Deep Crustal Conditions Research Articles

Transition from carbonatitic magmas to hydrothermal brines: Continuous dilution or fluid exsolution?

Carbonatites are the most important primary sources for the rare earth elements (REEs). While fractional crystallization of carbonate minerals results in the enrichment of volatiles, alkalis, and REEs in the remaining melts, the transition from carbonatitic magmas to hydrothermal brines remains unclear. Here, we investigated the pressure-temperature-composition (P-T-X) properties of the Na2CO3-H2O system up to 700°C and 11.0 kbar using a hydrothermal diamond anvil cell and a Raman spectrometer. Our results show that Na2CO3 becomes increasingly soluble under high P-T conditions, leading to the disappearance of melt-fluid immiscibility and the continuous transition from Na2CO3 melts to hydrothermal brines under deep crustal conditions. Given the abundance of Na2CO3 in highly evolved carbonatitic systems, we suggest that the continuous melt-fluid transition in deep-seated carbonatites results in REEs being sufficiently concentrated in the brine-melts to form economic ore bodies, whereas in shallow systems, REEs preferentially partition into carbonatitic magmas over synmagmatic brines and disperse in carbonatite rocks that underwent limited fractionation.

Experimental determination of liquidus H2O contents of haplogranite at deep-crustal conditions

The liquidus water content of a haplogranite melt at high pressure (P) and temperature (T) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (≤0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700–1000 °C, 0.6–1.5 GPa). To improve the high-P constraints on H2O concentration at the granite liquidus, we performed experiments in a piston–cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi3O8)—orthoclase (Or: KAlSi3O8)—quartz (Qz: SiO2)—H2O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H2O was homogenized well above the liquidus T, and T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T and then raised T until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H2O are 935–985, 875–900, 775–800, 725–775, and 650–675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab36.25Or36.25Qz27.5 (wt% basis) at 1.0 GPa is $$T = - 0.0995 w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 3} + 5.0242w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 2} - 88.183 w_{{{\text{H}}_{ 2} {\text{O}}}} + 1171.0$$ for $$0 \le w_{{{\text{H}}_{ 2} {\text{O}}}} \le 17$$ wt% and $$T$$ is in °C. We present a revised $$P - T$$ diagram of liquidus minimum H2O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H2O (<5.8 wt%) and higher temperature, our results plot on the high end of the extrapolated water contents at liquidus minima when compared to the previous estimates. As a consequence, amounts of metaluminous granites that can be produced from lower crustal biotite–amphibole gneisses by dehydration melting are more restricted than previously thought.

A seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite-facies rocks of Calabria (Southern Italy)

Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting.

Brine-assisted anatexis: Experimental melting in the system haplogranite–H2O–NaCl–KCl at deep-crustal conditions

The first-melting temperature of haplogranite in the presence of H2O–NaCl–KCl solutions was determined experimentally at deep-crustal conditions of 6–14kbar and 700–900°C. Minimum melting occurs at fluid K/(K+Na) of 0.2–0.25. Melting temperature rises strongly with increasing salinity: at 10kbar, first melting occurs at 800°C with a H2O mole fraction (XH2O) of 0.62±0.02, similar to the salinity of fluid inclusions in minerals of some granites and migmatites. At 900°C and 10kbar, XH2O is 0.33±0.01 for first melting. All minimum melts are granitic over the entire P−T−XH2O range, with SiO2 melt concentrations of 74±2wt% on a H2O-free basis and Al2O3 and alkalis typical for granites. Normative quartz is near 30% for all liquids. The K/(K+Na) ratios of minimum melts increase strongly with decreasing H2O activity: at 10kbar and XH2O=1, K/(K+Na) in the melt is 0.25, whereas at XH2O=0.34 it is 0.55. This “brine trend” is similar to, but more pronounced than, the trend described by decreasing H2O activity in H2O–CO2 fluids, and it better explains the compositions of K-rich granites. Minimum-melting curves in the presence of brines of constant XH2O have strongly positive dP/dT slopes at P>2kbar, in contrast to vertical or negative dP/dT slopes for those in the presence of H2O–CO2 fluid. There is therefore a wide P–T range over which migrating low-H2O-activity brines may generate subsolidus, deep-crustal metasomatism in the form of K2O and silica enrichment. Moreover, once melting occurs, rising accumulations of granitic magma may be fluid saturated and even increase their melting capacity with decreasing depth because of the strong pressure dependence of H2O activity in salt solutions. Our results offer an explanation for mid-crust migmatization and granite production: rising hot brines may provoke rock melting at some threshold of decreasing depth in the range 15–20km. Because of their enhanced capacity for metasomatism, leading to eventual melting at granulite facies conditions of temperature, pressure and H2O activity, concentrated brines should be considered as possibly important agents in crustal evolution.

