Fluid-absent Metamorphism Research Articles

Oxygen isotope composition of granulites from dabieshan in eastern China and its implications for geodynamics of yangtze plate subduction

The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ18O values relative to the normal mantle δ18O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ18O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ18O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ18O values before the granulite-facies metamorphism by interaction with a certain 18O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

Signification des fluides carboniques dans les granulites et les skarns du Sud-Est de Madagascar

In the granulites of southeast Madagascar, extensive skams are the signature of intense fluid circulation coeval with regional metamorphism. Fluid inclusions in different minerals from gneisses and skarns are CO 2-rich ( X CO 2 ≥ 0.8). Corresponding isochores are in good agreement with mineral thermobarometry. Such fluids, with high P CO 2 and P O 2 and low P H 2O , are in equilibrium with the observed mineral assemblages. Contrary to some assumptions that granulite fades were produced through fluid-absent metamorphism, these results demonstrate equilibrium between granulite mineral paragenesis and a CO 2-rich fluid phase.

Kinetics of pressure solution creep in quartz: theoretical considerations

The kinetics of pressure solution creep are formulated using chemical potentials generalized to nonhydrostatic states. Solving a coupling equation of diffusion and reaction on a spherical quartz grain with diameter d and grain boundary width w, the flow law of pressure solution creep is derived. As extreme cases, the flow law becomes: ϵ ̇ = (αν 2 SiO 2 KDw)(ν H 2O RTd 3) −1σ for the diffusion-controlled case and becomes: ϵ ̇ = (βν 2 SiO 2 κ +)(ν H 2O RTd) −1σ for the reaction-controlled case, where ϵ is strain rate, σ is deviatoric stress, ν is the molar volume, D is the diffusion coefficient through a wet grain boundary, K is the equilibrium constant, κ + is the rate constant of dissolution, R is the gas constant, T is temperature, and α and β are shape factors. Using the reaction constants determined by Rimstidt and Barnes (1980) and the grain boundary diffusion coefficients estimated by Nakashima (1995), the strain rate of pressure solution creep in metamorphic conditions for quartzose rocks is estimated as 10 −9∼13, 10 −8∼11, and 10 −7∼11 s −1 at 150, 250, and 350°C, respectively. These values, compared with the duration of regional metamorphism, suggest rapid pressure solution and dewatering in subduction zones followed by fluid-absent metamorphism.

Scapolite phase equilibria and carbon isotopes: constraints on the nature and distribution of CO 2 in the lower continental crust

Recent applications of scapolite phase equilibria and carbon isotopes are reviewed, in order to evaluate the role played by CO 2 in the genesis of granulites in the lowermost crust. Scapolite occurs mainly in mafic granulites from exposed high grade terranes, most of which have had a prior upper crustal history before equilibration at 9–10 kbar, and in high pressure (11–15 kbar) xenoliths and orthogneisses which likely represent mantle derived material that has not interacted with supracrustal lithologies. Calculated equilibrium CO 2 activities are sufficiently high (> 0.7) to be consistent with the presence of a CO 2-rich fluid phase accounting for the low H 2O activities in scapolite-bearing granulites and some xenoliths. CO 2 activities less than ca. 0.5 or the absence of scapolite are consistent with fluid absent metamorphism. The measured isotopic composition of carbon ( δ 13C PDB) in scapolite is only a tight constraint on the source of carbon in scapolite for mafic xenoliths and high pressure orthogneisses. Carbon isotope values for these lithologies (− 7 to − 2‰) overlap the range of values for CO 2 dissolved in or exsolved from basalts, and are consistent with a magmatic origin for the CO 2. The carbon isotope composition of scapolite in granulites from exposed terranes (− 10 to − 4‰) is more equivocal, and may instead be related to pregranulite facies carbonate present in the rock. Minimum values of volumetric fluid/rock, calculated to account for the modal abundance and composition of scapolite in granulites (on the order of 0.001 to 0.1), are much lower than those required for the desiccation of amphibolites to granulites (0.1 to 0.5). Thus scapolite should form in response to infiltration of such large volumes of CO 2, if P, T, and bulk compositional constraints are met. However only seven of twenty granulite occurrences contain primary carbonate-bearing scapolite in mafic granulites. It appears that CO 2 occurs in the lower crust only locally in a CO 2-rich fluid phase or dissolved in mafic melts, and not on a pervasive scale or at the high volumetric fluid/rock required the model of carbonic metamorphism.

Fluid-absent melting and the roles of fluids in the lithosphere: a slanted summary?

