Degree Of Melt Polymerization Research Articles

Understanding the extreme differentiation of granitic magmas through elemental geochemistry and Fe isotope signatures

The petrogenesis of extremely fractionated igneous rocks is closely linked to the evolution of the continental crust and the formation of rare metal mineralization. Extremely fractionated magmas evolve via fractional crystallization, hydrothermal evolution, and volatile-rich fluid–melt interaction, but the specific effects of these magmatic and magmatic–hydrothermal processes remain unclear. To understand the mechanisms by which extremely fractionated granitic magmas evolve, we characterized the geochemical and Fe isotope signatures of a suite of granites and pegmatites from the Baishitouquan (BST) granitic pluton in northwest China. The BST granitic system evolved from a high SiO2 and volatile-poor system to a low SiO2 and volatile-rich system as it transitioned from an evolutionary stage dominated by magmatic processes to one dominated by magmatic–hydrothermal processes. This transition is supported by changes in the degree of melt polymerization (NBO/T) and other geochemical indicators of fluid activity (e.g., TE1,3 (REE tetrad effect), Zr/Hf, Nb/Ta, and F/Cl ratios), suggesting that extremely fractionated granites form as a result of a combination of fractional crystallization and volatile-rich fluid–melt interactions. The bulk-rock samples of the BST pluton exhibit significant Fe isotope variability, with δ56Fe values ranging from 0.40 ± 0.02‰ in the least evolved leucogranite (Zone-a) to 0.65 ± 0.02‰ in the most evolved topaz albite granite (Zone-e). Notably, the increase in bulk-rock δ56Fe values during magma evolution is accompanied by a decrease in SiO2 contents, which is opposite to the general trend during granitic magma evolution. Given the correlations between δ56Fe values and indicators of magmatic differentiation (e.g., Fe2O3T, Rb, (Na + K)/(Mg + Ca), and (La/Lu)N), the variability in bulk-rock Fe isotope composition of granites is likely due to changes in magma composition, with garnet removal being the most likely mechanism fractionating Fe isotopes. Rhyolite-MELTS modeling, coupled with geochemical and petrographic characterization, indicate that the high concentrations of F and H2O in felsic magmas likely play critical roles in structurally modifying granitic melts by dramatically reducing their viscosity and enhancing fluid flow. The distinct geochemical–isotopic features of granites can, therefore, be used as tracers for identifying magmatic–hydrothermal processes and, potentially, rare metal mineralization.

Cerium speciation in silicate glasses: Structure-property relationships

Ascertaining elements oxidation state, coordination, and bonding environment provides an understanding of the parameters controlling the elements structural behavior, and in turn, the possibility of developing novel applications. Cerium doped materials are widely used for technological applications, mainly because of the strong UV absorption and the characteristic short decay time of the intense emission in the UV–Vis ranges. In this study, Ce speciation in different glass systems has been evaluated and related to variations in optical and physical properties, as well as to variations in glass network connectivity. Data obtained from X-ray Absorption Spectroscopy, Raman, and Photoluminescence Spectroscopy are presented. By using a multispectroscopy approach, we determined the Ce3+/Ce4+ redox ratio in silicate glasses/melts and the influence of the two different Ce species on structure and properties.Our study indicates that reduced Ce species are favored with an increase in the degree of melt polymerization but hindered in alkaline-rich bulk chemistries. Consequently, the oxygen ion activity (here represented by the theoretical optical basicity) might be used to estimate the Ce redox ratio's evolution. On the contrary, we show that glass luminescence properties cannot be tailored by merely adjusting the bulk chemistry. Furthermore, we propose an alternative approach for detecting and quantifying the different Ce species in glasses, and we show the occurrence of a non-symmetric Raman vibration at ~880 cm−1, whose intensity is well correlated with the Ce3+/Ce4+ redox ratio. We assign this band to the interaction of the SiO4 tetrahedra with Ce4+-O polyhedral. Thus, cerium preferentially links to the silicate network, and we examined the possibility of using Raman spectroscopy to detect and quantify the different Ce species.

