Time-integrated Fluid Fluxes Research Articles

Mantle hydration initiated by Ca metasomatism in a subduction zone: An example from the Chandman meta-peridotite, western Mongolia

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H2O, whereas Ol hydration was characterized by gains of Si and H2O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18–1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3–1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism.

Geological records of transient fluid drainage into the shallow mantle wedge.

Pore fluid pressure on subduction zone megathrusts is lowered by fluid drainage into the overlying plate, affecting subduction zone seismicity. However, the spatial and temporal scales of fluid flow through suprasubduction zones are poorly understood. We constrain the duration and velocity of fluid flow through a shallow mantle wedge based on the analyses of vein networks consisting of high-temperature serpentine in hydrated ultramafic rocks from the Oman ophiolite. On the basis of a diffusion model and the time-integrated fluid flux, we show that the channelized fluid flow was short-lived (2.1×10-1 to 1.1×101years) and had a high fluid velocity (2.7×10-3 to 4.9×10-2meters second-1), which is close to the propagation velocities of seismic events in present-day subduction zones. Our results suggest that the drainage of fluid into the overlying plate occurs as episodic pulses, which may influence the recurrence of megathrust earthquakes.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Field-Based Evidence for Intra-Slab High-Permeability Channel Formation at Eclogite-Facies Conditions During Subduction.

Fluid release from subducting oceanic lithosphere is a key process for subduction zone geodynamics, from controlling arc volcanism to seismicity and tectonic exhumation. However, many fundamental details of fluid composition, flow pathways, and reactivity with slab‐forming rocks remain to be thoroughly understood. In this study we investigate a multi‐kilometer‐long, high‐pressure metasomatic system preserved in the lawsonite‐eclogite metamorphic unit of Alpine Corsica, France. The fluid‐mediated process was localized along a major intra‐slab interface, which is the contact between basement and cover unit. Two distinct metasomatic stages are identified and discussed. We show that these two stages resulted from the infiltration of deep fluids that were derived from the same source and had the same slab‐parallel, updip flow direction. By mass balance analysis, we quantify metasomatic mass changes along this fluid pathway and the time‐integrated fluid fluxes responsible for them. In addition, we also assess carbon fluxes associated with these metasomatic events. The magnitude of the estimated fluid fluxes (104–105) indicates that major intra‐slab interfaces such as lithological boundaries acted as fluid channels facilitating episodic pulses of fluid flow. We also show that when fluids are channelized, high time‐integrated fluid fluxes lead to carbon fluxes several orders of magnitude higher than carbon fluxes generated by local dehydration reactions. Given the size and geologic features of the investigated metasomatic system, we propose that it represents the first reported natural analogue of the so‐called high permeability channels predicted by numerical simulations.

Limited channelized fluid infiltration in the Torres del Paine contact aureole

AbstractThe metamorphic rocks from the Torres del Paine contact aureole (Patagonia, Chile) show field, petrographic, and geochemical evidence for small amounts of igneous fluid infiltration due to the solidification of the granite complex. Hydrogen isotope ratios (D/H) in the contact aureole first decrease while approaching the intrusion and subsequently increase toward the granite contact. Initial decrease with metamorphic grade is due to preferential loss of the 2H isotopes due to Rayleigh fractionation during prograde dehydration reactions. The infiltration of high-δD fluids from the intrusion increases δD within the last 150 m. In contrast, (18O/16O) ratios show no systematic changes, indicating that neither oxygen loss by Rayleigh fractionation nor oxygen exchange by fluid infiltration was significant enough to dominate original variations seen in the oxygen isotope ratio of the protolith. Calculated volume of fluid using the position of the hydrogen isotope exchange front gives a relatively low time-integrated fluid flux of about 4 m3/m2 at the contact for the igneous fluid. These small amounts of fluid flux are in agreement with whole rock oxygen isotope data that are not affected in the contact aureole. Chlorine content of metamorphic biotite crystals, in contrast to oxygen isotopes, supports infiltration of igneous fluids. Indeed, relatively high-Cl concentrations in biotite were measured in some samples close to the intrusion (up to 0.2 wt%), while chlorine concentrations in biotite are constant everywhere else in the entire contact aureole, having low concentrations (0.01–0.06 wt%). The absence of a well-marked Rayleigh fractionation trend in Cl concentrations with increasing metamorphism is surprising since chlorine strongly fractionates into the fluid. This is best explained by slow diffusive exchange of chlorine in biotite in the cooler outer aureole. Hence recrystallization of biotite would be required to modify its Cl composition. Biotite grains from samples close to the intrusion with high-Cl content also have lower Ti content (0.4 pfu) than biotite (0.5 pfu) from other samples containing biotite with lower Cl content located at the same distance from the contact. Since Ti content in biotite is a function of temperature, this is a good indication that magmatic fluid infiltration started post peak, early during cooling of the metamorphic rocks. Episodes of fluid flow appear to have been nearly continuous during cooling as evidenced by numerous retrogression textures, such as secondary muscovite (above 470 °C) or chlorite + muscovite intergrowth after cordierite or biotite (slightly below 470 °C). This might be related to crystallization of subsequent batches of granites or the onset of minor fluid convection during cooling of the aureole. Nevertheless, only minor secondary muscovite has been found, and fresh cordierite is present throughout the aureole confirming small amounts of fluid infiltration.The time-integrated fluid flux computed from the hydrogen isotope exchange front is two orders of magnitude lower than values computed for metacarbonate in many other contact aureoles, suggesting low permeabilities of pelitic rocks. In conclusion, Cl contents and hydrogen isotope compositions of hydrous minerals provide a sensitive tool to identify small fluid-rock interaction events, much more sensitive than oxygen isotope compositions of the whole rock or minerals.

