Abstract Soils are highly heterogeneous entities in which organic and inorganic as well as living and non-living building blocks interact to form biogeochemical interfaces. While processes at these interfaces occur at the micrometer or submicrometer scale, they are thought to influence the behavior of soils at the global scale, e.g. soils as carbon sinks. Analytical methodologies with a high resolution are, therefore, required in order to investigate these processes with the final goal to understand biogeochemical-interface formation mechanistically. In the present study, sorption experiments of water-extractable organic matter on model minerals, such as boehmite and illite, were performed. Adsorption of organic matter on the minerals was quantified by conventional bulk-scale methods and compared with data from nanoSIMS measurements. From the data obtained, scaling factors have been developed which permit the quantification of organic matter in the secondary ion images provided by nanoSiMS.

Abstract One of the key aims of geochronology, and the subject of the papers in this Special Publication, is the linkage of isotopic ages to petrological and textural information. A close link between the two types of information greatly improves the constraints available from geochronology on the nature and rates of lithospheric processes such as metamorphism and deformation. There have been several key advances in this area over the past 10–20 years, relating to increased precision and accuracy of isotopic ages but also, and crucially, to the spatial resolution available to geochronologists. This resolution now approaches that on which petrological, chemical and textural information is obtained. We also, in this introduction, identify the barriers that have impeded further progress, which relate both to technical issues as well as to problems of understanding. Finally we set the papers in this volume in the context of the preceding discussion and outline the key ways in which these papers point towards further progress in the future.

Abstract Monazite is the mineral of choice in pelitic rocks for providing time constraints on metamorphic rocks and metamorphic processes. However, unlike rock-forming mineral chronometers such as garnet, the petrogenesis of monazite is relatively poorly understood. Consequently, although it is possible to generate precise monazite ages, the significance of the age in metamorphic rocks is often uncertain. In this contribution, we show how the petrogenesis of monazite can be linked to pressure and temperature information. Four complementary approaches, each illustrated by examples, are discussed: (i) the textural relationships of accessory minerals are used to relate the petrogenesis of monazite to that of the rock-forming mineral assemblage, and through this to P-T; (ii) monazite composition, in particular Y content, is used to relate monazite to the rock-forming mineral assemblage, and thus, to P-T; (iii) the bulk compositional control on monazite stability has been empirically determined and this relationship allows the temperature of initial monazite growth to be estimated in a given bulk composition; (iv) monazite-xenotime thermometry is utilized to provide estimates of the temperature of monazite growth. Either individually or combined, these approaches successfully enable monazite age data to be placed in a P-T framework.

Abstract Rare earth element (REE) analysis of zircon coupled with high spatial resolution U-Pb geochronology and imaging is emerging as a useful petrogenetic tool capable of providing a link between dated zircon growth and contemporaneous magmatic conditions or metamorphic reactions. Notable amongst recent studies has been the link established between zircon growth in equilibrium with garnet and characteristic heavy REE depletion in zircon which has enabled the dating of specific pressure-temperature (P-T) evolution. Links to other REE-bearing minerals have yet to be established but have similar potential for dating P-T pathways. A brief review of recent applications in both igneous and metamorphic zircons is presented here to illustrate the range of investigations in this rapidly developing area. The method is then applied to two case studies from the late-Archaean Lewisian Composite Terrane of the Outer Hebrides, NW Scotland. In both cases, these new zircon REE analyses reveal previously unknown events in the early history of these samples, as well as clarifying the relationship of different zircon growth phases in relation to the geochronology.

Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M 2 metamorphic episode. Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

Abstract Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl. 147 Sm/ 144 Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H 2 SO 4 leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.

Abstract In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 °C and 420 to 550 °C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during high pressure eclogite-facies recrystallization and at about 175 ± 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.

