A. L. Perchuk

Lu-Hf and Ar-Ar chronometry supports extreme rate of subduction zone metamorphism deduced from geospeedometry

Abstract Recent diffusion modeling of eclogitic garnets from the Great Caucasus, Russia, and Yukon, Canada, have shown that the preservation of garnet growth zoning in rocks that have equilibrated at high temperature (680–700 °C) is possible only if rates of pressure and temperature change on the burial and/or exhumation paths are in the order of several cm/year and several hundreds of °C/Ma. In order to confirm this observation, we performed Lu–Hf and Sm–Nd dating of garnet and Ar–Ar dating of mica on the same samples that were used for geospeedometry measurements in an earlier study. In both localities, garnet grew during prograde metamorphism at 690±40 °C and >1.5 GPa (Yukon) and 680±40 °C and >1.6 GPa (Great Caucasus). In contrast, phengite formed soon after the main eclogitic foliation at 520±50 °C (Yukon) and 600±40 °C (Great Caucasus). Garnet of the Yukon samples yielded Lu–Hf ages of 252±7, 255±7, 257±6 and 264±6 Ma that fall within error of phengite Ar–Ar integrated ages of 261±2 (laser spot date) and 256±3 Ma (age of mineral separates). No Sm–Nd ages were measured on the Yukon samples. For Great Caucasus samples, all Sm–Nd ages with the exception of one garnet–whole rock pair yielding a Sm–Nd age of 311±22 Ma are poorly constrained. In contrast, the Lu–Hf garnet chronometer yields ages of 322±14, 316±5 and 296±11 Ma that again fall within error of the phengite Ar–Ar mean age of 303±5 Ma. Because the geospeedometry approach provides information on cooling rates, information on the closure temperature of a given isotopic system can be extracted from the analytical solution of Dodson [Contrib. Mineral. Petrol. 40 (1973) 259] using appropriate sets of experimentally determined diffusion data. The results of these calculations indicate that uncertainties of more than 200 °C are to be expected for the Sm–Nd and Lu–Hf closure temperatures for both the Great Caucasus (750±150 °C) and Yukon samples (710±120 °C). In all cases, calculated closure temperatures are equivalent to or in the upper range of peak metamorphic temperatures. With respect to Ar, calculated closure temperatures of 570 °C for the Yukon eclogites and 560–600 °C for the Great Caucasus eclogites are within error of the temperatures of the early stage of cooling and/or exhumation. These results indicate that the eclogitic rocks experienced a minimum cooling and exhumation of about 150 °C and 25 km in a time interval smaller than the errors on the ages. The fact that garnet and phengite yield indistinguishable Lu–Hf and Ar–Ar ages is in good agreement with the observation deduced from geospeedometry that the time elapsed at eclogitic conditions should be extremely short (of the order of 1 Ma). Considering the exceptional precision of the age information obtained on eclogitic garnet using the Lu–Hf technique and that Lu–Hf, Ar–Ar and geospeedometry approaches were carried out the same samples, these results suggest that the time-scale resolution required for unraveling rates of high-pressure metamorphism remains out of reach of current thermochronological methods.

Rates of thermal equilibration at the onset of subduction deduced from diffusion modeling of eclogitic garnets, Yukon-Tanana terrane, Canada

Well-preserved eclogitic rocks near Faro (Yukon-Tanana terrane, Canada) underwent a prograde evolution from ~510 °C and ♢1.1 GPa to 690 °C and ♢1.5 Gpa followed by nearisobaric cooling to ~540 °C. A remarkable feature of the garnet porphyroblast cores is the presence of minute garnet “inclusions” of distinctly different composition. Preservation of a sharp compositional gradient at the interface between the host and the inclusion garnets and results of diffusion modeling indicate that the counterclockwise pressure-time evolution took place on a very short time scale of about 0.2 m.y. Minimum rates of heating (950 °C/m.y.) and burial (7 cm/yr) calculated along the prograde part of the path are in good agreement with values extracted from thermal models of newly initiated subduction zones.

Unusual Inclusions in Garnet from the Diamond-Bearing Gneiss of the Erzgebirge, Germany

