COMBINING CRYSTALLOGRAPHIC ORIENTATION RELATIONSHIPS AND MICROSTRUCTURES TO DETERMINE MINERAL INCLUSION ORIGINS

Description

Crystallographic orientation relationships (CORs) provide important insights into the origins of mineral inclusions, as they contain information about how (if at all) the lattices of host and inclusions interacted during inclusion formation. Large datasets have revealed complex orientation relationships in some mineral inclusion–host systems, including the presence of multiple CORs and CORs that are not completely fixed with respect to both lattices. This variety suggests that multiple inclusion-forming processes that generate CORs may exist, requiring new information to differentiate between them. Electron backscatter diffraction (EBSD) allows hundreds of point measurements of the crystallographic orientation of mineral inclusions to be acquired in a few days. Digital processing allows statistical treatment of data for the comparison of multiple datasets using one set of candidate CORs. Most importantly, EBSD allows in-situ measurements of host-inclusion CORs in polished samples, delivering highly spatially resolved data about CORs that can be combined with microstructural and compositional information to differentiate between multiple hypotheses of inclusion origins. New questions that can be addressed include 1) the relationship between host crystallography and shape-preferred orientations (SPOs) of inclusions, 2) the relationship between the SPO and COR of each inclusion, and 3) potential variations in host-inclusion COR characteristics between different microstructural and compositional domains of host crystals. EBSD datasets of CORs between rutile inclusions and host metapegmatite garnet from two localities with different geological histories illustrate the new insights that combining COR and microstructural analysis can provide. Inclusion abundance and habit vary in concentric and sector zones of garnet, with concentric zone boundaries parallel to garnet crystal facets. Inner zones bear equant rutile inclusions < 1μm in diameter, whereas outer zones feature acicular rutile inclusions <1μm in diameter and tens of μm in length, following SPOs parallel to garnet crystallographic directions. Multiple CORs coexist in all microstructural domains. The relative frequencies of different CORs vary, both between zones of inclusions exhibiting the same habit and between zones of differing habits. For acicular rutile inclusions, the host-inclusion COR is systematically linked to the inclusion elongation direction. In some zones, the SPO of needles does not follow all crystallographically equivalent garnet directions equally and instead depends on the orientation of the garnet facet. The correlation of spatial variations of rutile inclusion habit, SPO, and COR frequencies in the metapegmatite garnets with magmatic growth zoning contradicts an exsolution model of inclusion formation. In contrast, the observed heterogeneity can be well explained by heterogeneous nucleation of rutile at the growing garnet-melt interface, followed by intergrowth and finally engulfing of inclusions by garnet. Correlating statistically treated COR characteristics with microstructural and compositional data represents a powerful way to test hypotheses about mineral inclusion origins.

Period	1 Sept 2021
Event title	3rd European Mineralogical Conference: Mineralogy in the modern world
Event type	Conference
Location	UnknownShow on map
Degree of Recognition	International