Abstract Integration of petrographic observations, mineral chemistry, garnet Sm-Nd isochrons, and MnNaCaKFMASH pseudosection phase equilibria models constructed with THERMOCALC provides quantitative metamorphic pressure-temperature-time (P-T-t) paths which allow determination of assemblage/reaction history for pelites. Examples are presented for the Cretaceous to Tertiary magmatic arc of the North American Coast Plutonic Complex. Metamorphism in the western Coast Plutonic Complex of southeastern Alaska and in the North Cascades of Washington resulted from at least three widespread events from > 100 Ma to c. 60 Ma, and in both areas partly resulted from crustal thickening, evidenced by local occurrences of kyanite after andalusite. Pressure-temperature pseudosections constructed from bulk rock compositions and the intersection of garnet core composition isopleths provide estimates for the pressure and temperature of garnet core growth. Intersections of these isopleths indicate garnet growth 18 to 85 °C above the predicted garnet-in reaction temperatures. Rim and near-rim garnet compositions and matrix mineral chemistry provide estimates for near-peak metamorphic conditions. Finite pressure-temperature-time paths of garnet zone metamorphism were determined from the combined core growth and pressure-temperature conditions determined from near-rim garnet and matrix mineral compositions. The western Coast Plutonic Complex near the Stikine River, southeastern Alaska, displays a complex pattern of regional metamorphism overprinted by contact metamorphic aureoles. Many of the c. 90 Ma aureoles contain andalusite, andalusite plus sillimanite, or andalusite plus kyanite with complex replacement textures. A pseudosection constructed for a contact metamorphic rock on Kadin Island (95GL11c), predicts that garnet grew c. 555 ± 10 °C and 4.8 ± 0.7 kbar, above the garnet-in line and the aluminium silicate triple-point pressure. These results suggest that andalusite in samples from this aureole likely grew prior to garnet and that the pressure may have increased by ≤ 1 kbar during metamorphism. The southern part of the North Cascades in Washington also contain complex aluminium silicate replacement textures with early andalusite and later kyanite and sillimanite. A sample (96NC67), collected near the andalusite-bearing aureole of the Mt Stuart batholith, contains sillimanite and c. 10 mm garnet crystals containing staurolite inclusions in their cores. Temperatures estimated from the garnet core of this sample are within the pseudosection staurolite stability field, compatible with initial garnet growth significantly above the garnetin line. The garnet rim thermometry estimate of c. 668 ± 59 °C for this sample is c. 85 °C higher than the core growth temperature. The calculated P-T-t path provides important information for interpreting regional and contact metamorphism. An extensive region NE of the Mt Stuart batholith in the North Cascades underwent a significant pressure increase; however, the timing and nature of medium- to high-pressure metamorphism is controversial. Quantitative P-T-t paths constructed for garnet growth along the NE margin of the batholith indicate that 87-85 Ma garnet growth was younger than the nearby Mt Stuart batholith (93.5 ± 1.4 Ma, U-Pb zircon). Garnet core and rim segments are isochronous indicating a short interval for garnet growth. P-T-t paths indicate that garnet growth occurred in the sillimanite stability field during a maximum pressure increase of 1 to 2 kbar, after rocks passed through the andalusite stability field (Mt Stuart contact metamorphism). Careful sampling, hand-picking, acid leaching, and isotopic analysis of garnet provide geologically consistent ages with uncertainties of ≤ 1.0 Ma. Thermodynamic modelling in the MnNaCaKFMASH system provide reasonable P-T predictions for pelite mineral stability that can be integrated with isotope ages to provide quantitative P-T-t paths. The P-T-t paths developed for both regional and contact metamorphic rocks allow critical evaluation of tectonic models and of interpretations for mineral textures.

Abstract Drill core samples of garnet-clinopyroxene granulite at Tirschheim and a reference sample at Waldheim (Saxon Granulite Massif, Germany) endured the same P-T conditions, but developed variable mineral assemblages due to differences in bulk chemistry, reaction progress, deformation and retrogression. Titanite formed during peak-metamorphic conditions of 22–24 kbar and 1020–1050 °C. Dating titanite from the various samples should yield the same age for all. The observed age variation, which exceeds the duration of the entire metamorphic cycle, originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post-peak re-equilibration. (1) Pb inheritance observed in some peak-metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization. Instead, metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral. (2) Titanite that formed at peak-metamorphic conditions is characterized by high A1 contents and X F ≈ 0.8−1. Texturally comparable titanite that re-equilibrated during cooling (reduced Al contents and X F ) yields too young U-Pb ages. The age of such re-equilibrated titanite does not correspond to the age of the event indicated by the texture.

Abstract The Sonnblick Dome forms a large antiformal structure within the Pennine of the SE Tauern Window. The structural and metamorphic evolution of this area is relatively well constrained. However, mineral age determinations of the Alpine event within the Zentralgneis granitoid basement remain ambiguous due to a lack of isotopic equilibration during the Alpine event. In this paper we use a newly developed technique, namely Rb-Sr microsampling, in an attempt to place more reliable age constraints upon Alpine deformation. The new age determinations range between c. 22 Ma and 28 Ma. The oldest ages are interpreted to result from shearing related to the earliest stages of dome formation while the youngest ages probably represent late fabric development during uplift of the Zentralgneis complex. A clustering of six fabric ages around 25.5 ± 0.3 Ma is thought to represent the peak of deformation activity associated with formation of the dome. These new data can be fitted into a model for the metamorphic and structural evolution of the SE Tauern Window. In most of the rocks studied isotopic disequilibrium is apparent between and within minerals. It is clear that large feldspar augen did not attain isotopic equilibrium despite amphibolite facies metamorphism. In fact, even where chemical equilibrium was achieved isotopic homogenization did not always occur. The Rb-Sr data from white mica suggest that isotopic equilibration may occur during isochemical rotation of grains suggesting differing mechanisms for chemical and isotopic movement within grains. The data presented herein suggest that small-scale, in-situ sampling methods may provide the most reliable method for deformation chronology in basement gneisses.

