrological variations inside the peridotite, including different varieties of serpentines, but also inside the surrounding rocks, particularly the carbonate-bearing ones which comprise marbles and dolomites.