Mean grain diameters from thin sections: Matching the average to the problem

AbstractIt is common practice to estimate a mean diameter for spherical or sub-spherical particles or vesicles in a rock by multiplying the average diameter of the approximately circular cross-sections visible in thin section by a factor of 1.273. This number-weighted average may be dominatedby the hard-to-measure fine tail of the size distribution, and is unlikely to be representative of the average particle diameter of greatest interest for a wide range of geological problems or processes. Average particle size can be quantified in a variety of ways, based on the mass or surfacearea of the particles, and here we provide exact relations of these different average measures to straightforward measurements possible in thin section, including an analysis of how many particles to measure to achieve a desired level of uncertainty. The use of average particle diameter isillustrated firstly with a consideration of the accumulation of olivine phenocrysts on the floor of the 135 m thick picrodolerite/crinanite unit of the Shiant Isles Main Sill. We show that the 45 m thick crystal pile on the sill floor could have formed by crystal settling within about a year.The second geological example is provided by an analysis of the sizes of exsolved Fe-rich droplets during unmixing of a basaltic melt in a suite of experimental charges. We show that the size distribution cannot be explained by sudden nucleation, followed by either Ostwald ripening or Browniancoalescence. We deduce that a continuous process of droplet nucleation during cooling is likely to have occurred.VCH is supported by a Natural Environment Research Council studentship. MBH acknowledges support from the Natural Environment Research Council [grant number NE/J021520/1].This is the final version of the article. It first appeared from GeoScience World via https://doi.org/10.1180/minmag.2016.080.10

The effect of cooling rate on immiscible silicate liquid microstructure: An example from the Palaeogene dykes of Northeast England

AbstractThe migration and accumulation of immiscible silicate liquids may play a significant role in the differentiation of crustal magma bodies and the formation of some economic mineral deposits. However, our understanding of the processes that control the segregation of these liquids is currently limited by the short timescales of petrological experiments. Detailed microstructural investigations of Palaeogene basaltic dykes from Northeast England, coupled with simple 1D thermal models, constrain the effects of cooling rate on the microstructure of unmixed immiscible silicate liquids under natural conditions. The size of unmixed Fe-rich droplets within a continuous silicic phase is related to the cooling rate by a power law, with droplet diameter increasing with decreasing cooling rate, accompanied by an increase in the number of droplets. Fe-rich droplet coarsening is a result of diffusion-controlled growth. The average apparent aspect ratio and grain size of matrix plagioclase crystals indicate that nucleation and growth of these grains probably occurred in a static (or only weakly convecting) fluid dynamical regime.NER

REE mineralisation within the Ditrău Alkaline Complex, Romania: Interplay of magmatic and hydrothermal processes

The Ditrău Igneous Complex (north-east Romania) is a tilted Mesozoic alkaline intrusion (~19 km diameter), with enrichments in rare earth elements (REE), niobium, and molybdenum. It has the potential to contribute to a secure and sustainable European REE mining industry, ensuring supply security for these critical metals. The complex comprises a sequence of ultramafic rocks, alkali gabbros, diorites, syenites, nepheline syenites and alkali granites. These units have been significantly modified by sub-solidus interaction with late-stage magmatic fluids and are cut by secondary mafic dykes. The complex was subsequently cut by REE-mineralised carbonate-rich veins. Geochemical and petrological data, including apatite mineral chemistry, from the alkaline igneous rocks, dykes and veins within the Ditrău Complex, have been used to assess the interplay of magmatic processes with late-stage magmatic and hydrothermal fluids, and the effects of these processes on element remobilisation and concentration of critical metals. Only limited critical metal enrichment was achieved by magmatic processes; the REE were preferentially incorporated into titanite and apatite in ultramafic cumulates during primary crystallisation, and were not enriched in evolved magmas. A hydrothermal system developed within the Ditrău Complex magma chamber during the later stages of magmatic crystallisation, causing localised alteration of nepheline syenites by a sodium-rich fluid. Mafic dykes subsequently acted as conduits for late stage, buoyant potassic fluids, which leached REE and HFSE from the surrounding syenitic rocks. These fluids percolated up and accumulated in the roof zone, causing the breakdown of nepheline to K-rich pseudomorphs and the precipitation of hydrothermal minerals such as zircon and pyrochlore within veins. REE mineralisation within the Ditrău Complex is hosted in the latest hydrothermal phase, mineralised carbonate-rich veins, which cross-cut the complex. Monazite is the main REE-bearing phase, it crystallised from a late REE- and carbonate-rich fluid with pH controlled REE deposition.NERC funded Warwickshire Geological Conservation Group: Holloway Award British Geological Survey: BUFI funding EURARE project, funded by the European Community’s Seventh Framework Programme under grant agreement no. 30937