Re and Os isotopes of the central Oregon Cascades and along the arc indicate variable homogenization and mafic growth in the deep crust

The rhenium–osmium isotopic system has the potential to track the maturation of arc crust at lower crustal levels because the 187Os/188Os in magmas is sensitive to the age and composition of crust with which they interact. We here present Re and Os isotopic data for a suite of samples from the extensional central Oregon Cascade arc that includes diverse, primitive (high Mg) basalts and low- and medium-K basaltic andesites from North Sister and Little Brother volcanoes. The basaltic andesites contain higher 187Os/188Os than the two most common basalt types, calc-alkaline basalt and low-K tholeiites (0.17–0.19 vs. 0.14–0.15), indicating interaction with mafic crust. The evolution of 187Os/188Os in the deep crust depends on Re partitioning that in turn depends on sulfide and magnetite stability and oxygen fugacity. We estimate bulk mineral-melt partition coefficients for Re to be ∼4 at fO2≈QFM and ∼10 at fO2≈+1 to +3 ΔQFM in keeping with near liquidus phase equilibria at deep crustal conditions (Mercer and Johnston, 2008) and on fO2 of central Oregon primitive magmas (Rowe et al., 2009). Modeling fractional crystallization of the central Oregon primitive basalts indicates that the deep arc crust is likely to have 187Re/188Os similar to the range measured in North Sister basaltic andesites (∼55–260). The age of the mafic crustal contaminant in the central Oregon Cascades is then estimated to be 9–29Ma and is younger than the start of Cascade arc volcanism at ∼45Ma in central Oregon. The range in 187Os/188Os at the Cascade volcanic centers Mt. Lassen and Mt. Adams is greater than is observed for central Oregon arc magmas. We estimate a range in mafic crustal contaminant ages of 8–340Ma for Mt. Lassen and 0–70Ma for Mt. Adams and are older than suggested by previous studies (Hart et al., 2002, 2003). Mafic contaminant age ranges reflect the degree to which the crust is homogenized, or the extent to which the composition of the crust is a product of recent arc magmatism vs. pre-existing protoliths. Intra-arc extension in central Oregon facilitates homogenization and leads to assimilation of relatively young arc crust. Where extension is less narrowly focused, the range in Re and Os isotopes is greater and reflects isotopic heterogeneities, including variably aged crust at Mt. Lassen or variable enrichment of Re and Os in the lithosphere beneath Mt. Adams.

Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

AbstractThe ultrasonic technique for measuring travel times of compressional and shear waves using dual‐mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single‐mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp = 5.60 km/s, ∂Vp/∂P = 0.090 (km/s)/GPa, Vs = 3.37 km/s, ∂Vs/∂P = 0.05 (km/s)/GPa, σ = 0.216, and Vp/Vs = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂Vp/∂T = −0.518 × 10−3 (km/s)/K and ∂Vs/∂T = −0.182 × 10−3 (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual‐mode transducers, it is possible to accurately determine Vp/Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.

