Wet or dry? The difficulty of identifying the presence of water during crustal melting

Schwindinger, Martin; Weinberg, Roberto F.; Clos, Frediano

doi:10.1111/jmg.12465

Cited by 40 publications

(15 citation statements)

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“…Similar residual pelitic compositions to the SiO 2 -depleted samples in the MIC contact aureole are reported elsewhere in LP-HT settings where H 2 O-flux melting is interpreted to have occurred, including the El Hoyazo granulite enclaves, Mt. Stafford metapelitic migmatites, Famatian orogeny in Argentina, and Kangaroo Island migmatites (Acosta-Vigil et al 2010, 2012Cesare, Mariani, & Venturelli, 1997;Schwindinger et al 2019;Sola et al 2017). These studies suggest that H 2 O-flux melting may be a relatively common process in some LP-HT settings that can easily be overlooked.…”

Section: Extensive Melt Production Controlled By H 2 O-flux Meltingsupporting

confidence: 81%

“…These studies suggest that H 2 O‐flux melting may be a relatively common process in some LP‐HT settings that can easily be overlooked. As noted by Sola et al () and Schwindinder et al (), melt production in these settings estimates may be severely underestimated if peak P–T conditions and dehydration melting reactions are the only parameters used, and the estimated total volume of melt produced is ignored.…”

Section: Discussionmentioning

confidence: 98%

“…Adding a relatively small proportion of H 2 O can significantly increase both melt production and the modal abundance of residuum minerals, without altering the melt composition, residuum mineral assemblage, and mineral compositions (e.g. Schwindinger, Weinberg, & Clos, ; Sola et al ). Here, H 2 O‐flux melting is consistent with both petrographical and field observations and modelling that indicate (a) peak temperatures of ~800°C and (b) extensive melt production and loss.…”

Section: Discussionmentioning

confidence: 99%

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Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Broadwell

Caddick

Beard

2019

Journal Metamorphic Geology

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Contact metamorphism associated with mafic intrusives is one of several mechanisms that has been invoked to produce extensive high‐temperature (HT) metamorphism and associated partial melting of the crust. Indisputable evidence for polymetamorphism in these settings can be difficult to decipher because both melt loss and retrogression (i.e. rehydration) can erase or obscure the records of earlier HT metamorphism by modifying HT mineral parageneses and compositions. Here, a combination of detailed field and petrographical observations, inverse mineral thermometry, and thermodynamic forward modelling is used to delineate the polymetamorphic history of migmatites from the Smith River Allochthon (SRA) in the central Appalachians. Bulk rock geochemical data suggest that some metapelitic samples lost a significant amount of melt during interpreted contact metamorphism with the Rich Acres gabbro, resulting in a residual bulk composition (<50 wt% SiO2, ~30 wt% Al2O3). Garnet cores (Grt1) in SiO2‐depleted samples are interpreted to grow during this HT contact metamorphism, with Fe‐Ti oxide thermometry on spinel inclusions in Grt1, cordierite–garnet thermometry, and thermodynamic forward modelling constraining peak P–T conditions during contact heating of the migmatites to ~800ºC and ∼0.5 GPa. This is associated with an inferred peak assemblage prior to melt loss of crd+kfs+pl+grt+bt+spl (mag+usp+hc)+ilm+sil+qtz+melt. Garnet in SiO2‐depleted samples has a distinct high‐Ca rim (Grt2), which appears to record a younger metamorphic event. A combination of substantial melt loss and later rehydration appears to be a major control on the ability of SiO2‐depleted samples to faithfully record evidence for this polymetamorphism. The tectonic implications of this younger metamorphic event are not entirely clear, but it appears to record renewed burial and heating of the SRA sometime after the Taconic orogeny, which may be related to either the neo‐Acadian or Alleghanian orogenies.

