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.

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Measurement of crystal size distributions

Preface 1. Background 2. Microstructures of sedimentary rocks 3. Microstructures of igneous rocks 4. Microstructures of metamorphic rocks 5. Microstructures of deformed rocks Mineral symbols used in this book Glossary of microstructural terms.

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A Practical Guide to Rock Microstructure

Idealized conceptual models of supercontinent cyclicity must be tested against the geologic record using pre-Pangean reconstructions. We integrate tectonostratigraphic records and paleomagnetic data from Siberia, Laurentia, and Baltica to produce a quantitative reconstruction of the core of the Nuna supercontinent at 1.9–1.3 Ga. In our model, the present southern and eastern margins of Siberia juxtapose directly adjacent to, respectively, the arctic margin of Laurentia and the Uralian margin of Baltica. Consistent tectonostratigraphic records of the three cratons collectively indicate the history of Nuna9s assembly and breakup. According to this reconstruction, the late Mesoproterozoic transition from Nuna to Rodinia appears to have been much less dramatic than the subsequent late Neoproterozoic transition from Rodinia to Gondwana.

https://people.earth.yale.edu/sites/default/files/files/Evans/46_11b-Nuna.pdf

Assembly and breakup of the core of Paleoproterozoic-Mesoproterozoic supercontinent Nuna

holocrystalline under a wide spectrum of cooling regimes implies batch system. Instead, the CSDs of each system reflect a combination that cooling and crystallization can be uncoupled and considered of kinetic and dynamic influences on crystallization. Heterogeneous separately. This is tantamount to realizing that the Avrami number nucleation and annexation of small crystals by larger ones, enis large in most igneous systems. Crystallization automatically trainment of earlier grown and ripened crystals, rate of solidification adjusts through nucleation and growth to the cooling regime, and front advance, and protracted transit of a well-established mush all aspects of the ensuing crystal population reflect the relative roles column are some of the eVects revealed in the observed CSDs. There of nucleation and growth, which reflect the cooling regime. The may be an overall CSD evolution, reflecting the maturity of characteristic scales of crystal size, crystal number, and crys- the magmatic system, from simple straight nonkinked CSDs in tallization time are intimately tied to the characteristic rates of monogenetic systems to multiply kinked, piecewise continuous CSDs nucleation and growth, but it is the crystal size distributions (CSDs) in well-established systems such as Hawaii and Mount Etna. This that provide fundamental insight on the time variations of nucleation is not unlike the evolution of CSDs in some industrial systems. and growth and also on the dynamics of magmatic systems. Crystal Finally, the fact that comagmatic CSDs are not often captured size distributions for batch systems are calculated by employing evolving systematically through large changes in nucleation rates, the Johnson‐Mehl‐Avrami equation for crystallinity related to even in low crystallinity systems, may suggest that magma is always exponential variations in time of both nucleation and growth. The laced with high population densities of nuclei, supernuclei, and slope of the CSD is set by the diVerence a ‐ b, where a and b crystallites or clusters that together set the initial CSD at high are exponential constants describing, respectively, nucleation and characteristic population densities. Further evolution of the CSD growth. The batch CSD has constant slope and systematically occurs through sustained heterogeneous nucleation and rapid anmigrates to larger crystal size (L) with increasing crystallinity. The nealing at all crystallinities beginning at the liquidus itself and diminution in nucleation with loss of melt is reflected in the CSD operating under more or less steady (not exponentially increasing) at late times by a strong decrease in population density at small rates of nucleation. crystal sizes, which is rarely seen in igneous rocks themselves. Observed CSDs suggest that a ‐ b ~6‐10 and that b ~0. That is, growth rate is approximately constant and nucleation rate apparently increases exponentially with time. Correlations among CSD slope, intercept, and maximum crystal size for both batch and open systems suggest that certain diagnostic relations may be useful in interpreting the CSD of comagmatic sequences. These systematics are explored heuristically and through the detailed

On the Interpretation of Crystal Size Distributions in Magmatic Systems

[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, fO2 = NNO + 2, H2O-saturated conditions for an equilibration period, decompressed to final pressures, Pf, ranging from 175 to 5 MPa, and then held for 0.3–931 hours. According to the plagioclase liquidus curve in PH2O-T space for the relevant melt composition, these decompressions impose effective undercoolings, ΔTeff, of 34–266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 ≤ Pf < 150 MPa (ΔTeff = 34–93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < Pf < 50 MPa (ΔTeff = 125–241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low Pf. Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ΔTeff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at ∼100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleation-dominated regime.