The presence or absence, nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks. Despite this, confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings. However, experimental and field investigations constrain these roles, and we are able to make some valid generalizations. Studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments. This commonly indicates internal lithological control of volatile activities. Under fluid-present conditions, crustal fluids do not appear to mix readily; fluid flow is generally structurally or lithologically controlled. The transition from amphibolite to granulite facies is characterized by a decrease in a H2O, as well as a temperature increase. In many terranes, these phenomena correlate with the appearance of migmatites. Some granulites are not migmatitic but have chemical and mineralogical characteristics of melt-depleted restites. This indicates efficient extraction of melts. Incongruent melting reactions involving the breakdown of crystalline hydrates, in crustal rocks, produce granitoid melts plus denser restites that are nearly or actually anhydrous. Experimental and theoretical treatments of fluid-absent melting show that micas and amphiboles decompose at 700 to 900°C, for mid- to lower-crustal pressures. The melts resemble the compositions of S- and I-type granitoids. Pervasive aqueous fluids could not generally have been present during this melting, since exposed granulite terranes are not characterized by high melt proportions, and the magmas themselves are water-undersaturated. Likewise, the general presence of very H 2O-poor pervasive fluids is unlikely, since this would allow only subsolidus dehydration at normally attainable crustal temperatures. In this case the amphibolite-granulite transition is possible at relatively low temperatures, and the resultant rocks would not contain evidence of in-situ melting. The granulite-granite connexion and effective crustal differentiation seem viable only during high-temperature, fluid-absent metamorphism. Data on continental geotherms show that the lower crust is not entirely granulitic. Granulites are produced in regions of anomalously high heat flow (the mantle connexion). Thus, inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes. Mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities, and probable fluid-absent conditions. Mantle-derived basaltic magmas too are volatile-bearing and it is plausible that volatile-fixing phases also occur in the mantle. Volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals, as in the crust.

Calculation of CO2 activities using scapolite equilibria: constraints on the presence and composition of a fluid phase during high grade metamorphism

Thermodynamic and phase equilibrium data for scapolite have been used to calculate CO2 activities (aCO2) and to evaluate the presence or absence of a fluid phase in high-grade scapolite bearing meta-anorthosite, granulites, calc-silicates, and mafix xenoliths. The assemblage scapolite-plagioclase-garnet±quartz may be used to calculate or limit aCO2 by the reaction Meionite+Quartz = Grossular+Anorthite+CO2. Granulites from four high-grade terranes (Grenville Province, Canada; Sargut Belt, India; Furua Complex, Tanzania; Bergen Arcs, Norway) yield aCO2=0.4-1, with most >0.7. For scapolite-bearing granulites from the Furua Complex, in which aCO2≥0.9, calculated H2O activities (aH2O) based on phlogopite dehydration equilibria are uniformly low (0.1–0.2). The aCO2 calculated for meta-anorthosite from the Grenville Province, Ontario, ranges from 0.2 to 0.8. For Grenville meta-anorthosite also containing epidote, the aH2O calculated from clinozoisite dehydration ranges from 0.2 to 0.6. Calc-silicates from the Grenville, Sargur, and Furua terranes mostly yield aCO2< 0.5. The presence of calcite and/or wollastonite provides additional evidence for the low aCO2 in calc-silicates. Samples from six xenolith localities (Lashaine, Tanzania; Eifel, W. Germany; Lesotho; Delegate, Gloucester, and Hill 32, Australia) yield a wide range of aCO2 (0.1 to >1). The calculated fluid activities are consistent with metamorphism (1) in the presence of a mixed CO2−H2O fluid phase in which CO2 is the dominant fluid species but other C−O−H−S species are minor, (2) in the absence of a bulk fluid phase (“fluid-absent metamorphism”), or (3) in the presence of a fluid-bearing melt phase. The results for many granulites and Grenville meta-anorthosite are consistent with the presence of a CO2-rich, mixed CO2−H2O fluid phase. In contrast the relatively restricted and low values of aCO2 for calc-silicates require an H2O-rich fluid or absence of a fluid phase during metamorphism. The range of values for xenoliths are most consistent with absence of a fluid phase. The primary implication of these results is that a CO2-rich fluid accounts for the reduced aH2O in scapolite-bearing granulites. However, scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase, and the presence of scapolite is not a priori evidence of a CO2-rich fluid phase. In addition, close association of scapolite-free mafic granulites with scapolite-bearing granulites having identical mineral compositions in the Furua Complex, and the absence of scapolite from most granulite terranes implies that a CO2-rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes.