Abstract By using a set of granitic whole rock samples, originating from the Regensburg Forest (Germany) and dated (approximately 350 Ma old) previously by means of thermal ionization mass spectrometry (TIMS), the capability of dynamic reaction cell (DRC) inductively coupled plasma mass spectrometry (ICPMS) for Rb-Sr age determination was demonstrated. With DRC-ICPMS, chemical separation of Sr from Rb during the sample pretreatment is no longer required as interference-free determination of the 87 Sr/ 86 Sr isotope ratio can be accomplished by monitoring the signals of the SrF + adduct ions, formed as a result of the selective reaction between the Sr + ions extracted from the ICP and the reaction gas CH 3 F. The mass discrimination was established to depend strongly on the matrix composition. This drawback could be overcome by using the United States Geological Survey reference material G-2 as an external standard. Results obtained by DRC-ICPMS (age and initial 87 Sr/ 86 Sr ratio) showed an excellent agreement with both (a) experimental values obtained by means of quadrupole-based and sector field ICPMS after isolation of Sr via cation exchange chromatography and (b) TIMS literature values. In addition, DRC-ICPMS offers a smaller combined uncertainty on the isotope ratio results as a result of (a) an improved internal isotope ratio precision (<0.1% RSD when also using Ne as an additional non-reactive collision gas) and (b) the fact that, in contrast to quadrupole-based and sector field ICPMS, no correction for the remaining overlap between the signals of 87 Sr + and 87 Rb + is required.

Abstract Quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources. Most common are predictions made on the basis of laboratory kinetic data. However, the applicability of such laboratory-based predictions has long been questioned and many observations in the field now suggest much slower rates. Here, published quantitative field-based constraints on high temperature (>400 °C) reaction rates are assembled from a variety of sources. Reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory-based predictions. A general rate law for regional metamorphism has been derived which best describes these field-based data: log 10 ( R net ) ≅ 0.0029 T − 9.6 ± 1 where R net is the net reaction rate (g/cm 2 /a) and T is temperature (°C). At the same time, natural reaction rates attending contact metamorphism differ from laboratory-based predictions by less than two orders of magnitude, and are in close agreement at higher temperatures. Thus, while existing laboratory-based kinetic data may be judiciously applied to some contact metamorphic systems, laboratory-based kinetic predictions clearly misrepresent regional metamorphism. To explain this kinetic discrepancy, regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited (or transient) availability of aqueous fluid in the intergranular medium. The general field-based rate law may be applied to regional metamorphic, and other environments (i.e. ultrahigh pressure or ultrahigh temperature metamorphism), if similar system characteristics (mainly, low aqueous fluid content) can be inferred.

Abstract The rate of melt segregation in regional migmatite terrains can be estimated by various lines of research. Firstly, the segregation rate of a melt batch, with a volume below the melt percolation threshold, cannot exceed the melt production rate and is therefore limited by the heating rate derived from geothermometry and geochronology (method A). Other estimates come from physical models for melt percolation (method B) and from the degree of (dis)equilibrium reached between melt and source rocks (method C). The first method is restricted by the current time resolution of isotopic techniques. Results from the second and third approaches depend heavily on assumed values of melt viscosity and other parameters (B); on the correct recognition of (dis)equilibrium trace element distributions (C); and on the migmatization model used (B and C). The validity of method C is undermined by the mathematical equivalence of trace element models for five different scenarios: (1) disequilibrium melting (with or without melt escape) followed by in situ crystallization of non-segregated melt; (2) equilibrium melting, followed by equilibrium crystallization and major melt escape; (3) disequilibrium melting, followed by equilibrium crystallization and minor melt escape; (4) pervasive retrograde re-equilibration; and (5) subsolidus differentiation. Hence, trace element data in support of model 1 (implying fast melt segregation rates) are equally consistent with models 2 to 4 (implying slow melt segregation rates), and even with melt-absent model 5. The level of trace element saturation reached during accessory phase dissolution into melt may not provide better answers, as it is largely controlled by textural constraints, e.g. shielding of accessories by porphyroblasts. We conclude that the way forward is to directly couple microtextural and microgeochemical information with time constraints. This requires high-resolution (space and time) geochronology, possibly with more advanced methods than presently available.