Oxygen Isotope Fractionation between Phenocrysts and Melt: Equilibrium Estimation for the Alkaline Lavas of Changbaishan Volcano (Northeast China)

The mechanisms of the occurrence of nonequilibrium oxygen isotope fractionation between olivine and plagioclase phenocrysts and the groundmass of rocks (from alkaline basalts to alkaline rhyolites) of Changbaishan volcano are considered. The basaltic trachyandesite–trachyte–comendite–pantellerite rock series shows a systematic increase of the δ18O values of feldspar (from 5.4 to 7.8‰) and the rock groundmass (from 5.6 to 7.1‰). In contrast, the δ18О values of olivine decrease from 5.35‰ typical of mantle peridotites to significantly lower values (3.88–4.14‰). This is accompanied by changes in olivine composition from the Mg-rich (Fo = 74–79) to almost pure fayalite (Fo = 1), thus indicating a positive correlation between the Mg–number and δ18О of olivine. It is shown that feldspar and the rock groundmass are close to the oxygen isotopic equilibrium, whereas olivine–rock groundmass and olivine–feldspar pairs did not approach the oxygen isotopic equilibrium. The main mechanisms leading to the oxygen isotopic disequilibrium in the mineral–melt system are considered. An approach to the estimation of oxygen isotope equilibrium between the phenocrysts and melts is proposed. It takes into account the degree of melt polymerization (NBO/T ratio). This approach makes it possible to explain the positive correlation between δ18О values and olivine Mg-numbers in the Changbaishan lavas, as well as the occurrence of low δ18O values (down to +3‰) in ferrous olivine from alkaline–salic rocks. The δ18О values observed in the olivine and the rock groundmass reflect changes in the degree of melt polymerization during crystal fractionation. The proposed model does not require an additional geological process, in particular, the contribution of a low–δ18О material in the formation of the Changbaishan differentiated rock series.

Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.

Water-melt interaction in hydrous magmatic systems at high temperature and pressure

Experimental data on the structure and properties of melts and fluids relevant to water-melt interaction in hydrous magmatic systems in the Earth's interior have been reviewed. Complex relationships between water solubility in melts and their bulk composition [Al/Si-ratio, metal oxide/(Al + Si) and electron properties of metal cations] explain why water solubility in felsic magmas such as those of rhyolite and andesite composition is significantly greater than the water solubility in basalt melts. The silicate solubility in aqueous fluid is also significantly dependent on composition with metal oxide/(Al + Si) and electron properties of the metal cations, the dominant variables. Hydrogen bonding is not important in hydrous fluids and melts at temperatures above 500°C to 550°C and does not, therefore, play a role in hydrous magmatic systems. The properties of hydrous melts and aqueous solutions are governed by how the silicate speciation (Qn species, where n is the number of bridging oxygen in an individual species) varies with bulk composition, silicate composition, temperature, and pressure. The reactions that describe the interactions are similar in melts, fluids, and supercritical fluids. The degree of melt polymerization caused by dissolved water varies with melt composition and total water content. Silicate- and alkali-rich felsic magmatic melts are more sensitive to water content than more mafic magmas. Transport and configurational properties of hydrous magmatic melts can be modeled with the aid of the Qn speciation variations. Liquidus and melting phase relations of hydrous systems also can be described in such terms, as can minor and trace element partition coefficients. Stable isotope fractionation (e.g., D/H) can also be rationalized in this manner. Critical to these latter observations is the high silicate concentration in aqueous fluids. These components can enhance solubility of minor and trace elements by orders of magnitude and change the speciation of H and D complexes so that their fractionation factors change quite significantly. Data from pure silicate-H2O systems cannot be employed for these purposes.

