Lukas P. Baumgartner

Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy

Abstract Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590–630°C, P=2.7–2.9 GPa) of the Zermatt–Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu–Hf and Sm–Nd garnet geochronology in conjunction with petrologic estimates of the prograde P–T path. We report a Lu–Hf garnet–omphacite–whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm–Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438] obtained from the same sample and mineral material. The Sm–Nd and Lu–Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269–287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668–684] from Lago di Cignana and elsewhere in the Zermatt–Saas ophiolite, lie within a range of ∼50–38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm–Nd and Lu–Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu–Hf age that is weighted toward the older garnet core. Sm–Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu–Hf ages. The difference in Sm–Nd and Lu–Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm–Nd and Lu–Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23–0.47 cm/yr. These values correlate with continuous shortening rates of 0.4–1.4 cm/yr between the European plate and the African–Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt–Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions.

Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precision SmNd and RbSr geochronology

Sm‐Nd isotope data from garnets in ultrahigh-pressure (coesite-bearing) eclogite from the Western Alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the Zermatt‐Saas ophiolite complex. We report an exceptionally well-constrained Sm‐Nd isochron age of 40:62:6 Ma from the Lago di Cignana eclogite. Difficulties in dating Alpine eclogites using this method were overcome by using an HF acid-leaching procedure on garnet to remove mineral inclusions. The Zermatt‐Saas rocks reached greenschist-facies conditions around 38 2M a, on the basis of Rb‐Sr whole rock‐phengite isochrons. These data, combined with existing estimates of the pressure and temperature conditions of the eclogite, give an estimate for the initial exhumation rate of 10 to 26 km=m.y., depending on the inferred pressure for the recrystallization of phengite, followed by slow exhumation of 0.3 km= m.y. from 34 to 14 Ma. The initial exhumation rates at Lago di Cignana are among the highest yet determined for high-pressure terranes, and are well constrained because the age has been determined directly on the high-pressure assemblage. The rapid exhumation rates presented here place critical constraints on collisional tectonic models.

Intercrystalline stable isotope diffusion: a fast grain boundary model

We formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals, and the intragranular distribution of stable isotopes in each mineral, as functions of mineral grain sizes and shapes, diffusivities, modes, equilibrium isotopic fractionations, and the cooling rate of a rock. One of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes. This Fast Grain Boundary (FGB) model describes interdiffusion between any number of mineral grains, assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled. The model can be used for a rock containing any number of minerals, and number of grain sizes of each mineral, several grain shapes, and any thermal history or domain size desired. Previous models describing stable isotope interdiffusion upon cooling have been based on Dodsons equation or an equivalent numerical analogue. The closure temperature of Dodson is the average, bulk temperature recorded between a mineral and an infinite reservoir. By using Dodsons equation, these models have treated the closure temperature as an innate characteristic of a given mineral, independent of the amounts and diffusion rates of other minerals. Such models do not accurately describe the mass balance of many stable isotope interdiffusion problems. Existing models for cation interdiffusion could be applied to stable isotopes with some modifications, but only describe exchange between two minerals under specific conditions. The results of FGB calculations differ considerably from the predictions of Dodsons equation in many rock types of interest. Actual calculations using the FGB model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate. Closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient, and may differ from that predicted by Dodsons equation by hundreds of degrees C. Either or both of two minerals may preserve detectable zonation, which may in some instances be larger in the faster diffusing mineral. Rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone. The results of FGB diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations, concordant or discordant, recorded within any rock in which diffusion could have acted over observable scales. The predictions of this model are applicable to thermometry, evaluation of open or closed system retrogression, and determination of cooling rates or diffusion coefficients.

Incremental growth of the Patagonian Torres del Paine laccolith over 90 k.y.

