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Abstract Nepheline syenites from the ∼1.2 Ga Ilímaussaq Complex of southern Greenland are examined to assess the utility of anisotropy of magnetic susceptibility (AMS) fabrics as proxies for silicate petrofabrics. Mineral lamination is a relatively common structural feature in cumulate rocks, including in the Ilímaussaq intrusion, but there is little consensus on the process (or processes) responsible for its formation. The Ilímaussaq AMS data are combined with rock magnetic experiments and electron backscatter diffraction (EBSD) measurements to characterize the magnetic mineralogy and compare the magnetic fabrics obtained to the silicate petrofabric. The data show that Na-amphibole (arfvedsonite) is most likely the dominant control on the AMS fabrics in the coarse-grained nepheline syenites (referred to as kakortokites), and that the AMS fabric is inverse relative to the observed silicate fabric. The EBSD data for a kakortokite sample suggests that the petrofabric is defined by arfvedsonite and is wholly planar, with evidence of only weak cross-lineation of c axes. The fine-grained nepheline syenites (lujavrites), two of which have a well-developed lamination carried by Na-pyroxene (aegirine), appear to have composite AMS fabrics that are considered to be a consequence of a mixed aegirine (normal) and arfvedsonite (inverse) response. The combined datasets shed light on the mechanisms of fabric acquisition in both lithologies. In the kakortokites, the AMS fabrics and silicate crystallographic preferred orientations, as well as the lack of observed microstructural evidence for subsolidus intra-crystal deformation, support models invoking gravitationally controlled crystal mats in the development of the macro-rhythmic layering of these rocks. In the lujavrites, the strong planar fabrics revealed by both the AMS and EBSD datasets, with some evidence of subsolidus deformation, point to fabric formation and perhaps even aegirine crystallization at the postcumulus stage. The combination of EBSD and AMS fabric datasets is a powerful means of deciphering the processes responsible for mineral alignment in igneous cumulates.
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Towards an integrated texture toolkit, 1: unveiling the complex relationship between crystal shape and fabric in EBSD data
Rock textures observed via thin section are skewed from their true 3D nature. This is due to various cut effects—artifacts introduced due to the lower dimensional nature of the thin section relative to the rock. These cut effects can be corrected, and several methods have been developed to invert crystal shape and crystal size, but with each process performed separately and sequentially. With the ongoing adoption of electron backscatter diffraction (EBSD) by petrologists, an additional data stream has now become available: the 3D orientation of 2D grain sections. For EBSD analysis, no stereological corrections are typically applied for interpreting the data. This study tests whether this orientational information is skewed due to a fabric cut effect. We test this by numerically generating synthetic crystal datasets representative of several crystal shapes and population sizes. We find that EBSD orientational data has a fabric cut effect since crystals oriented with long axes perpendicular to the thin section are more likely to be sampled compared to those with long axes oriented parallel to it. This effect must be accounted for to interpret the true 3D fabric accurately. Towards this end, we develop two new tools for working with EBSD-derived fabric: (1) a simple first-order test for determining if a measured fabric exceeds that of the fabric cut effect, and (2) a method of inverting cut fabrics that provides robust error estimations. We demonstrate the applicability and accuracy of these methods using a range of synthetic examples and a natural sample. With these newly developed tools, there is clear potential for a new textural toolkit framework, to further our ability to correct for the various cut effects while also providing accurate uncertainty estimates.
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- Award ID(s):
- 2112035
- PAR ID:
- 10566819
- Publisher / Repository:
- Springer-Verlag
- Date Published:
- Journal Name:
- Contributions to Mineralogy and Petrology
- Volume:
- 179
- Issue:
- 4
- ISSN:
- 0010-7999
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00410-024-02128-x