40Ar/39Ar Constraints on the tectonic history and architecture of the ultrahigh‐pressure Sulu orogen

AbstractNew 40Ar/39Ar ages are presented from the giant Sulu ultrahigh‐pressure (UHP) terrane and surrounding areas. Combined with U‐Pb ages, Sm‐Nd ages, Rb‐Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K‐feldspar 40Ar/39Ar ages. Subsequent unroofing at rates of 5–25 km Myr−1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40Ar/39Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr−1. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr−1, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr−1, suggesting relatively shallow crustal depths.

Ion association in concentrated NaCI brines from ambient to supercritical conditions: results from classical molecular dynamics simulations.

Highly concentrated NaCl brines are important geothermal fluids; chloride complexation of metals in such brines increases the solubility of minerals and plays a fundamental role in the genesis of hydrothermal ore deposits. There is experimental evidence that the molecular nature of the NaCl–water system changes over the pressure–temperature range of the Earth's crust. A transition of concentrated NaCl–H2O brines to a "hydrous molten salt" at high P and T has been argued to stabilize an aqueous fluid phase in the deep crust.In this work, we have done molecular dynamic simulations using classical potentials to determine the nature of concentrated (0.5–16 m) NaCl–water mixtures under ambient (25°C, 1 bar), hydrothermal (325°C, 1 kbar) and deep crustal (625°C, 15 kbar) conditions. We used the well-established SPCE model for water together with the Smith and Dang Lennard-Jones potentials for the ions (J. Chem. Phys., 1994, 100, 3757). With increasing temperature at 1 kbar, the dielectric constant of water decreases to give extensive ion-association and the formation of polyatomic (NanClm)n-m clusters in addition to simple NaCl ion pairs. Large polyatomic (NanClm)n-m clusters resemble what would be expected in a hydrous NaCl melt in which water and NaCl were completely miscible. Although ion association decreases with pressure, temperatures of 625°C are not enough to overcome pressures of 15 kbar; consequently, there is still enhanced Na–Cl association in brines under deep crustal conditions.

Evidence for xenon silicates at high pressure and temperature

We present evidence for the stability of xenon silicate phases at deep crustal conditions. High pressure‐temperature investigations of the Xe‐SiO2 system were conducted using diamond‐anvil cells combined with laser heating and external resistive heating. A new compound was synthesized at 0.7–1.5 GPa and 1500–1750° as evidenced by Raman spectroscopy. Upon temperature quenching, the material undergoes a back transformation to another Xe‐SiO2 phase stable up to 2.14 GPa. X‐ray diffraction data show that this second phase has a cubic structure [a = 8.03(±0.03) Å] and a bulk modulus of 20(±0.6) GPa. The phase was found to have a minimum of 4 wt% Xe from electron microprobe analysis. These results indicate that the chemical affinity of Xe and silica increases at high pressures and temperatures. Hence Xe can be potentially hosted by silicates in the crust, which helps to resolve the geochemical paradox of the ‘missing Xe’ in the atmospheres of the Earth and Mars.

Geochronological constraints on metamorphism, magmatism and exhumation of deep-crustal rocks of the Kramanituar Complex, with implications for the Paleoproterozoic evolution of the Archean western Churchill Province, Canada

