A recent fascinating development in the study of high-grade metamorphic basements is represented by the finding of tiny inclusions of crystallized melt (nanogranitoid inclusions) hosted in peritectic ...phases of migmatites and granulites. These inclusions have the potential to provide the primary composition of crustal melts at the source. A novel use of the recently-published nanogranitoid compositional database is presented here. Using granulites from the world-renowned Ivrea Zone (NW Italy) on which the original melt-reintegration approach has been previously applied, it is shown that reintegrating melt inclusion compositions from the published database into residual rock compositions can be a further useful method to reconstruct a plausible prograde history of melt-depleted rocks. This reconstruction is fundamental to investigate the tectonothermal history of geological terranes.
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•Nanogranitoid inclusions have the potential to provide the primary composition of crustal melts at the source.•A novel use of the nanogranitoid compositional database is presented here.•Reintegrating melt inclusion compositions from the published database into residual rock compositions can be a further useful method to reconstruct a plausible prograde history of melt-depleted rocks.
Estimates on the geological carbon cycle are subject to large uncertainties that can be reduced by thorough observation of rocks. In this contribution, we focus specifically on C–O–H fluid-melt-rock ...interactions in graphitic metapelitic granulites and on their bearing to the carbon budget of granulitic roots of continents. We provide robust microstructural and thermometric constraints on the coexistence of anatectic silicate melts and C–O–H fluids up to near ultrahigh temperature conditions in the archetypal crustal section of Ivrea-Verbano Zone (IVZ, Italian Alps). Fluid inclusions in garnet are investigated before and after high-temperature experiments, and contain considerable proportions of CO2, CH4, N2, but lower H2O than predicted for graphitic systems at granulite facies. When comparing and contrasting the melt compositions obtained by Perple_X and rhyolite-MELTS with natural melts from IVZ, a much better match is obtained by the former, questioning the choice of rhyolite-MELTS for modelling melting equilibria of metasedimentary rocks and for quantifying carbon budget of the lower crust. Overall, data show that assuming only a limited extent of fluid-melt immiscibility in the deep crust contradicts the evidence from natural rocks and prompts to an incomplete view of actual carbon behavior and carbonic fluids. The available experimental dataset on CO2 solubility in felsic melts cannot be used to interpret the volatile budget of melt inclusions in graphitic migmatites and granulites, as most solubility experiments were conducted under carbonate-saturated (i.e. highly oxidizing) conditions which maximize CO2 content of melt, compared to graphitic (i.e. more reducing) protoliths. As a consequence, thermodynamic models still cannot account for all the complexities related with interactions among H2O–CO2–CH4 ternary fluids, H2O- and CO2-bearing anatectic melts and graphite-bearing residues in graphitic metapelites. Targeted experimental studies are therefore crucial to boost substantial computational efforts, before any precise estimates on carbon budget and fluxes in the lower anatectic crust can be made.
•C-bearing melt and fluid inclusions coexist in same clusters of peritectic garnet in granulites.•Fluid-melt immiscibility in graphitic rocks may occur up to near ultrahigh temperature conditions.•Care is required in considering CO2 solubility experiments as synthetic analogues of natural suprasolidus graphitic systems.•Thermodynamic models still cannot account for all complexities of fluid-melt-rock interactions at suprasolidus conditions.•Reliable estimates of carbon budget in lower crust cannot be made without new experimental studies and computational tools.
•Experiments, models and nanogranitoids are used concurrently and comparatively.•Water-fluxed melting does not produce a specific geochemical signature in melt.•Granite compositions are produced at ...low- to medium-P, whatever the fluid regime is.•The abundance of peraluminous granites does not imply a dry nature of orogenic crust.
The diverse fluid regimes during melting of the metasedimentary crust have been often discriminated on the basis of the composition of anatectic granitoids, with granites indicating fluid-absent melting conditions and trondhjemitic compositions suggesting the addition of external water in the source region. The lack of abundant metasedimentary-derived trondhjemites in the geological record is supposed to prove the minor role of water-fluxed melting in the crustal maturation. In terms of trace elements, instead, Rb, Sr and Ba contents and their ratios have been commonly used to discriminate dehydration vs. water-fluxed melting scenarios. Here I show that reconciling results of melting experiments, thermodynamic modeling and nanogranitoid study brings out a different picture. Equilibrium thermodynamics cannot properly reproduce melt compositions of the selected benchmark experiments, with the latter having trondhjemitic compositions mainly for the metastable behavior of muscovite during laboratory runs. The formation of sufficient volumes of extractable trondhjemitic melts is related to high pressure melting conditions (≥8 kbar at 700°C and ≥11 kbar at 750°C) or K-poor bulk rock compositions, rather than to the only presence of water. At low- to medium-pressure, crustal melts are granites in composition, whatever the fluid regime is. It is inferred that the abundance of anatectic peraluminous granites (compared to metasedimentary-derived trondhjemites) does not imply a dry nature of the orogenic crust. Likewise, the use of LILE (Rb, Sr and Ba) signatures may lead to erroneous conclusions on the fluid regime of the deep continental crust.