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An experimental study of the kinetics of decompression-induced crystallization in silicic melt

Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland

Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada

Microprobe analyses of phenocrysts and groundmass, and crystal-size distributions of phenocrysts of pahoehoe natrocarbonatite lavas of the 1963 eruption of Oldoinyo Lengai have been determined. Nyerereite phenocrysts are homogeneous, with average composition Nc41Kc9Cc50 (neglecting F, Cl, P2O5, and SO3) where Nc=Na2CO3, Kc=K2CO3, and Cc= (Ca,Sr)CO3. Gregoryite phenocrysts have turbid, pale brown, oscillatorily zoned cores (average composition Nc77Kc5Cc18) with 0–30% oriented inclusions of exsolved nyerereite. Overgrowths on gregoryites (30 μm wide) are relatively sodic (Nc81Kc4Cc15) and are free of inclusions. Cores and rims are rich in SO3 (4%) and P2O5 (2%). Blebs of pyrite-alabandite mixtures (≤100 μm) occur in the groundmass. The groundmass has the simplified composition Nc65Kc15Cc20, less calcic than the composition of the 1-kbar nyerereite+gregoryite +liquid cotectic in the ternary system Nc-Kc-Cc. Groundmass quench growth of alkali halides + carbonate was followed by slower growth of coarse-grained and irregular gregoryite +KCl+BaCO3. Crystal size distributions of gregoryite and nyerereite in one sample are linear, implying little loss or gain of phenocrysts by crystal settling. AverageGτ is 0.15 mm, compared toGτ=0.03 mm for combeite phenocrysts from consanguineous nephelinite. Assuming an equal residence time (τ) for both lavas, the apparent crystal growth rate (G) in carbonate melt is 5 times greater than in peralkaline undersaturated silicate melt. Data from experiments with natrocarbonatite and related synthetic systems indicate that Na−K−Ca carbonatite magmas which crystallize calcite cannot fractionate to nyerereite+gregoryite +liquid assemblages. Natrocarbonatites plot in the liquidus field of nyerereite, and minor fractionation of nyerereite to produce the erupted lavas is indicated. The term natrocarbonatite has been inappropriately applied to other eruptive rocks with calcite phenocrysts, and the only known occurrence of gregoryite-bearing natrocarbonatite is Oldoinyo Lengai. Natrocarbonatite probably originates by liquid immiscibility from strongly peralkaline nephelinites, which have also been erupted at Oldoinyo Lengai.

Petrology and genesis of natrocarbonatite

In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples

Shombole, a nephelinite-carbonatite volcano in south Kenya, erupted silicate lavas, carbonatite dikes and tuffs, and pyroclastic rocks similar to those at other East African alkaline centres. Shombole lavas containing cpx + nepheline + accessory minerals range from perovskite-bearing nephelinites (43% SiO2, volatile-free) to sphene-bearing and phonolitic nephelinites (46–49% SiO2) and phonolites (49–56% SiO2) and have low peralkalinity ([Na+K]/Al ≈ 1.15) which does not correlate with SiO2. Early fractionation of olivine and clinopyroxene strongly depleted Ni and Cr concentrations (≈10 ppm); fractionation of perovskite, melanite, sphene, and apatite produced negative correlations of all REE with SiO2. Many lavas contain cognate intrusive xenoliths and xenocrysts and oscillatory zoning is a common feature of clinopyroxene, nepheline, and melanite crystals, indicating recycling of intrusive material. Irregular calcite-rich bodies in many samples are interpreted as quenched immiscible Ca-carbonatite liquid, and [Ca-carbonate]-silicate liquid immiscibility is observed in experiments with one nephelinite. Chemical variation in the Shombole suite can be modeled as a combination of crystal fractionation (clinopyroxene and heavy minor phases) and retention of neutral density nepheline derived from disaggregated xenoliths entrained during emplacement of dike swarms. Six newly analyzed lavas from Oldoinyo L'engai, northern Tanzania, are geochemically similar to Shombole nephelinites except that they have relatively high Na2O+K2O (average 18% vs 12%) and Zr (average 680 ppm vs 400 ppm). They are extremely peralkaline and are not typical of nephelinites from other centres. Three with [Na+K]/Al≈1.5 contain euhedral wollastonite phenocrysts; three with [Na+K]/Al≈2.0 contain combeite (Na2Ca2Si3O9) phenocrysts and pseudomorphs after wollastonite. Both types contain abundant sodalite phenocrysts (+nepheline+clinopyroxene+melanite+sphene). Seven other wollastonite nephelinite samples from L'engai have been described, but it is a lava type rarely seen in other centres. Combeite has been described from only two other locations (Mt. Shaheru, Zaire; Mayener Feld, Eifel). The hyperalkaline L'engai nephelinites have compositions similar to those of experimental silicate liquids immiscible with natrocarbonatite. Textural evidence for both carbonate-silicate (as carbonate globules) and silicate-silicate (as two optically discrete glasses with distinct compositions) liquid immiscibility is observed in the samples.

Tony D. Peterson

Papers

Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland

Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada

Petrology and genesis of natrocarbonatite

In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples

Peralkaline nephelinites. I. Comparative petrology of Shombole and Oldoinyo L'engai, East Africa