Crystal Size Distribution Curves Research Articles

Magma Mixing and Mingling during Pluton Formation: A Case Study through Field, Petrography and Crystal Size Distribution (CSD) Studies on Sirsilla Granite Pluton, India

Abstract Field, petrography, and crystal size distributions (CSD) of different lithological variants from Sirsilla granitic pluton (SGP), southern India, is described here to understand operative magmatic processes. The SGP contains many mafic microgranular enclaves (MMEs) and syn-plutonic dykes. The contact relationship between MMEs and the host granite is often diffusive or gradational and rarely sharp, implying disaggregation and under-cooling of MMEs. Petrographic features like resorption textures, quartz ocelli, and the poikilitic nature of the large K-feldspar grains enclosed within plagioclase indicate interaction and magma mixing/mingling processes in an open magma chamber. Bladed biotite and acicular apatite grains in MMEs are due to rapid crystallization during the magma mingling process. The CSD curves generated for plagioclase provide an inverse relationship between population density and crystal size. Multiple crystal populations, i.e., a gently sloping line for the core samples and a steeply sloping line for margin samples, are interpreted to be caused by the mafic - felsic magma mixing and mingling processes.

Magma chamber evolution of the Ardestan pluton, Central Iran: evidence from mineral chemistry, zircon composition and crystal size distribution

AbstractThe Oligo–Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh–Dokhtar Magmatic Assemblage of Iran. The Ardestan pluton is composed mainly of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2–2.8 apfu) over very restricted Fe# ratios (0.42–0.56) which are characteristic of continental arc magmatic suites. High Ti2O contents of biotite (<6.1 wt.%) suggest a magmatic origin. Ti-in-biotite geothermometery gives a mean crystallisation temperature of 730 ± 56°C, slightly higher than calculated TZr.Ti°C (716 ± 50°C) and similar to the average TZr.sat°C (735 ± 26°C). These results are consistent with the low bulk-rock SiO2 contents, which provide minimum estimates of temperature and indicate zircon crystallised from a fractionated magma. Zircons from the Ardestan pluton have high (Sm/La)N (>10) ratios suggesting a magmatic origin. T–$f_{{\rm O}_{\rm 2}}$ calculations of oxygen fugacity between –13.6 to –16.9 indicate oxidising crystallisation conditions between the Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers. Tight linear trends of log (XF/XOH), log (XCl/XOH) and log (XCl/XOH) vs. XMg represent a narrow range of $f_{{\rm H}_2O}$, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of basic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.

Variations in Composition, Texture, and Platinum Group Element Mineralization in the Lower Group and Middle Group Chromitites of the Northwestern Bushveld Complex, South Africa

Small-scale variations in mineral chemistry, textures, and platinum group element (PGE) mineralization were investigated in the Lower and Middle Group chromitite layers LG6, LG6a, MG1, MG2, and MG2 II from vertical drill core profiles at the Thaba mine in the northwestern limb of the Bushveld Complex. We present detailed geochemical profiles of chromite composition and chromite crystal size distribution curves to shed light on the processes of chromite accumulation and textural modification as well as mineralization. Multiple samples within each layer were assayed for PGE concentrations, and the respective platinum group mineral association was determined by mineral liberation analysis (MLA). There is strong evidence for postcumulus changes in the chromitites. The crystal size distribution curves suggest that the primary chromite texture was coarsened by a combination of adcumulus growth and textural equilibration, while compaction of the crystal mush played only a minor role. Mineral compositions were also modified by postcumulus processes, but because of the very high modal amount of chromite and its local preservation in orthopyroxene oikocrysts, that phase retained much primary information. Vertical variations of chromite composition within chromitite layers and from one layer to another do not support the idea of chromite accumulation from crystal-rich slurries or crystal settling from a large magma chamber. Instead, we favor a successive buildup of chromitite layers by repeated injections of relatively thin layers of chromite-saturated magmas, with in situ crystallization occurring at the crystal mush-magma interface. The adcumulus growth of chromite grains to form massive chromitite required addition of Cr to the layers, which we attribute to downward percolation from the overlying magma. The PGE concentrations are elevated in all chromitite layers compared to adjacent silicate rocks and show a systematic increase upward from LG6 (avg 807 ppb Ir + Ru + Rh + Pt + Pd + Au) to MG2 II (avg 2,062 ppb). There are also significant internal variations in all layers, with enrichments at hanging and/or footwalls. The enriched nature of chromitites in PGEs compared to host pyroxenites is a General feature, independent of the layer thickness. The MLA results distinguish two principal groups of PGE mineral associations: the LG6, LG6, and MG1 are dominated by the malanite series, laurite, and PGE sulfarsenides, while the MG2 and MG2 II layers are characterized by laurite and PGE sulfides as well as Pt-Fe-Sn and PGE-Sb-Bi-Pb alloys. Differences in the PGE associations are attributed to postcumulus alteration of the MG2 and MG2 II layer, while the chromitites below, particularly LG6 and LG6a, contain a more pristine association.