ABSTRACT
Suprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. ...The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt‐depleted rocks by means of phase equilibria modelling. It involves the stepwise down‐temperature reintegration of a certain amount of melt into the residual bulk composition along an inferred P–T path, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt‐reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets of P–T pseudosections were constructed progressing step by step, to lower temperatures along the inferred P–T paths. Melt‐reintegration was done following one‐step and multi‐step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass–balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e. melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt‐reintegration approach is applied to highly residual (SiO2 <55 wt%) rocks, or to rocks for which H2O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating).
Structural and microstructural analyses and phase equilibria modeling of migmatitic amphibolite-facies metapelites from the late Carboniferous Calamita Schists, on the Island of Elba, Italy, show how ...the interplay between partial melting and regional (far-field) deformation assisted deformation at very shallow (P ≤ 0.2 GPa) crustal levels. Partial melting was caused by the heat supplied by an underlying late Miocene intrusion (Porto Azzurro pluton) and occurred by biotite continuous melting. The produced melt remained in situ in patches, likely experienced limited migration in stromatic migmatites, and crystallized as a K-feldspar + plagioclase + quartz assemblage. Deformation in the presence of melt occurred by melt-enhanced grain boundary sliding, producing well-foliated high-strain zones with weak evidence of subsolidus deformation at the microscale where the original melt was present. Melt crystallization caused strain hardening and forced subsolidus deformation into localized mylonitic shear zones. The localized character of retrograde deformation was likely determined by the heterogeneous distribution/ingress of fluids in the aureole that locally assisted strain localization, enhancing dislocation creep and reaction softening.
•K-feldspar + plagioclase patches record in-situ partial melting in the upper crust.•Melting was caused by granite emplacement and occurred in the andalusite field.•Deformation is distributed in the partially molten rocks.•Melt crystallization causes strain localization into mylonitic shear zones.
Abstract Large Igneous Province eruptions coincide with many major Phanerozoic mass extinctions, suggesting a cause-effect relationship where volcanic degassing triggers global climatic changes. In ...order to fully understand this relationship, it is necessary to constrain the quantity and type of degassed magmatic volatiles, and to determine the depth of their source and the timing of eruption. Here we present direct evidence of abundant CO 2 in basaltic rocks from the end-Triassic Central Atlantic Magmatic Province (CAMP), through investigation of gas exsolution bubbles preserved by melt inclusions. Our results indicate abundance of CO 2 and a mantle and/or lower-middle crustal origin for at least part of the degassed carbon. The presence of deep carbon is a key control on the emplacement mode of CAMP magmas, favouring rapid eruption pulses (a few centuries each). Our estimates suggest that the amount of CO 2 that each CAMP magmatic pulse injected into the end-Triassic atmosphere is comparable to the amount of anthropogenic emissions projected for the 21 st century. Such large volumes of volcanic CO 2 likely contributed to end-Triassic global warming and ocean acidification.
Abstract
Exceptional magmatic events coincided with the largest mass extinctions throughout Earth’s history. Extensive degassing from organic-rich sediments intruded by magmas is a possible driver of ...the catastrophic environmental changes, which triggered the biotic crises. One of Earth’s largest magmatic events is represented by the Central Atlantic Magmatic Province, which was synchronous with the end-Triassic mass extinction. Here, we show direct evidence for the presence in basaltic magmas of methane, generated or remobilized from the host sedimentary sequence during the emplacement of this Large Igneous Province. Abundant methane-rich fluid inclusions were entrapped within quartz at the end of magmatic crystallization in voluminous (about 1.0 × 10
6
km
3
) intrusions in Brazilian Amazonia, indicating a massive (about 7.2 × 10
3
Gt) fluxing of methane. These micrometre-sized imperfections in quartz crystals attest an extensive release of methane from magma–sediment interaction, which likely contributed to the global climate changes responsible for the end-Triassic mass extinction.
The Euganean Hills (NE Italy) magmatic district represents the final volcanic activity of the Veneto Volcanic Province. Alkaline to subalkaline magmatic suite dominated by intermediate to felsic ...volcanic rocks characterises the latest volcanic activity of the Euganean Hills. Magmatic (intrusive and volcanic) enclaves are common in Euganean Hills trachytes. We used the ability of clinopyroxene to record variations of P, T, and fO2 to reconstruct the geological history of the volcanic enclaves and trachytic host. Despite similar major and trace elements composition, clinopyroxene from host is higher in Ca and Na (and Fe3+) and lower in Mg than enclaves and is slightly enriched in trace elements but with the same pattern distribution. Minor differences in geochemistry and crystal structure of clinopyroxene from enclaves and trachytic host suggest similar parental magmas that differs by small degrees of fractional crystallisation. Clinopyroxene geobarometry performed combining X-ray diffraction with mineral geochemistry for volcanic enclaves–trachytic host combined with amphibole geobarometry for intrusive enclaves and crystal mushes points to a crystallisation pressure range between 4.8–2.0 kbars. Our data support the model of a complex system of magma chambers at intermediate to shallow crustal level where mafic magma accumulated, evolved by fractionation processes and mixed.
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