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Section: Extensive Melt Production Controlled By H 2 O-flux Meltingsupporting

confidence: 81%

Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

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Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Broadwell

Caddick

Beard

2019

Journal Metamorphic Geology

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“…Unfortunately, the term has found traction in the literature and has been used to describe many examples where melting was induced at P – T conditions above the water‐saturated solidus (e.g. Aguilar, Liesa, Reche, & Powell, 2016; Collins et al., 2016; Fornelli et al., 2002; García‐Arias et al, 2015; Genier et al., 2008; Milord et al., 2001; Sawyer, 2010; Schwindinger et al., 2019; Sola et al., 2017; Weinberg & Hasalová, 2015). Whereas some of these examples specify that the target rocks were dry and not melt‐bearing prior to the addition of H 2 O (i.e.…”

Section: Discussionmentioning

confidence: 99%

“…Despite this, there has been a recent resurgence of interest in the role that H 2 O may play in crustal melting and the generation of granitic melt (e.g. Collins, Huang, & Jiang, 2016; Schwindinger, Weinberg, & Clos, 2019; Weinberg & Hasalová, 2015). The compilation of Weinberg and Hasalová (2015) presents numerous examples of anatectic terranes where melting through the addition of H 2 O is inferred to have occurred.…”

Section: Introductionmentioning

confidence: 99%

Mineral equilibrium constraints on the feasibility of diffusive H₂O‐fluxed melting in the continental crust

Tafur

Diener

2020

Journal Metamorphic Geology

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Generation of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H2O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low‐P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H2O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H2O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H2O added. aH2normalO remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher aH2normalO. Our results demonstrate that, at the site of melting, melting is driven by diffusion of H2O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water‐rich fluid phase. Melting will initiate and proceed as long as a μH2normalO gradient exists between the H2O source and target lithology. Our calculations show that an ordinary magma, such as an I‐type magma with typical H2O content, has a μH2normalO high enough to be a viable H2O source, allowing diffusive H2O‐fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H2O, which necessitates a continuously advecting H2O source such as a magma conduit or melt‐bearing shear zone. A magmatic H2O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear‐zone hosted magma conduits are localized features, diffusive H2O‐fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.

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Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective

Bartoli

Acosta-Vigil

Cesare

et al. 2019

Contrib Mineral Petrol

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Despite melt inclusions in migmatites and granulites have provided a wealth of information on crustal anatexis in different geological scenarios, yet a complete compositional study (including trace elements and H2O) is missing for the Himalayas. In this contribution, we focus on nanogranitoids occurring in peritectic garnet of migmatites from Kali Gandaki valley in central Nepal (Greater Himalayan Sequence, GHS). Their microstructural position proves that these melts were produced at 650-720 °C and 1.0-1.1 GPa, during the Eohimalayan prograde metamorphism (41-36 Ma) associated to crustal thickening.Nanogranitoid compositions (mostly granodiorites, tonalities and trondhjemites) resemble those of experimental melts produced during H2O-present melting of metasedimentary rocks.They have variable H2O concentrations (6.5-14.4 wt%), which approximate the expected minimum and maximum values for crustal melts produced at the inferred P-T conditions. These compositional signatures suggest that melt formation occurred in the proximity of the Click here to access/download;Manuscript;Bartoli_et_al.May2.docx Click here to view linked References H2O-saturated solidus, in a rock-buffered system. The low to very low contents of Zr (3-8 ppm), Th (0.1-1.2 ppm) and LREE (4-11 ppm) along with the weak to moderate positive Eu anomalies (Eu/Eu* =1.2-3.3), the high B concentrations (200-3400 ppm) and the high U/Th ratio (up to 21) point to the lack of equilibration between melt and accessory minerals and are consistent with melting of plagioclase at low temperature. Kali Gandaki nanogranitoids have recorded the beginning of melting in a H2O-present scenario that, in other GHS localities, may have produced voluminous crustal melts. We demonstrate that compositional comparison with nanogranitoids may be useful to reconstruct the petrogenesis of Eohimalayan granitoids.

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Wet or dry? The difficulty of identifying the presence of water during crustal melting

Cited by 40 publications

References 82 publications

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Mineral equilibrium constraints on the feasibility of diffusive H₂O‐fluxed melting in the continental crust

Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective

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Wet or dry? The difficulty of identifying the presence of water during crustal melting

Cited by 40 publications

References 82 publications

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Mineral equilibrium constraints on the feasibility of diffusive H2O‐fluxed melting in the continental crust

Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective

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Mineral equilibrium constraints on the feasibility of diffusive H₂O‐fluxed melting in the continental crust