Early Cretaceous Vein-Related Garnet Granulite in Fiordland, Southwest New Zealand: A Case for Infiltration of Mantle-Derived CO2-Rich Fluids

Regionally extensive two-pyroxene granulite facies orthogneisses of Early Cretaceous age in Fiordland, southwest New Zealand, are criss-crossed by garnet-bearing feldspathic veins (and dikes) having associated marginal reaction zones of garnet granulite. The two-pyroxene granulites resulted from fluid-absent meta-morphism of a suite of synkinematic primary anhydrous intrusions. Subsequent restricted formation of garnet granulite in feldspathic compositions, and locally eclogite in ultramafic compositions, proceeded chiefly via reactions involving hornblende breakdown, and occurred in response to sharply increased load pressure and local lowering of water activity. The restricted occurrence of carbonate scapolite and -rich fluid inclusions in the vein areas suggests that water activity was lowered by infiltrating carbonic fluids. Infiltration occurred along pre-existing fracture systems, many of which were already filled with plagio-clase-rich veins and dikes. Published carbon isotope data for in scapolite within the veins indicates a mantle source. Open system behavior accompanying infiltration favored garnet stability by lowering bulk rock ferric/ferrous ratio and content. Fluid infiltration occurred at or near peak metamorphic pressure (~12 kbar at 650-700°C). Granulite metamorphism was of short duration (< 20 m.y.) and accompanied tectonic thickening in a subduction-related magmatic arc.

Constraints on granulite genesis from carbon isotope compositions of cordierite and graphite

Carbon dioxide is a common constituent in the channels of high-grade metamorphic cordierites1–6 Here we show that analysis of the stable isotopes of carbon in cordierite (crd) or graphite (gr) can provide a means to discriminate between two controversial models of granulite formation: (1) large-scale CO2 infiltration7, or (2) magmatic processes8–11, which might result in fluid-absent metamorphism. The fractionation of carbon isotopes between cor-dierites and graphites in high-grade rocks approximates the predic-ted equilibrium value. This suggests that cordierites can preserve peak metamorphic carbon isotope and channel gas compositions. Isotopically light carbon values in many granulite-facies cordierites (δ13Ccrd< −15) and graphites severely restrict theories of massive CO2 infiltration (δ13Cfluid≳−7). CO2rock ratios from a number of granulite terranes are estimated to be ≲0.01 by weight (∼0.016 on a molar oxygen basis), unless the rocks contain sub-stantial additional carbon. These data emphasize the importance of local fluid buffering in the granulite facies, consistent with dehydration by partial melts.

Stable isotope evidence for fluid-present and fluid-absent metamorphism in metapelites from the Damara Orogen, Namibia

A large difference of about 5‰ in the O-isotopic composition of quartz characterizes the transition from very low grade or even anchimetamorphic slates to pelites metamorphosed at greenschist facies conditions. A further increase in temperature does not result in systematic isotopic shifts (mean around +15.5‰). However, immediately at the isograds marking the transition from greenschist to amphibolite facies and amphibolite facies to higher amphibolite facies the quartz δ18O values are significantly higher by >1‰. This increase is restricted to the area immediately at the isograds.

Fluid-absent metamorphism

Many interpretations of metamorphic parageneses tacitly assume that a free fluid phase was always present during metamorphic equilibration (fluid-present metamorphism), as opposed to the circumstance that, while volatile species are certainly released from minerals during metamorphism, these species may not saturate the grain boundaries to produce free fluid (fluid-absent metamorphism). H 2 O-activity has meaning for both fluid-present and fluid-absent metamorphism and may be evaluated if an equilibrium may be written involving H 2 O and the temperature and rock-pressure of equilibration are known. P H 2 O has only a physical meaning for fluid-present metamorphism. Although the effects of an aqueous fluid on displacing reactions in P–T space are well known, as are the enhanced kinetic and deformational effects of excess fluid, there is actually little compelling evidence to suggest that the presence of aqueous fluids is common during metamorphism of the middle and lower crust, and the mantle, for the duration of most dynamic processes, including partial melting. Possible exceptions are discontinuous dehydration reactions which affect large compositional ranges of metamorphic rock over narrow pressure–temperature intervals and which could locally produce a free liquid if production rate exceeds escape rate. Evidence from stable isotopes and fluid inclusions obviously points to the accumulation of free aqueous fluid in definite locations for limited duration, but cannot, unfortunately, be used as evidence for generalized fluid transport during regional or contact metamorphism. Both mineralogical and textural criteria presently used to infer ubiquitous fluid, must be examined in terms of crustal volume and duration of dynamic processes to deduce the presence or absence of free fluid. If volatile components ever saturate grain boundaries, then the fluid-phase may only exist locally and perhaps only for short periods of time. Flow of metamorphic fluid may be focused into narrow regimes that could be quite widely spaced. Fluid-present metamorphism should not be considered as typical of whole crust evolution.