Michael D. Higgins

Measurement of crystal size distributions

Abstract Studies of crystal size distributions (CSD) can reveal much about how rocks solidify and under what conditions. Data from two-dimensional sections can be readily acquired at many different scales, from electron microscope images, thin sections, slabs, outcrops, and so on, but the conversion to true, three-dimensional values is complex. The widely used Wager method does not have a good theoretical basis and does not give accurate results. A modification of the Saltikov correction method is proposed here that is more accurate and can account for different crystal shapes and fabrics. Population densities determined by this method differ by factors of 0.02 to 100 from those determined by the Wager method. Published CSDs determined using other methods can be recalculated if the crystal shape and fabric parameters can be estimated. The method has been incorporated into a new program, CSDCorrections.

Closure in crystal size distributions (CSD), verification of CSD calculations, and the significance of CSD fans

Abstract Crystal size distribution (CSD) measurements are susceptible to the closure problem, just like chemical compositions. In its simplest form this means that the total crystal content of a rock cannot exceed 100%. Where chemical or thermal effects limit the total quantity of a single phase, closure can occur at lower volumetric phase proportions. This means that parts of the CSD diagram [ln(population density) vs. size] are not accessible. If the volumetric phase proportion is constant, then straight CSDs will appear to rotate around a point at small sizes giving a fan of CSDs. These fans are significant and do show changes in crystal sizes that can be interpreted in terms of magmatic processes. However, the correlation between the slopes (or characteristic lengths) and intercepts of individual CSDs in a family is not significant, but just a consequence of the constant phase proportion effect. Many other graphs, such as characteristic length vs. volumetric phase proportion, can give more information on magmatic processes.

Magma dynamics beneath Kameni volcano, Thera, Greece, as revealed by crystal size and shape measurements

Size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from Kameni volcano, Greece, can be modelled by mixing two populations of crystals, each with overlapping linear crystal size distributions (CSD)—termed microlites and megacrysts. The magmas bearing the microlites and megacrysts started to crystallise 6–13 and 24–96 years, respectively, before each eruption. The dates of initiation of crystallisation of the megacrysts indicate that they are left-overs of earlier injections of new magma into a shallow chamber: some magma remains after each eruption and continues to crystallise. New magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma. Eruption followed 6–13 years after mixing. Such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied, just topped up from time to time.

Large-scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the Daly gap

The dearth of intermediate magmatic compositions at the Earths surface, referred to as the Daly gap, remains a major issue in igneous petrology. The initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of Fe- and Si-rich liquids in igneous rocks. This work presents a record of large-scale magmatic differentiation due to immiscibility in the tholeiitic Sept Iles intrusion (Canada), one of the largest layered plutonic bodies on Earth. Gabbroic cumulate rocks from the Critical Zone of this intrusion show a bimodal distribution in density and P2O5 content, despite identical major element chemistry of the principal magmatic phases. Immiscibility is supported by the presence of contrasting Fe-rich and Si-rich melt inclusions trapped in cumulus apatite. Phase diagrams and well-documented occurrences of small-scale immiscibility confirm that liquid-liquid unmixing and the separation of Fe-rich and Si-rich liquids may contribute significantly to the Daly gap along the tholeiitic liquid line of descent.

Textural coarsening in igneous rocks

The initial growth of crystals in magma is driven by kinetic forces, and the resulting textures can be preserved in rapidly cooled igneous rocks. However, crystals in such rocks have a high surface area with respect to their volume, and hence an excess surface energy. This energy can be dissipated by textural equilibration. At advanced stages, this is represented by textural coarsening, in which smaller crystals dissolve simultaneously with the growth of larger crystals. These textural changes occur commonly in slowly cooled plutonic rocks and may be important for the development of some volcanic rocks as well. Textural coarsening is clearly an important petrologic process, but may not have received the attention it deserves inasmuch as it does not change the chemical composition of the rock, and hence cannot be quantified by the geochemical methods that currently dominate petrology.

Three generations of anorthosite-mangerite-charnockite-granite (AMCG) magmatism, contact metamorphism and tectonism in the Saguenay-Lac-Saint-Jean region of the Grenville Province, Canada

Four A-type granitoid plutons from the Saguenay-Lac-Saint-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give UPb zircon igneous crystallisation ages of 1146 ± 3 Ma, 1082 ± 3 Ma, 1067 ± 3 Ma, and 1020+4−3 Ma. These ages indicate that the region has not been affected by the Ottawan period of the Grenville orogeny. A compilation of these and existing UPb age data reveals three periods of Mesoproterozoic magmatism in this region: 1160-1140 Ma, 1082-1050 Ma and 1020-1010 Ma. Each of these periods appears to have had some or all of the components of the well-known anorthosite-mangerite-charnockite-granite suite, and the first two periods can be correlated with magmatism elsewhere in the Grenville Province. Sparse UPb metamorphic ages also fall within these periods and are interpreted to reflect regional-scale contact metamorphism produced by the large plutons. In addition, the few ages of strike-slip movements on major shear zones also fall within these time periods. It is difficult to interpret these data as reflecting separate orogenic events, as there is little definitive evidence of calc-alkaline magmatism, thrusting or regional metamorphism in this region. Perhaps during the interval 1160 to 1010 Ma magmatism was driven by repeated upwelling of hot mantle, either in a plume or above a sinking lithospheric slab, but access to the mid-levels of the crust currently exposed was controlled by strike-slip movements on major vertical shear zones that resulted from plate-tectonic activity.

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

A fundamental dispute: A discussion of "On some fundamentals of igneous petrology" by Bruce D. Marsh, Contributions to Mineralogy and Petrology (2013) 166: 665-690

Marsh (Contrib Miner Petrol 166:665–690, 2013) again claims that crystal-free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions. He ignores an earlier critique of his unconventional inferences (Latypov, J Petrol 50:1047–1069, 2009) as well as a wealth of petrographic, geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions. Most tellingly, the cryptic variations preserved in the Skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization. A major flaw in Marsh’s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system. This inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere.

Crystal structure, mosaicity, and strain analysis of Hawaiian olivines using in situ X-ray diffraction

Abstract Deformation of olivine in a volcanic context is poorly constrained, although deformed olivine is abundant in some volcanic rocks, and its presence is important for the definition of the magmatic history of volcanic edifices such as Kilauea Volcano, Hawaii. Deformed olivines at Kilauea originate in the lower crust; therefore, the classic approaches and interpretations applied to mantle-derived olivine are not applicable. Deformed olivine crystals from Kilauea lava samples were examined using an in situ XRD technique. Our results validate and refine optical observations of olivine deformation. We also confirm the presence of deformation and quantify it for olivine crystals of any size, even for small crystals (0.15 mm). There are significant correlations between deformation intensity (strainrelated mosaicity) and olivine composition and crystal size. Although this technique does not allow the simple estimation of the P-T conditions of deformation, crystal formation, or magmatic history, some constraints are provided herein. In particular we estimate the threshold degree of mosaicity, above which we consider that a crystal underwent deformation. In situ XRD is shown to be an easyto- use, fast, low-cost, non-destructive technique and is less ambiguous than optical microscopy. For crystals optically exhibiting subgrain formation, analysis of asterism by in situ XRD has been used to reconstruct the mosaic spread of the original grain, and thus its original strain condition prior to subgrain formation.

Inherited correlation in crystal size distribution: COMMENT

The results and conclusions in the recent paper by [Pan (2001)][1] are based on an inadequate understanding of the concept and measurement of population density and the fundamentals of the method of crystal size distributions. Not only does Pan perform his example calculations incorrectly, which