Fluid-mediated selective dissolution of subducting carbonaceous material: Implications for carbon recycling and fluid fluxes at forearc depths

Subduction of crustal C governs the long-term global C cycling. The role of carbonates recycling in subduction zones and the related dissolution of C at various depths have been the subject of a large body of literature over the last decades. Much less is known about the contribution of carbonaceous material (CM) to the deep C cycling in subduction zones. This paper presents natural evidence for intense fluid-mediated leaching of CM in pelitic schists at high-pressure/low-temperature conditions relevant to the forearc region of subducting slabs. Manifestations of such process were identified along fluid pathways at various scales in the blueschist-facies subduction complexes of both Alpine Corsica and the Western Alps. Microstructural, whole-rock and Raman analyses across a selected metasomatic aureole were used to quantify the amount and mechanisms of C loss during fluid-rock interaction. In samples affected by intense fluid infiltration, >90% of the initial CM was removed from the rock. Microstructural and micro-Raman data indicate selective leaching of disordered CM relative to nearly crystalline graphite. The collected data allowed constraining the magnitude of fluid fluxes required to bleach the studied CM-bearing lithologies at different P-T-fO2 conditions, which corresponds to rather high time-integrated fluid fluxes in the order of ~106 m3/m2. In settings of large-scale fluid channelization, such as along regional-scale, lithological/tectonic boundaries or at the top of the subducted sedimentary pile, intense dissolution of subducted CM is expected. This process may thus exert a negative feedback on the sink of C phases into the deep mantle over the geological timescales and contribute to the release of isotopically light C from subducting slabs in forearc regions.

Microstructural analyses of a giant quartz reef in south China reveal episodic brittle-ductile fluid transfer

A long-lived hydrothermal system at the Heyuan fault, South China, has led to the development of a giant quartz reef, now partially exhumed along its length for more than 40 km. Systematic analyses and focused microstructural studies have been undertaken to unravel a complex formation history of repeated fracturing, hydrothermal fluid flow and sealing cycles, resulting in a dynamic permeability across the fault zone. The change in morphology and decreasing grain size with time further indicates the move from slow ductile opening to fast seismic events. Quartz-reef formation has been estimated to occur within a range of ~200–350 °C, based on evaluation of (i) quartz deformation microstructures; (ii) chlorite and mica geothermometry; and (iii) review of comparable quartz-reef studies. Additionally, a set of physico-chemical formation conditions have been identified which compose the ‘quartz-reef window’. These are: (i) significant volume of fluid; (ii) fluid sources from meteoric, metamorphic and/or from mantle origin; (iii) considerable Time-Integrated Fluid Fluxes; (iv) SiO2 oversaturation due to (a) temperature change, (b) sudden pressure drop, or (c) chemical change e.g. fluid mixing; (v) accommodation space to ‘grow’ the reef; (vi) channel permeability; and (vii) cap rock/seal to trap the fluid flow. The mechanism of quartz-reef growth is here interpreted as the brittle-ductile analogue of the brittle fault-valve model.

Intra-slab COH fluid fluxes evidenced by fluid-mediated decarbonation of lawsonite eclogite-facies altered oceanic metabasalts

The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20–60 kg CO2 per m3 of rock (i.e. ~0.7–2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20–50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

Two-stage fluid flow and element transfers in shear zones during collision burial-exhumation cycle: Insights from the Mont Blanc Crystalline Massif (Western Alps)

