Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)
О. А. АндрееваЕ. О. ДубининаIrina A. AndreevaV. V. YarmolyukA. Yu. BychkovAnastassia Y. BorisovaJianqing JiXin ZhouЕ. V. КоvalchukSergey Y. BorisovskyА. А. Аверин
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The balance of CO 2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO 2 -rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ 18 О and δ 13 С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO 3 , 24 wt% MgCO 3 and 3 wt% CaCO 3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.Keywords:
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Carbonate minerals
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Phenocryst
Liquidus
Supersaturation
Recrystallization (geology)
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Melt inclusions trapped in phenocrysts provide a unique picture of magma systems prior to modification by crustal processes. However, post-entrapment crystallization complicates their interpretation. Re-heating the phenocryst to the temperature of entrapment is a commonly applied method to recover the original melt composition. To understand the effects of re-homogenization, we compared the composition of re-heated and naturally quenched melt inclusions and inclusion compositions that had been subjected to over-heating and under-heating to examine the degree to which anomalous compositions were produced. Our results on plagioclase hosted inclusions from ocean floor basalts indicate that the general patterns represented by naturally quenched inclusions are the same as observed for rehomogenized inclusions. Most important, the range of minor elements described for plagioclase hosted inclusions from basalts is found in naturally quenched inclusions, and is therefore not a consequence of the re-homogenization process.
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Compositions of plagioclase-melt pairs are commonly used to constrain temperatures (T), dissolved water contents (H2O) and pressures (P) of pre-eruptive magma storage and transport. However, previous plagioclase-based thermometers, hygrometers, and barometers can have significant errors, leading to imprecise reconstructions of conditions during plagioclase growth. Here, we explore whether we can refine existing plagioclase-based hygrothermobarometers with either plagioclase-melt or melt-only chemistry (± T/H2O), calibrated using random forest machine learning on experimental petrology data (n= 1152). We find that both the plagioclase-melt and melt-only models return similar cross-validation root-mean-square errors (RMSEs), as the melt holds most of the P-T-H2O information rather than the plagioclase. T/H2O-dependent melt models have test set RMSEs of 25 °C, 0.70 wt.% and 76 MPa for temperature, H2O content and pressure, respectively, while T/H2O-independent models have RMSEs of 38 °C, 0.97 wt.% and 91 MPa. The melt thermometer and hygrometer are applicable to a wide range of plagioclase-bearing melts at temperatures between 664 and 1355 °C, and with H2O concentrations up to 11.2 wt.%, while the melt barometer is suitable for pressures of ≤500 MPa. An updated plagioclase-melt equilibrium model has also been calibrated, allowing equilibrium anorthite content to be predicted with an error of 5.8 mol%. The new P-T-H2O-An models were applied to matrix glasses and melt inclusions from the 1980 Mount St Helens (USA) and 2014-2015 Holuhraun (Iceland) eruptions, corroborating previous independent estimates and observations. Models are available at: https://github.com/kyra-cutler/Plag-saturated-melt-P-T-H2O-An, enabling assessment of plagioclase-melt equilibrium and characterisation of last-equilibrated P-T-H2O conditions of plagioclase-saturated magmas.
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In order to quantify H2O content and chemical composition of island arc low-K tholeiite magma that crystallized Ca-rich plagioclase, melt inclusions of a typical island arc tholeiite from Izu-Oshima volcano (34°N 44', 139°E 24') were analyzed. Composition of studied plagioclase ranges widely from An83 to An95. Composition of studied melt inclusions also shows wide variation, which suggests that the melt inclusions represent various stages of crystallization differentiation at Izu-Oshima volcano. Ca/Na ratios of plagioclase-hosted melt inclusions are comparable with compositions of aphyric lava, which preclude an exotic origin for the Ca-rich plagioclase. Analyzed H2O content of the melt inclusions ranges from 0.2 to 2.4 wt.% (0.2 to 1.4 wt.% for plagioclase-hosted melt inclusions and 0.8 to 2.4 wt.% for olivine-hosted melt inclusions). Ca/Na partition coefficient between plagioclase and hydrous basaltic melt, KD(Ca-Na)plag-melt, is empirically calibrated based on experimental data as ln KD(Ca-Na)plag-melt = 4100/T(K) -800* P(GPa)/T(K) + 2.2* ln (Al2O3melt(wt.%)/Sio2melt(wt.%)) + 0.33* √ H2Omelt(wt.%). Based on the equilibria between host plagioclase and melt inclusion and taking effect of overgrowth into consideration, 3 to 6 wt.% H2O in melt is required. The lower H2O content of the analyzed melt inclusions is probably due to the leakage of volatiles through the host crystal during decompression, eruption and quench. Variation in estimated H2O content in the melt at the time of crystallization of plagioclase (3 to 6 wt.%) can be due to polybaric crystallization from H2O-saturated melt.
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Abstract Compositions of plagioclase‐melt pairs are commonly used to constrain temperatures (T), dissolved water contents (H 2 O) and pressures (P) of pre‐eruptive magma storage and transport. However, previous plagioclase‐based thermometers, hygrometers, and barometers can have significant errors, leading to imprecise reconstructions of conditions during plagioclase growth. Here, we explore whether we can refine existing plagioclase‐based hygrothermobarometers with either plagioclase‐melt or melt‐only chemistry (±T/H 2 O), calibrated using random forest machine learning on experimental petrology data ( n = 1,152). We find that both the plagioclase‐melt and melt‐only models return similar cross‐validation root‐mean‐square errors (RMSEs), as the melt holds most of the P‐T‐H 2 O information rather than the plagioclase. T/H 2 O‐dependent melt models have test set RMSEs of 25°C, 0.70 wt.% and 76 MPa for temperature, H 2 O content and pressure, respectively, while T/H 2 O‐independent models have RMSEs of 38°C, 0.97 wt.% and 91 MPa. The melt thermometer and hygrometer are applicable to a wide range of plagioclase‐bearing melts at temperatures between 664 and 1355°C, and with H 2 O concentrations up to 11.2 wt.%, while the melt barometer is suitable for pressures of ≤500 MPa. An updated plagioclase‐melt equilibrium model has also been calibrated, allowing the equilibrium anorthite content to be predicted with an error of 5.8 mol%. The new P‐T‐H 2 O‐An models were applied to matrix glasses and melt inclusions from the 1980 Mount St Helens (USA) and 2014–2015 Holuhraun (Iceland) eruptions, corroborating previous independent estimates and observations. Models are available at https://github.com/kyra‐cutler/Plag‐saturated‐melt‐P‐T‐H2O‐An , enabling assessment of plagioclase‐melt equilibrium and characterization of last‐equilibrated P‐T‐H 2 O conditions of plagioclase‐saturated magmas.
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