)ratios. It is known that such geotherms may occur onlyin subduction and collision zones [2], and the collisiongeotectonic setting is favorable for the formation of dia-mond-bearing metamorphic complexes [3, 4].According to formal criteria (mineralogy, structure,and texture), the diamond-bearing rocks of metamor-phic complexes are classified as metamorphic. How-ever, some petrological, geochemical, and experimen-tal data of recent years indicated their possible partial orcomplete melting [5–12]. It should be noted that thereare still no reliable criteria for the identification of suchmelts. The reason is that the exhumation rate of ultra-high pressure (UHP) complexes is lower than the rate ofphase transformations, and metamorphic reactionsobliterate, therefore, evidence for melting. On the otherhand, the knowledge of the physical state of rocks isvery important, because it strongly affects the mecha-nisms of very important processes, including diamondformation and rock exhumation. Petrological and min-eralogical studies played a key role in the discovery ofdiamondiferous metamorphic complexes and can beused to solve these important problems.This paper reports a discovery of unusual inclusionsin garnet from the diamond-bearing garnet–mica gneiss ofthe Saxonian Erzgebirge. The morphology and phasecomposition of these inclusions could hardly be explainedwithin the model of solid-phase evolution only.GEOLOGIC SETTING AND PETROLOGY (LITERATURE DATA)The Erzgebirge composes a large anticlinorium inthe Variscan basement of the Bohemian Massif in Cen-tral Europe [12, 13]. The core of the complex is com-posed of migmatite-bearing gneisses and schists host-ing eclogite and peridotite lenses. Diamond-bearinggarnet–mica gneisses occur as lenses (up to severalhundred meters long) in the same sequence [14]. Theappearance of these rocks is indistinguishable from thatof rocks of the same name widespread in moderatepressure complexes. Diamonds in the gneisses of theErzgebirge were discovered by chance owing to prob-lems with thin section polishing [15]. Subsequently, inaddition to diamond, another lesser known indicator ofhigh pressures was found in these rocks, a polymor-phous modification of rutile—titanium dioxide with thestructural type of αPbO

Geospeedometry and Time Scales of High-Pressure Metamorphism

Kinetic theory allows the calculation of a time scale for metamorphic events using the extent of relaxation of garnet growth zoning along a particular P-T trajectory. Eclogitic garnets from the Kokchetav Massif (Kazakhstan), the Great Caucasus (Russia), and the Yukon-Tanana terrane (Canada) experienced different metamorphic P-T histories and display different types of zoning patterns, which allowed testing of a variety of geospeedometric procedures. In all cases, the preservation of sharp compositional gradients and hence the limited degree of diffusive modification of garnet compositions can be explained if associated tectono-metamorphic processes were of very short duration. Results of diffusion modeling indicate rates of temperature and pressure change on the burial and/or the exhumation path on the order of several hundreds of °C/m.y. and several cm/yr, respectively. These extreme exhumation and cooling rates apply for rocks buried to a depth greater than, for example, 20 km, thus arguing for the existence of contrasted velocity fields for eclogitic block exhumation from deep versus shallow levels of the lithosphere.

Experimental modeling of mantle metasomatism coupled with eclogitization of crustal material in a subduction zone

Alteration of mantle wedge rocks under the influence of fluids and melts is a poorly known subduction-zone process. It was experimentally modeled using various materials analogous to the crust (glaucophane schist and amphibolite) and mantle (olivine and olivine + orthopyroxene) under the P-T conditions (800°C and 29 kbar) corresponding to a hot subduction zone. Schist or amphibolite was loaded into the lower part of a capsule and underwent partial (10–90%) eclogitization during the experiment with the formation of omphacite, garnet, and quartz, sometimes coexisting with Ca-Na amphibole and orthopyroxene. The eclogitization was accompanied by the release of aqueous fluid, which dissolved minerals and products of partial melting of the schist. Ascending fluid flows transported major components into the overlying peridotite. This resulted in the formation of a garnet-phlogopite-orthopyroxene reaction zone at the base of the peridotite layer; this zone accumulated Si and K, which was practically absent in the starting materials. The gain of Si, Al, and CO2 and loss of Mg resulted in the growth of new minerals in the olivine material: garnet, orthopyroxene, and magnesite. Under natural conditions, such a change would have been described as dunite transformation to garnet-bearing harzburgite. The experiments showed that the mineral and chemical composition of the suprasubduction mantle strongly depends on the transfer of components from a downgoing lithospheric slab.

Experimental simulation of orthopyroxene enrichment and carbonation in the suprasubduction mantle under the influence of H2O, CO2, and SiO2

Thermal and chemical gradients at the boundary of a subducting plate and suprasubduction mantle lead to a variety of poorly understood geologic processes. This paper reports the results of experiments simulating interaction between olivine (Ol, mantle analog) and carbonated glaucophane schist (Gls, analog of the upper layer of the subducted crust) under the P-t conditions of a ‘hot’ subduction zone. The experiments were carried out at a pressure of P = 25 kbar under temperature gradient conditions (tminGls = 720°C and tmaxOl = 1000°C) and at a constant temperature of t = 800°C corresponding to the boundary between the materials in the gradient experiment. A comparison of experimental data obtained using two different methods showed that the temperature gradient experiment reproduced the character of schist-olivine interaction under isothermal conditions and provided additional information on the effect of temperature on mineral reactions. Orthopyroxene occurs in the experimental products in different textural positions, forming a layer (with or without magnesite) at the base of the olivine zone or separate grains and intergrowths with magnesite at the boundary of olivine grains. The development of orthopyroxene and carbonate redeposition are described by reactions between olivine and aqueous fluid components, SiO2 and CO2, at

The Problem of CO 2 Recycling in Subduction Zones

a_{SiO_2 } = 0.23

Fluid-magmatic interaction between Glaucophane schist and olivine: Experimental modeling under the conditions of a thermal gradient

and