Abstract Rates of melt production and segregation in migmatites can be assessed using geochronology of accessory phases. Here we report on the distribution and growth patterns of accessory phases and their coupling with major phase microtextures and chemical zoning patterns in garnet. We propose that migmatization in SW Finland involved partial melting and subsequent moderate retrograde re-equilibration. The latter process has three major effects: (i) it partially obscures geochemical signatures formed during earlier equilibrium or disequilibrium melting; (ii) it excludes the possibility of very fast (< 100 years per batch) melt segregation rates in migmatites where restite-melt back reaction has operated; (iii) trace element distributions between leucosome and host rock cannot be used to infer melt segregation rates. Garnet MnO patterns show flat cores, suggesting high-grade equilibration, and sharply increasing concentrations at the rims, attributed to garnet-melt back reaction. Trace element patterns (Zr, Y and heavy rare earth elements) also document retrogression at the rims, but in addition preserve earlier growth zoning related to incongruent melting. We report retrograde zircon and xenotime growth associated with garnet resorption, here related to restite-melt back reaction in leucosome, melanosome and mesosome. Hence, geochronological studies of migmatite terranes should take into account that the youngest zircon material is not restricted to leucosome, but can also form overgrowths on corroded grains in melanosome and mesosome.

Abstract Kimberlites are extraordinary natural phenomena, ascending through the Earth’s lithosphere, entraining xenoliths, to erupt at the surface within hours to days of their inception deep within the lithospheric mantle. With the realization that some Ar/Ar phlogopite grain core ages may be indicative of geological events, we have undertaken high spatial resolution Ar/Ar dating of phlogopites in xenoliths and megacrysts from Kimberley, Monastery and Letseng in southern Africa, and Malaita, in the Solomon Islands, to est whether other mantle phlogopite cores may yield meaningful ages. Modelling of Ar diffusive loss profiles from phlogopite grain boundaries to cores provides information on both the eruption age and the duration of outgassing within the kimberlite magma, and hence yields estimates on diatreme ascent rates. The ascent durations are very similar for all of the southern African pipes studied, yielding durations of 0.9–6.9 days, assuming an average kimberlite magma temperature of 1000 °C. These can be compared to estimates from phlogopite xenoliths from Siberian diamond-bearing kimberlites yielding ascent durations of 2–15 hours (assuming the same magma temperature).

Abstract Recent advances in microscale 40 Ar/ 39 Ar geochronology have revealed argon concentration gradients in naturally deformed muscovite that are incompatible with volume diffusion uniquely, and have been interpreted to result from intragranular defect-enhanced diffusion. Defects and heterogeneously spaced stacking faults observed by transmission electron microscopy in such muscovites are evaluated as potential fast pathways for argon diffusion. Two-dimensional defects, such as stacking faults, are of particular interest for noble gas diffusion because of the net dilatation effect that a stacking fault is able to generate in minerals. In micas, partial dislocations (and the area between them known as stacking faults) within the interlayer displace the potassium atoms from a stable hexagonally centred position between opposing tetrahedral layers to an unstable position relative to one of the tetrahedral layers such that repulsive forces lead to a localized net dilatation effect within the interlayer. Such a dilatation effect may have direct consequences for argon retention in micas. Numerical modelling of the effects stacking faults have on argon diffusion was performed on the basis of the calculated interlayer spacing, measured isotope data, and observed linear stacking fault density. These calculations result in effective diffusivity ratios defined by volume diffusion to defect-enhanced diffusion of 10 6 to 10 7 , which are comparable with diffusivity ratios in other materials (ceramics or metals). In the absence of defects causing physical grain size reduction (e.g. kink bands or subgrain boundaries), stacking faults are potentially the main defect in sheet silicates exerting a measurable influence on intragranular argon diffusion. Stacking-fault-enhanced argon diffusion differs from pipe diffusion, whose significance on bulk diffusion depends on high dislocation densities, by the small volume fraction of dislocations required to affect bulk diffusivities. In contrast to pipe diffusion, the linked occurrence of dislocations and stacking faults within mica interlayers represents a potentially significant volume fraction, even in samples that do not have high apparent dislocation densities.

Abstract Isotope geochemistry has produced many technical developments in the past decade or so that have revolutionized the potential information available on the tectonics of metamorphic belts from geochronology. These include the ability to date minerals and rocks on small spatial scales, scales that at last approach those from which other types of information — structural and petrological — are obtained. However, interpreting the new data, and their integration with the other datasets available, is not straightforward and requires careful chemical and textural observations that go hand-inhand with the geochronology. The increasing realization of the importance of this approach has led to a number of symposia at international conferences devoted to this topic in recent years. The set of papers in this book emanates from one such symposium and describes recent progress in integrating this new information with other datasets from metamorphic petrology on a mineral and sub-mineral scale.