Crystal size distribution of amphibole grown from hydrous basaltic melt at 0.6–2.6 GPa and 860–970 °C

We carried out three series of amphibole crystallization experiments from hydrous basaltic melt to calibrate the dependence of crystal growth rate on temperature and pressure in amphibole-bearing igneous rocks. One series of 100 h duration multi-anvil experiments were carried out at a constant pressure of 0.6 GPa and variable temperatures from 860 to 970 °C. The second series was conducted at a constant temperature of 970 °C and variable pressures from 0.6 to 2.6 GPa. The third series examined the time dependence at 970 °C and 0.6 GPa, with durations from 1 to 100 h. A verification experiment showing both reproducibility and the ability of these three series to predict behavior at novel conditions was performed in a piston cylinder at 1.0 GPa and 900 °C for 63 h. All experiments yielded mostly amphibole in a quenched glass of granitic to granodioritic composition. We used the two-dimensional thin section method to measure the crystal size distribution (CSD) of amphibole in the experimental products. Concave-down CSD curves at small sizes indicate a textural coarsening process during the crystallization. The CSD data were inverted using canonical CSD theory for CSD growth rate; maximum and average growth rates of amphibole were also inferred directly from the maximum and average grain size and crystallization time. The maximum growth rate is, of course, always larger than the average growth rate, which is in turn slightly larger than the CSD growth rate, suggesting that CSD growth rate is an adequate measure of the average growth rate of a mineral in magmatic rocks. The CSD growth rate increases with increasing temperature in the isobaric series and with increasing pressure at constant temperature. However, the growth rate is negatively correlated with crystallization time at constant temperature and pressure. Based on the experimental results, a functional form for evaluating growth rate at known pressure and temperature from an observed amphibole CSD was developed and applied to a diorite collected from the eastern Tianshan Mountains, Xinjiang Uygur autonomous region, NW China. The estimated growth rate of amphibole is between 1.6 × 10^(–9) mm/s and 5.6 × 10^(–7) mm/s, and combined with petrological constraints on pressure and temperature, the corresponding crystallization time was between 0.1 and 4.3 yr in the natural diorite.

Non-isothermal crystallization behavior of spinel crystals in FeO-SiO2-TiO2-V2O3-Cr2O3 slag

The growth behaviour of spinel crystals in vanadium slag with high Cr2O3 content was investigated and clarified by statistical analyses based on the Crystal Size Distribution (CSD) theory. The results indicate that low cooling rate and Cr2O3 content benefit the growth of spinel crystals. The chromium spinel crystals firstly precipitated and then acted as the heterogeneous nuclei of vanadium and titanium spinel crystals. The growth mechanisms of the spinel crystals at the cooling rate of 5 K/min consist two regimes: firstly, nucleation control in the temperature range of 1873 to 1773 K, in which the shapes of CSD curves are asymptotic; secondly, surface and supply control within the temperature range of 1773 to 1473 K, in which the shapes of CSD curves are lognormal. The mean diameter of spinel crystals increases from 3.97 to 52.21 µm with the decrease of temperature from 1873 to 1473 K.