U–Pb and Sm–Nd geochronology establishes an important Paleoproterozoic (~1.9 Ga) history for the Kramanituar Complex, located in the interior of the Archean western Churchill Province, and provides further insight into the Snowbird tectonic zone, a crustal-scale (>2000 km) feature whose role during Precambrian time remains controversial. The Kramanituar Complex is a window of deep-crustal rocks, dominated by a granulite-facies metagabbroic suite of ~1902 Ma age, with minor supracrustal rocks and charnockite. These yield peak equilibrium conditions of 12–15 kbar and 850–900 °C, which are bracketed between ~1910 Ma, the age of prograde metamorphic monazite in sillimanite- and kyanite-bearing paragneiss, and 1901 Ma, the cooling ages of titanite and rutile in gabbroic and paragneissic rocks. Although mineral assemblages reflecting peak metamorphic conditions are widespread, some rocks record near-isothermal decompression to ~8 kbar and 800 °C. The timing of uplift and exhumation is tightly bracketed between magmatic crystallization of gabbroic anorthosite under granulite-facies conditions at 1902 Ma and widespread rutile cooling ages of 1901 Ma. Time-averaged cooling rates of >100 °C/Ma are estimated for gabbroic anorthosite and leucogranite. Rocks surrounding the Kramanituar Complex are mainly amphibolite-facies Archean plutonic and supracrustal rocks. Those to the north were at deep-crustal conditions coeval with the complex, whereas those to the south appear to preserve a record of older, mainly Neoarchean tectonometamorphic assemblages and fabrics. The southern boundary of the complex is inferred to be a normal fault that accommodated tectonic unroofing of the Kramanituar Complex at ~1.9 Ga. This geochronological data set highlights the protracted history of a segment of the geophysically defined Snowbird tectonic zone, a structure along which latest significant magmatism and tectonometamorphism has been interpreted as either ~2600 or ~1800 Ma. Penetrative ~1900 Ma activity in the east-trending Chesterfield segment, as documented at the Kramanituar Complex, suggests that the complex may have represented a favorably oriented segment of a crustal-scale Archean fault that was reactivated during Paleoproterozoic time, possibly in response to collision between the Slave and Churchill cratons (Thelon Orogen) at ~1.97–1.9 Ga.

Hypersaline fluids in Precambrian deep-crustal metamorphism

An outstanding debate about the evolution of Precambrian granulite facies terranes concerns the role of fluids in deep-crustal metamorphism. One line of thought ascribes dryness and large-ion element (LILE) depletion to dehydration melting of rocks containing biotite and amphibole, without important participation of a low-density fluid phase, and removal of the granitic melts to the upper crust during metamorphic episodes. This mechanism is supported for the highest-temperature granulite terranes by experimental rock-melting studies. An alternative approach emphasizes field evidence for alkali metasomatism and silica mobility on outcrop and thin-section scales, which seemingly demonstrate fluid-driven processes. The debate concerns whether or not fluids could have coexisted with partial melts in migmatitic terranes and have been important agencies in destroying silicate hydrates and transporting LILE, especially Rb and U, out of the lower crust. Nearly ubiquitous CO2-rich fluid inclusions in granulite facies rocks have been cited by many workers as evidence that important granulite facies fluids were carbonic, in contrast to the H2O-dominated fluids of lower-grade metamorphism. However, the low solubilities of silicate constituents in CO2-rich fluids and the low wetting ability (high dihedral angles) of such fluids relative to silicate mineral grain boundaries, inhibiting infiltration, have been revealed in experimental studies over the last two decades. These features seriously detract from the appeal of carbonic fluids as important agencies in deep-crustal metamorphism. Recent experimental work in the system NaClKclH2O at deep crustal temperatures and pressures demonstrates that concentrated brines have appropriate low H2O activity, high infiltration ability, and high alkali mobility (especially Rb affinity). Limited anatexis may be promoted by hyperfusible solutes such as F and B, but the low H2O activity restricts rock melting. Improved thermodynamic properties for biotite indicate that the H2O activities necessary for orthopyroxene stability in quartzofeldspathic rocks, in lieu of biotite, are considerably higher than estimates based on previously available data, and that brines which participate in granulite facies metamorphism at deep-crustal conditions could be only moderately concentrated, similar in character to those of fluid inclusions observed in many different kinds of rocks. Recent observations of brine inclusions in granulites support the concept that polyionic salt solutions, immiscible with CO2 in the high-grade metamorphic temperature range (700–850°C) are feasible granulite facies fluids. The importance of hypersaline fluids in high-grade metamorphic processes may have been greatly underestimated from previous fluid inclusion studies. Of the several conceptual sources of hypersaline fluids in the crust, volatile-rich alkaline basalts seem plausible because of their additional ability to deliver heat for metamorphism. The postulated magmatic emanations may split into concentrated brines and immescible CO2 during ascent. They transport alkalis and LILE upward in the crust. Alkalic basalts in modern continental settings are characteristic of oblique plate convergence or continential distension, and may represent remelting of subcontinental mantle previously enriched in volatiles and alkalis by subduction processes. A possible Phanerozoic example is the interior western U.S.A. This analogy leads to a ‘Basin and Range’ hypothesis (Hopgood and Bowes, 1990, Tectonophysics 174, 279–299), in which the ancient granulite facies terranes represent zones of cratonal deformation. Granulites of the uplifted and deeply eroded Precambrian mobile belts may record zones of extraordinary thermal and physicochemical activity effected by primitive plate tectonic processes involving continental collision and disruption.

