Abstract Owing to advances in microanalytical techniques over the last 15 years, there is a growing database on the volatile contents of subduction-related magmas as recorded in melt (glass) inclusions trapped in phenocrysts in volcanic rocks. Basaltic magmas from subduction zones show a wide range of water contents, ranging from as high as 6–8 to 3000 ppm, suggesting that no melt inclusions sample undegassed arc magmas. The Cl and S contents of arc basaltic magmas are greater than midocean ridge basalts, indicating that these volatiles are also recycled from subducted sediment and altered oceanic crust back into the mantle wedge. Comparison of the fluxes of volatiles subducted back into the mantle along subduction zones and returned from the mantle to the surface reservoir (crust, ocean, and atmosphere) via magmatism suggests that there is an approximate balance for structurally bound H2O and Cl. In contrast, ∼50% of subducted C appears to be returned to the deep mantle by subduction, but uncertainties are relatively large. For S, the amount returned to the surface reservoir by subduction zone magmatism is only ∼15–30% of the total amount being subducted. Dacitic and rhyolitic magmas in arcs contain 1–6 wt.% H2O, a range that overlaps considerably with the values for basaltic magmas. Either basaltic parents for these differentiated magmas are relatively H2O-poor, or intermediate to silicic arc magmas form through open-system processes involving variable amounts of crustal melting, mixing with basalt and basaltic differentiates, and fluxing of CO2-rich vapor from mafic magma recharged into silicic magma bodies. Consideration of H2O–CO2 relations and gaseous SO2 emissions for intermediate to silicic arc magmas shows that such magmas are typically vapor-saturated during crystallization in the middle to upper crust. Gas emissions thus reflect migration and accumulation of volatiles within complex open magmatic systems.

Volatiles in subduction zone magmas : concentration and fluxes based on melt inclusion and volcanic gas data

http://www.geo.mtu.edu/EHaz/ConvergentPlatesClass/wallace/Wallace_2005_SOTA.pdf

Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data

This paper introduces Petrolog3, software for modeling (1) fractional and equilibrium crystallization, (2) reverse fractional crystallization at variable pressure, melt oxidation state and melt H2O contents, and (3) postentrapment reequilibration of melt inclusions in olivine. Petrolog3 offers an algorithm that allows calculations with a potentially unlimited number of (1) mineral-melt equilibrium models for major and trace elements and (2) models describing melt physical parameters such as density and viscosity, melt oxidation state, and solubility of fluid components in silicate melts. The current version of the software incorporates 46 mineral-melt equilibrium models for 8 minerals; 3 models describing distribution of trace elements between minerals and melt; 4 models of melt oxidation state; 1 model for H2O solubility in silicate melts; and 4 models describing melt density and viscosity. The idea behind the program is to provide the community of igneous petrologists and geochemists with a user-friendly interface for using any combinations of available mineral-melt equilibrium models for computer simulation of the crystallization process.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2011GC003516

Petrolog3: Integrated software for modeling crystallization processes

The equations for the chemical potentials of fayalite and forsterite components in olivine and liquid may be used to derive a thermodynamic expression for the exchange coefficient \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) (defined as the molar Mg/Fe2+ of the liquid divided by Mg/Fe of coexisting olivine) This expression, well known in the literature, shows that \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) is a function of temperature, pressure, and compositions of the olivine and liquid Quantitative application of this equation requires knowledge of the free energies and volumes of liquid and crystalline fayalite and forsterite, and a description of the non-ideality of Fe–Mg mixing in olivine Independent measurements of these parameters reported in the literature have been used to constrain the influence of temperature, pressure and olivine composition on iron–magnesium partitioning The other requirement for calculation of \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) is knowledge of the ratio of activity coefficients of Fe2+ and Mg in the liquid phase (γFe2+/γMg), but no independent predictive model for this exists To assess and predict the variation of γFe2+/γMg as a function of liquid composition, a database of almost 400 olivine–liquid pairs at 1 bar and 200 experiments at pressures to the upper stability limit of olivine has been considered Measured values of \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) in this data base vary from 017 to 045, and the data were chosen to cover wide ranges of temperature and olivine composition, in addition to liquid composition Within the framework of the thermodynamic equation, γFe2+/γMg is found to be a function of silica content and alkali content of the liquid Water is also inferred to have a direct influence at high pressure A predictive model for \( K_{D} {\kern 1pt} ^{{{\text{Mg}} - {\text{Fe}}}}_{{{\text{Ol}} - {\text{Liq}}}} \) is proposed which recovers all input data at the 1σ level of uncertainty It is used to assess equilibrium in published experimental studies of mantle melting and to illustrate the influence of individual parameters such as temperature and pressure Application of the model to the field of melt inclusions is also discussed