Quantitative textural investigation of trachyandesites of Damavand volcano (N Iran): Insights into the magmatic processes

Damavand volcano is a dormant stratovolcano in northern Iran in the middle of the Alborz Mountains. Investigation of the magmatic processes responsible for the eruption of the volcano and the conditions of the magma chamber is important in order to understand the volcanism of this system. Owing to their higher abundance and younger age, trachyandesitic rocks are the main components of this volcano. To get insights into the crystallization of these rocks, we carried out a quantitative and qualitative petrographic study of three main volcanic units erupted between 63 and 66.5 years ago. Crystal Size Distribution (CSD) studies can reveal details about magmatic processes. Measuring 4732 individual plagioclase crystals and conducting a CSD study, revealed a non-straight and concave-up CSD curve for nearly all of the studied volcanic units which suggests the occurrence of similar physico-chemical processes responsible for their magmatism. Plagioclase crystals occur as microlites and phenocrysts; the phenocrysts show either oscillatory zoning or sieve textures. Each segment of the CSD curves are consistent with a particular plagioclase texture in all the studied volcanic units. The presence of different plagioclase textures and the concave-up shape of the CSD curves suggests the variation of the physico-chemical conditions of the magma chamber during the magmatism of the Damavand in this time period. Mixing of magmas with different crystal populations can be an alternative for this phenomena.

Understanding magmatic processes at Telica volcano, Nicaragua: Crystal size distribution and textural analysis

Telica volcano in Nicaragua currently exhibits persistent activity with continuous seismicity and degassing, yet it has not produced lava flows since 1529. To provide insight into magma chamber processes including replenishment and crystallization, crystal size distribution (CSD) profiles of plagioclase feldspar phenocrysts were determined for Quaternary Telica basalts and basaltic andesites. Textural analysis of 14 highly crystalline lavas (>20 vol% phenocrysts) indicates that the samples are dominated by sieve-textured plagioclase feldspar phenocrysts whose origin requires thermochemical disequilibrium within the magmatic system. The CSD curves display an inverse relationship between phenocryst length and population density. Concave-up patterns observed for the Telica lava samples can be represented by linear segments that define two crystal populations: a steeply sloping segment for small crystals ( 1.5 mm in length. The two crystal populations may be explained by magma replenishment and a mixing model in which a mafic magma is introduced to a stable chamber that is petrologically and geochemically evolving. Residence times calculated using the defined linear segments of the CSD curves suggest these magmatic processes occur over timescales on the order of decades to centuries. The crystal size distribution and textural analysis advocate for the current persistent activity as being consistent throughout Telica’s historic and prehistoric eruptive periods and driven by replenishment of mafic magma.

Quantitative Study on Crystallization-Induced Scaling in High-Concentration Direct-Contact Membrane Distillation

A novel method was proposed to quantify the scaling formation in high-concentration direct-contact membrane distillation (DCMD). The crystallization kinetics was studied using sodium chloride as a model scalant in a polyvinylidene fluoride (PVDF) hollow-fiber module. Combined with experimental investigations, a newly established mathematical method, namely, crystallization on membrane surface (COMS) modeling, was developed and verified in terms of the membrane distillation (MD) performance and deposited crystal characteristics (median size and total number). With the experimental results of the feed concentration and DCMD performance test, the critical point of crystal deposition on the membrane surface associated with a major water flux decline was observed. On the basis of the crystal size distribution curves obtained at various operation times, the crystallization kinetic constants were determined. Combining the derived crystallization kinetic expressions with MD transport equations, the COMS modeling ...

Nucleation and crystal growth of zeolite A synthesised from hydrogels of different density

Various factors affect the process of zeolite formation and thus the characteristics of the ultimate material. The present study is devoted to the influence of the density of the starting aluminosilicate hydrogels, i.e. the amount of the solid phase in the system, on the characteristics of the final product. Different density (viscosity) of the five alumosiliate gels was obtained by changing the ratio SiO2/H2O, while the ratios Na2O/H2O and SiO2/Al2O3 remained constant. The freshly prepared hydrogels were hydrothermally treated at 80 °C under static conditions. The reaction course was followed by a set of complementary analyses including XRD, chemical analysis, microscopy and thermogravimetry. The linear rate of crystal growth was calculated according to Zhdanov and Samulevich's method. The crystal size distribution curves of the end products were measured and the corresponding curves of nuclei distribution within gel matrix and specific number of crystals were calculated. The observed effect of “local ordered structure” (potential nuclei) formation was commented on from the viewpoint of the autocatalytic nucleation theory.