Solubility and partitioning behavior of Au, Cu, Ag and reduced S in magmas

Experiments have been conducted at 200MPa, 800–1030°C, and fO2 0.8 log units below the Ni–NiO buffer to determine the solubility of Au, Cu and Ag in silicate melts and pyrrhotite. The metal activities were imposed by using Au965Ag020Cu015 alloy capsules. Sulfur-free and sulfur-bearing systems were studied with otherwise identical melt compositions to assess the relative effect of S on the solubility of these metals. The data show that the major element composition of the silicate melt only moderately affects the solubility of Au, Ag and Cu between basalt and dacite, yielding solubilities identical within 50% relative. In comparison, solubilities in the rhyolite melts are lower by a factor of 1.5–2.5 for Cu and higher by up to a factor of 5 for Ag, depending on the aluminum saturation index of the melt. The solubilitiy of Ag significantly increases with increasing peraluminousity above an aluminum saturation index of 1. The effect of melt composition is significant on the solubility of Au in S-bearing melts, in part due to its effect on the sulfur concentration at sulfide saturation. The effect of S is the most pronounced in peralkaline rhyolites and mafic melts, and minimal in peraluminous rhyolites. The solubilities of all three metals significantly decrease with decreasing temperature. The concentration of sulfur in the melt at sulfide saturation and its volatile/melt partition coefficient are primarily determined by the FeO activity in the melt and the activity coefficients of dissolved FeS species, which appear to correlate with the degree of melt polymerization. The volatile/melt partition coefficient of reduced S increases from 79±4 (1σ) to 635±80 as the melt composition changes from basalt to slightly peraluminous rhyolite, and from 225±13 to 776±148 as the aluminium saturation index increases from 0.7 to 1.1 in rhyolites.At 1000°C, pyrrhotite/silicate melt partition coefficients for Cu increase from 540±30 (1σ) to 1140±110 from basalt to dacite, whereas the partition coefficients for Ag are nearly constant at 50±10. The partition coefficients of Au increase from 180±20 to 900±210 from basalt to dacite. Pyrrhotite/rhyolite melt partition coefficients for Cu, Ag and Au increase by about an order of magnitude as temperature drops from 1000°C to 800°C.At typical S concentrations for arc magmas (500–2000μg/g), the budget of Ag in the magma will not be controlled by pyrrhotite, whereas primary control on the Au and Cu budget by pyrrhotite may only be relevant in intermediate to felsic magmas.

Amorphous materials: Properties, structure, and durability: Solubility and solution mechanisms of NOH volatiles in silicate melts at high pressure and temperature--amine groups and hydrogen fugacity

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (fH₂), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1-2.5 GPa pressure and 1300-1500 °C temperature ranges. The fH₂ was controlled at values defined by the magnetite-hematite (MH), Mn3O4-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H2O.

The importance of melt composition in controlling trace-element behaviour: an experimental study of Mn and Zn partitioning between forsterite and silicate melts

Partition coefficients for Mn 2+ and Zn 2+ between forsterite and melts in the system Fo-Ab-An at temperatures of 1150–1400°C and 1-bar pressure have been determined experimentally. For a given temperature the values of the Nernst partition coefficients ( D Mn and D Zn) depend on the composition, and hence structure, of the melt phase, however, for a given degree of melt polymerization (expressed as the number of non-bridging oxygens per tetrahedral cation, NBO/T), the temperature dependence of the partition coefficients is less than the error in measurement. In these systems, melt composition is therefore more important than temperature in controlling trace-element behaviour. The exchange partition coefficients K D( Mn Mg ) and K D( Zn Mg ) are also more dependent on NBO/T than on temperature. Previous attempts to extract the ΔS and ΔH for the exchange reaction by regressing ln K D vs. 1 T are not valid as they ignore the changes in K D controlled by melt composition. Our data imply that for this system ΔH is small and ΔS varies with melt composition. In deploymerized melts the Nernst partition coefficients are ∼0.8 for Zn 2+ and ∼0.6 for Mn 2+ whereas in highly polymerized melts, values up to 4.5 for Zn 2+ and 3.0 for Mn 2+ have been found. In these highly polymerized systems small changes in the degree of polymerization are accompanied by large changes in the partition coefficients. This implies that fixed values of mineral/melt partition coefficients are unlikely to be adequate in trace-element modelling studies of highly differentiated igneous rocks.