The thermodynamics of iron and magnesium partitioning between olivine and liquid: criteria for assessing and predicting equilibrium in natural and experimental systems

https://eprints.utas.edu.au/681/1/CG2002.pdf

Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications

We describe and model a potential re-equilibration process that can affect compositions of melt inclusions in magnesian olivine phenocrysts. This process, referred to as “Fe-loss”, can operate during natural pre-eruptive cooling of host magma and results in lower FeOt and higher MgO contents within the initially trapped volume of inclusion. The extent of Fe-loss is enhanced by large temperature intervals of magma cooling before eruption. The compositions of homogenised melt inclusions in olivine phenocrysts from several subduction-related suites demonstrate that (1) Fe-loss is a common process, (2) the maximum observed degree of re-equilibration varies between suites, and (3) within a single sample, variable degrees of re-equilibration can be recorded by melt inclusions trapped in olivine phenocrysts of identical composition. Our modelling also demonstrates that the re-equilibration process is fast going to completion, in the largest inclusions in the most magnesian phenocrysts it is completed within 2 years. The results we obtained indicate that the possibility of Fe-loss must be considered when estimating compositions of parental subduction-related magmas from naturally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions affected by Fe-loss will inherit not only erroneously low FeOt contents, but also low MgO due to the inherited higher Mg##of the residual melt in re-equilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclusions affected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheating will increase depending on the degree of re-equilibration, and can reach up to 50 °C in cases where complete re-equilibration occurs over a cooling interval of 200 °C.

/pdf/re-equilibration-of-melt-inclusions-trapped-by-magnesian-4cwia4p4t0.pdf

Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications

Paleozoic igneous rocks and mineral deposits are keys to revealing the tectonic setting of the Qoltag belt in Eastern Tianshan, Xinjiang Province. Investigation in petrology, mineralogy and geochemistry indicates that the Hongyuntan granitoids at the southern margin of the Qoltag area are typical Cale-Alkaline volcanic arc granitoids, and have genetic relation with melting of the lower crust affected by subduction of the oceanic crust. U-Pb age determinations of Zircon by LAM-ICP/MS show the Hongyuntan granitoids were intruded at 328.5 ± 9.3 Ma of Middle Carboniferous. By contrast, magmatism in the Middle Tianshan, south of the Qoltag area is not important. In the late Paleozoic of the Qoltag area, mineral deposits show zonal distribution with its southern margin being dominated by Fe deposits of the volcanosedimentary and hydrothermal type, and its northern margin being dominated by Cu-Mo deposits of the porphyry type. In combination with distribution of the volcanosedimentary strata and tectonic evolution of the Qoltag and its surrounding areas, it is suggested that a Qoltag ocean occurred in the Paleozoic. In early Paleozoic, the Paleo-Qoltag ocean subducted southward beneath the northern margin of the Tarim plate, resulting in the Middle Tianshan island arc. While in late Paleozoic, the Qoltag ocean changed in subduction polarity and decent northward beneath the Junggar-turpan-Hami block. The Hongyuntan granitoids were thus formed in this volcanic arc setting above the subducting Qoltag ocean crust of late Paleozoic in association with other Hercynian magmatic rocks, volcanosedimentary strata, and zonally distributed mineral deposits.

Geochronology, geochemistry and tectonic significances of the Hongyuntan granitoids in the Qoltag area, Eastern Tianshan

Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins. Trace element, fluid/melt inclusion and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti and CO2-rich fluid inclusions and give δ18O values (5.1-6.2‰) of mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 ± 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; δ18O 4.6-5.8‰), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59-4 Ma). In contrast, a metamorphic sapphire-ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO2-poor, but melt inclusions suggest some alkaline melt interaction. The δ18O values (5.1-6.2‰) overlap magmatic sapphire values. Interactions at contact zones (T = 780-940°C) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.

FN Della-Pasqua

Papers

Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications

Geochronology, geochemistry and tectonic significances of the Hongyuntan granitoids in the Qoltag area, Eastern Tianshan

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

Extreme effusive eruptions: Palaeoflow data on an extensive felsic lava in the Mesoproterozoic Gawler Range Volcanics

Geological and metallogenic relations of mineral deposits in SE Asia: geochronological studies