Settling and compaction of olivine in basaltic magmas: an experimental study on the time scales of cumulate formation

A series of centrifuge-assisted settling experiments of 30 vol % olivine in 70 vol % basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates. The settling experiments were performed in a centrifuging piston cylinder at 200–1,500g, 1,270–1,280 °C, and 0.8–1.1 GPa on previously annealed and texturally equilibrated samples. The mechanical settling of the dense olivine suspension occurs at about 1/6 the speed of simple Stokes settling, resulting in a sedimentation exponent n = 4.1(6) in agreement with predictions from analogue systems. The porosity (φ m ) of the orthocumulate resulting from gravitational settling of crystals is about 54 % and formation times of olivine orthocumulates result to 0.1–10 m day−1 (for an initial crystal content of the melt of 1–5 % and grain sizes of 2–10 mm). After mechanical settling, olivine grains rest on each other, and further compaction occurs through pressure dissolution at grain contacts, olivine reprecipitation where olivine is in contact with melt, and concomitant expulsion of excess liquid from the cumulate layer. With centrifugation at 400g for 50 h, porosities as low as 30.3 vol % were achieved. The olivine content at the bottom of the gravitational cumulate is 1 − φm ~ log(Δρ · h · a · t), where Δρ is the density difference between crystals and melt, h the crystal layer thickness, a the acceleration, and t the time of centrifuging. Compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction. The compaction limit, that is the lowermost porosity to be reached by this mechanism, is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3–5 % porosity for the experiments. Crystal size distribution curves and a growth exponent n of 3.1(3) indicate that diffusion-controlled Ostwald ripening is the dominant crystal growth mechanism. The above relationship, combined with a linear scaling for grain size as appropriate for reaction-controlled pressure solution creep, allows calculation of formation times of adcumulates. If chemical compaction is dissolution–reprecipitation limited, then single layers of natural olivine adcumulates of ½ m thickness with 70–75 vol % olivine at the base (as observed in the Rhum layered intrusion) would have typical formation times of 0.4–3 years for grain sizes of 2–10 mm. This time scale compares favourably with characteristic cooling times of sills. If a greater than 20-m-thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt, then compaction proceeds to the compaction limit within a few years. It can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades, a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 %. In conclusion, gravitational settling and gravitation-driven chemical compaction are feasible cumulate-forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions.

Magmatic processes inferred from chemical composition, texture and crystal size distribution of the Heikongshan lavas in the Tengchong volcanic field, SW China

The products of volcanic eruption contain abundant information about the magmatic processes in a magma chamber. In this paper, we study the magmatic processes associated with the Heikongshan volcano in the Tengchong volcanic field (TVF), Yunnan Province of southwestern China, through chemical composition, texture and crystal size distribution (CSD) analysis of volcanic lavas. The Heikongshan lavas comprise five flow units associated with three different episodes of volcanic eruption since the middle Pleistocene. Bulk rock chemical analyses indicate that unit I is a basaltic trachyandesite, units II–IV are of trachyandesite, and unit V is composed of trachyte. The textures of these lavas can be further classified into three main groups accordingly, each represented by different phenocryst and groundmass assemblages, reflecting the changes of physical environment in the magma chamber. CSD analysis of plagioclase phenocrysts yields a mixture of CSD curves, some of which are nearly straight lines, while others are weakly concave-up. These concave-up CSD curves are consistent with magma mixing between host magma and early residual dacitic magma. In specifics, the CSD curve of large plagioclase phenocrysts (>1.6mm) in unit I reflects the characteristic of early dacitic magma. Whereas, the CSD curves of small plagioclase phenocrysts (<1.6mm) in units II–IV yield straight lines, indicating a relatively steady open system in which they formed. In contrast, influenced by the plagioclase crystals in early dacitic magma, the CSD curves of large plagioclase phenocrysts (>1.6mm) in units II–IV deviated from their original trend and formed in concave-up shape. Finally, in the last episode when unit V formed, the magma temperature decreased, resulting in the formation of relatively large amount of small-sized plagioclase crystals. Such process is reflected in the CSD diagram by a relatively higher value of intercept and the steeper slope.