The Miocene Paine Granite in the Torres del Paine Intrusive Complex, southern Chile, is an extraordinary example of an upper crustal mafic and granitic intrusion. The granite intruded as a series of three sheets, each one underplating the previous sheet along the top of the basal Paine Mafic Complex. High-precision U/Pb geochronology on single zircons using isotope dilution–thermal ionization mass spectrometry yields distinct ages of 12.59 ± 0.02 Ma and 12.50 ± 0.02 Ma, respectively, for the first and last sheet of the laccolith. This age relationship is consistent with field observations. The zircon ages define a time frame of 90 ± 40 k.y. for the emplacement of a >2000-m-thick granite laccolith. These precise U-Pb zircon ages permit identification of the pulses in a 20 k.y. range. The data obtained for the Paine Granite fill the gap between 100 k.y. and 100–1000 yr pulses described in the literature for crustal magma chambers.

A model for coupled fluid-flow and mixed-volatile mineral reactions with applications to regional metamorphism

The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·102–5·104 mol/cm2. The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 104–106 cm3/cm2 at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcys Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10-4 m/year and minimum permeabilities are 10-10–10-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks.

A new look at stable isotope thermometry

Interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks. The Fast Grain Boundary (FGB) model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion (Eiler et al., 1991, 1992). This model assumes that grain boundary diffusion is much faster than volume diffusion, and it accounts for exchange among all minerals in a rock. Previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks. Modeling results are described for amphibolites and hornblende granites and gneisses; biotite granites, schists, and gneisses; pelitic and semi-pelitic rocks; garnet peridotites; anorthosites, gabbros, pyroxenites, and related rocks; and calc-silicate rocks. Examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase-pyroxene in pyroxene bearing anorthosites and garnet-quartz in garnetiferous quartzites. In contrast, the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures. Closed-system processes are capable of producing a variety of patterns of stable isotope resetting, discordance, mineral zonation, and fractionation reversals. Examples include large reversals of quartz-feldspar fractionations in micaceous rocks, and oscillatory zonation in feldspar from some quartz-rich rocks. These results permit reinterpretation of many studies of stable isotope thermometry, speedometry, and retrograde alteration history. FGB modeling of mineral zonation provides an important new guide to applying recently developed micro-analytical tools to slowly cooled rocks. Application of the FGB model to quartzo-feldspathic gneisses from the Adirondack Mountains, New York, demonstrates the usefulness of diffusion modeling in discriminating closed-system, diffusion controlled retrogression from open-system retrogression, and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity.

ONE- AND TWO-DIMENSIONAL MODELS OF FLUID FLOW AND STABLE ISOTOPE EXCHANGE AT AN OUTCROP IN THE ADAMELLO CONTACT AUREOLE, SOUTHERN ALPS, ITALY

Abstract Localized depletion of 18O and 13C in a thin subhorizontal marble layer in the Adamello contact aureole, Southern Alps, Italy, resulted from fluid infiltration focused along a crosscutting dike. Values of δ18O and δ13C in calcite from the 1 m long profile decrease systematically from sedimentary values of δ18O = 22‰(SMOW) and δ13C= 0‰(PDB) to δ18O = 12.5‰ and δ13C ≈ -7‰ near the dike. The presence of clinozoisite and garnet in the 5-15 cm thick marble layers near the granodiorite dike indicates H2O-rich fluid conditions (XCO₂ ≈ 0.01 ). The O and C isotope profiles were compared with one- and two-dimensional models of advective-dispersive isotope transport. Individually the isotope profiles fit one-dimensional transport models well. However one-dimensional models, using equilibrium fluidrock exchange or a kinetic formulation, do not explain the relative locations or shapes of the two isotope-exchange profiles given the petrologic constraint of XCO₂ ≈ 0.01 for the infiltrating fluid. Excellent agreement with the δ18O and δ13C data is obtained using a twodimensional model that specifies (1) a high-permeability zone in marble near the dike that focuses fluid flow parallel to the dike and (2) a lower permeability zone in marble away from the dike where isotope exchange is dominated by molecular diffusion. The combined constraints imposed by phase equilibria and two isotope tracers allow two-dimensional fluid flow to be inferred from one-dimensional data. The results emphasize that isotope distributions resulting from multidimensional flow may fortuitously fit one-dimensional transport models if isotope tracers are considered independently. The use of multiple tracers coupled to fluid-composition constraints is therefore essential to discriminate between various transport models.

Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks

Retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks. Mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed, however, leaving doubt as to the mechanism responsible for the bulk-mineral isotopic compositions commonly measured. Ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data. Two samples of magnetitebearing, quartzo-feldspathic Lyon Mountain gneiss from the Adirondack mountains, N.Y. were studied in detail. Conventional stable isotope analysis of both samples indicates that isotope thermometers are discordant and were reset by as much as 200°C from the estimated peak temperature of 750°C. The relative order of apparent temperatures recorded by various thermometers differs between the two samples, however, with Tqtz-fsp≫Tmt-qtz and Tmt-fsp in one sample and Tqtz-fsp<Tmt-qtz and Tmt-fsp in the other. Diffusion modeling using the Fast Grain Boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during cooling, whereas the latter is not. The modeling predicts that 0.5 mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1‰ (rims lower in δ18O than cores), and that the cores of smaller (0.1 mm) grains will be similarly lower than to the cores of large (0.5 mm) grains. Ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains (Δcore-rim=1.1±0.4‰) and predicted grain-size vs core composition variations, consistent with diffusion-controlled resetting of bulk mineral fractionations. In contrast, the second sample shows irregular inter-and intra-granular variations over an 8‰ range, consistent with open system alteration. These results provide direct documentation of the importance of interdiffusion in affecting stable isotope distributions in slowly cooled rocks. The correlations of bulk-mineral resetting with zonation show that bulk mineral data, when interpreted with detailed modeling, can be used to determinate what processes controlling retrogression.

Fast Grain Boundary: a FORTRAN-77 program for calculating the effects of retrograde interdiffusion of stable isotopes

Exchange of stable isotopes between coexisting minerals is recognized widely as an important factor in the interpretation of stable isotope geochemistry of plutonic and high-grade metamorphic rocks. Where retrogression has occurred without major recrystallization events, the rate limiting step for stable isotope exchange will be diffusion. The mathematics of diffusion are well known for many problems, but no analytical solution, including that for closure temperature, adequately describes the complex and highly variable controls of rate and mass balance that will dominate many diffusion processes in rocks. We have implemented a model describing diffusional exchange for rocks in which grain boundary diffusion is sufficiently rapid that a representative volume of rock (typically millimeter to centimeter) is able to have mutual equilibration of all grain boundaries for the time scale of cooling. This Fast Grain Boundary model explicitly links intracrystalline diffusion rates and abundances of all minerals in a rock, and allows study of the impact of rock type on stable isotope thermometry, retrogression, and zonation. The FORTRAN-77 program for the Fast Grain Boundary model presented here can be used with a personal computer to solve typical problems in minutes. Input includes the grain size(s), model abundance(s), diffusion coefficient, and fractionation factor for each constituent mineral, and a cooling rate for the rock. Output includes the diffusion profile and integrated (bulk) composition of every mineral in a rock, as well as the apparent temperatures that would be observed by applying bulk-mineral stable isotope thermometry to such a rock.

Stochastic permeability models of fluid flow during contact metamorphism

Stable isotope data from hydrothermally altered rocks often show significant scatter. Such scatter cannot be quantitatively interpreted by models in which each lithologic unit is assumed to have a uniform permeability. If a stochastic modeling approach is used instead, heterogeneous permeability maps can be constructed to approximate the statistical distributions of natural systems, and both overall isotopic trends and data scatter can be matched. Three models are presented for the Alta, Utah, contact aureole to show that the observed scatter in δ18O values is consistent with subhorizontal down-temperature fluid infiltration through carbonates with heterogeneous permeabilities. Infiltration through rocks with heterogeneous permeabilities creates irregular, lobate isotope patterns, so that the idealized isotope profiles predicted by one-dimensional homogeneous permeability models do not develop. Localized sampling is unlikely to yield an accurate estimate of the overall importance of fluid-rock interaction or of dominant flow directions. In heterogeneous systems, large-scale hydrothermal alteration and flow patterns can best be estimated from statistically unbiased and spatially distributed data sets.