Fluids, Melting, Granulites and Granites: A Controversy – Reply to the Commentary of J.D. Clemens, I.S. Buick and G. Stevens

Aranovich, L. Ya.; Makhluf, A. R.; Manning, Craig E.; Newton, Robert C.; Touret, Jacques

doi:10.1016/j.precamres.2016.03.020

Cited by 10 publications

(4 citation statements)

References 44 publications

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“…Petrologists have distinguished two cases of hydrous crustal melting: fluid-absent melting 2 , also called dehydration melting, where water exclusively results from breakdown of water-bearing minerals such as micas at temperature of ≥750–850 °C, or fluid-present melting, where in addition to water-bearing minerals, a free fluid phase (“free water”) embedded in the porosity at subsolidus conditions enables melting at lower temperatures (<750 °C). Both cases produce water-rich melts (with >5 wt% H 2 O), but fluid-present melting broadly tends to produce more abundant melts that are richer in water 6 . The presence of a widespread free fluid phase in the deep crust and its link with crustal melting remains a strongly debated issue, including in the Himalayas 1 , 6 – 8 .…”

Section: Introductionmentioning

confidence: 99%

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Melting conditions in the modern Tibetan crust since the Miocene

et al. 2018

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Abundant granitic rocks exposed in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes. However, complex field relations and superposition of multiple tectonic events make it difficult to determine the role of melting in orogenesis. In contrast, geophysical measurements image present-day crustal conditions but cannot discriminate between partial melt and aqueous fluids. Here we connect pressure–temperature paths of Himalayan Miocene crustal rocks to the present-day conditions beneath the Tibetan plateau imaged with geophysical data. We use measurements of electrical conductivity to show that 4–16% water-rich melt is required to explain the crustal conductivity in the north-western Himalaya. In southern Tibet, higher melt fractions >30% reflect a crust that is either fluid-enriched (+1% H2O) or hotter (+100 °C) compared to the Miocene crust. These melt fractions are high enough for the partially molten rocks to be significantly weaker than the solid crust.

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Section: Introductionmentioning

confidence: 99%

“…Both cases produce water-rich melts (with >5 wt% H 2 O), but fluid-present melting broadly tends to produce more abundant melts that are richer in water 6 . The presence of a widespread free fluid phase in the deep crust and its link with crustal melting remains a strongly debated issue, including in the Himalayas 1 , 6 – 8 .…”

Section: Introductionmentioning

confidence: 99%

Melting conditions in the modern Tibetan crust since the Miocene

et al. 2018

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“…Although there is still some debate as to whether or not granulites need externally derived fluids in order to initiate melting (Aranovich et al, 2016;Clemens et al, 2016), the conclusion from melting experiments (Gao et al, 2016) and from trapped melt inclusions in peritectic minerals (Bartoli et al, 2016;Stepanov et al, 2016;Ferrero et al, 2018) is that granulites are often associated with loss of granitic melt. Given that we can quantify relative K/Ca decreases, we are also able to place constraints on the amount of melt loss required to form our samples by modelling the partitioning of K and Ca between melt and residual minerals.…”

Section: Melt-loss Modellingmentioning

confidence: 99%

Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

Antonelli¹,

Paolo²,

Chacko³

et al. 2019

Geochem. Persp. Let.

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Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.

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“…This point of view has been recently challenged by some works, which have highlighted the potential role of free H 2 O and/or highly saline solutions (brines) during melting of the continental crust: water-fluxed melting (Weinberg & Hasalová, 2015a) and brine-assisted melting (Aranovich, Makhluf, Manning, & Newton, 2014). Not surprisingly, these studies have added fuel to the fire: see Clemens and Stevens (2015) vs. Weinberg and Hasalová (2015b), and Aranovich, Makhluf, Manning, Newton, and Touret (2016) vs. Clemens, Buick, and Stevens (2016). It is important to highlight the fact that, despite the opposing views on the prevailing fluid regime during crustal melting, fluid-present, and -absent processes are not mutually exclusive.…”

Section: Implications For the Fluid Regime In A High-grade Terranementioning

confidence: 99%

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

Carvalho

Bartoli

Ferri

et al. 2018

Journal Metamorphic Geology

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We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

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Fluids, Melting, Granulites and Granites: A Controversy – Reply to the Commentary of J.D. Clemens, I.S. Buick and G. Stevens

Cited by 10 publications

References 44 publications

Melting conditions in the modern Tibetan crust since the Miocene

Melting conditions in the modern Tibetan crust since the Miocene

Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

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