Deep crustal carbonates as CO2 fluid sources: Evidence from metasomatic reaction zones

Metasomatic reaction zones which developed at marble-pelitic schist contacts in a granulite facies terrane in West Greenland contain a consistent sequence of five mineralogical zones. Outward from the carbonates the zones are characterized by the assemblages grossular-diopside-meionite (I), meionite-anorthite-diopside (II), anorthite-diopside-edenitic hornblende (III), anorthite-enstatite (IV), plagioclase-almandine-sillimanite (V). Sphene is superceded by ilmenite between zones (II) and (III); quartz is present in all zones except zone I. Scapolite, plagioclase, clinopyroxene and mica exhibit a small degree of compositional variation which correlates with distance from the carbonate. These small compositional variations are superimposed on a strong CaO chemical potential gradient. Compositional features, zone distributions and CaO activity calculations demonstrate that the zones developed in response to CaO diffusion along a chemical potential gradient of 2 kcal/m. The CaO source appears to be carbonate rocks which release calcium as decarbonation reactions proceed. The maximum volume of CO2 released in this process, and that released during discontinuous reactions in the marbles, will contribute a total volume of CO2 approximately equivalent to the volume of carbonate in the rock. Calculations demonstrate that a terrane consisting of as little as 8% carbonate will release sufficient CO2 to result in complete dehydration of an amphibolite terrane, at deep crustal conditions. Dehydration through CO2 release will be accomplished either through rapid burial, which would prevent both equilibration of mineral assemblages and CO2 release at intermediate crustal levels, or through diffusion-driven metasomatic reactions which would lead to CO2 release primarily at the high temperatures of deep crustal environments. The latter process would be the dominant CO2 source at deep crustal levels if carbonate rocks occur predominately as relatively thin layers.

Peridotite and Gabbroic Structures in the Monte Maggiore Massif, Alpine Corsica

Penetratively deformed peridotites from the Monte Maggiore spinel/plagioclase lherzolite massif in Alpine Corsica are intruded by gabbroic dikes ranging in mineralogy from troctolites to Fe and Ti gabbros. The dikes cross-cut diffuse plagioclase segregations and veinlets which may represent trapped mafic melt within the residual peridotite. Elliptical shaped dunite pods with refractory compositions are often bounded by lace-like networks of plagioclase () which invade neighboring undepleted lherzolite. Lherzolite mineral compositions range from olivine , orthopyroxene , and clinopyroxene . Peridotite and gabbroic structures and textures suggest that the emplacement of the dikes was contemporaneous with the high temperature deformation of the peridotite, occurring under upper mantle or deep crustal conditions. Locally the intrusion of the dike magma followed preferential shear fracture planes related to flow structures in the enclosing peridotite. The similarity of differentiation patterns in the gabbroic dikes with gabbroic cumulates in the overlying Corsican ophiolitic section suggests that the Mt. Maggiore dikes may have fed these magma chambers.