The Mont-Blanc Massif was intensely deformed during the Alpine orogenesis: in a first stage of prograde underthrusting at c. 30 Ma and in a second stage of uplift and exhumation at 22-11 Ma. Mid-crustal shear zones of 1 mm–50m size, neighbouring episyenites (quartz-dissolved altered granite) and alpine veins, have localised intense fluid flow, which produced substantial changes in mineralogy and whole-rock geochemistry. Four main metamorphic zones are oriented parallel to the strike of the massif: (i) epidote, (ii) chlorite, (iii) actinolite-muscovite±biotite and (iv) muscovite±biotite. In addition, phlogopite-bearing shear zones occur in the chlorite zone, and calcite-bearing shear zones are locally found in the muscovite zone. The initial chemical composition of the granitic protolith is relatively constant at massif scale, which allows investigating compositional changes related to shear zone activity, and subsequent volume change and elements mobility. The variations of whole-rock composition and mineral chemistry in shear zones reflect variations in fluid/rock ratios and fluid’s chemistry, which have produced specific mineral reactions. Estimated time-integrated fluid fluxes are of the order of 106m3/m2. The mineral assemblages that crystallised upon these fluid-P-T conditions are responsible for specific major and trace element enrichments. The XFe (Fe/Fe+Mg) pattern of shear zone phyllosilicates and the δ13C pattern of vein calcite both show a bell-type pattern across the massif with high values on the massif rims and low values in the centre of the massif. These low XFe and δ13C values are explained by down temperature up-flow of a Fe-Mg-CO2-rich and silica-depleted fluid during stage 1, while the massif was underthrusting. These produced phlogopite, chlorite and actinolite precipitation and quartz hydrolysis, resulting in strong volume losses. In contrast, during stage 2 (uplift), substantial volume gains occurred on the massif rims due to the precipitation of quartz, epidote and muscovite from a local fluid hosted in the Helvetic cover. These two fluids advocate for the presence of an upper-crustal scaled fluid convection cell, with up-going fluids through the lower crust and likely down-going fluids in the 15km upper crust.

Fluids in the crust during regional metamorphism: Forty years in the Waterville limestone

![Figure][1] Research over the last four decades on carbonate rocks of the Waterville limestone, Maine, U.S.A., has contributed to the development of both concepts and methodologies for understanding fluid-rock interaction during regional metamorphism, including: (1) buffering of fluid composition by mineral reactions, (2) infiltration of carbonate rocks by aqueous fluids, (3) petrologic fluid-rock ratios, (4) infiltration-driven metamorphism, (5) one- and two-dimensional continuum models for coupled fluid flow and mineral reaction, (6) time-integrated fluid fluxes, and (7) channelized, horizontal fluid flow in the direction of increasing temperature within chemically isolated layers. Disagreement between the last concept and both hydrodynamic models for metamorphic fluid flow and empirical evidence for homogenization of fluid composition at a scale much larger than layer thickness motivated development of the latest models for coupled fluid flow and mineral reaction in the Waterville limestone. The new models consider a flow medium composed of layers that differ in the initial amounts and compositions of minerals, both horizontal flow in the direction of increasing temperature and vertical flow in the direction of decreasing pressure and temperature, significant but imperfect homogenization of fluid composition across layering by CO2-H2O interdiffusion, and infiltration by fluids that are spatially variable in composition on the kilometer scale with CO2 content increasing with increasing grade of metamorphism. The new models reproduce measured progress of the biotite-forming reaction in the Waterville limestone over a range of spatial scales spanning six orders of magnitude, from differences in reaction progress of up to a factor of ~100 between adjacent centimeter-thick layers to the coexistence of mineral reactants and products over a distance of ~13 km parallel to the metamorphic field gradient. Results imply that the present spatial distributions of reaction progress represent a steady state achieved when rocks closely approached equilibrium with the infiltrating fluid during metamorphism. The new models resolve what for many years appeared to be fundamental discrepancies among petrologic data for regionally metamorphosed carbonate rocks, hydrodynamic models of regional metamorphism, and the length scales of mass transport of volatiles across layering by diffusion. [1]: pending:yes

The impact of porosity waves on crustal reaction progress and CO2 mass transfer

Rocks below the brittle-ductile transition can deform viscously and compact while fluid percolates through (two-phase solid–fluid flow). We investigate chemical reaction systematics during two-phase flow using one-dimensional numerical models in which reactive H2O–CO2 fluid ascends down-temperature toward the surface, driving the retrograde reaction: 7CO2+3Calcite+Tremolite=5Dolomite+8Quartz+H2O. The reaction progress is compared to that predicted by fluid–rock reaction during “standard” Darcian flow. A range of layer thicknesses (km-scale) in one- and two-layer systems were investigated at pressure–temperature conditions below the brittle-ductile transition corresponding to ∼13–20 km depth. Model porosity waves of wavelength ∼5 km were generated repeatedly at the base of the flow region using a solitary wave solution with a prescribed initial wave amplitude A = (maximum porosity)/(background porosity of 10−3).Simulation of reaction progress and carbon transfer for the Darcian flow model and for porosity wave transport with A=5, 2.5 and 1.25 yield the following results. First, the overall reaction progress in the two fluid transport models is mainly controlled by the time-integrated fluid flux, and is not strongly dependent on the flow regime. The implication is that the fluid pressure gradient anomalies in the regional-scale porosity waves modeled herein play a negligible role in driving reaction progress. Second, although there are high fluid velocities and thus strong advection in large amplitude porosity waves, the kinetic parameters adopted from experiments predict that the fluid compositions approach local fluid–rock equilibrium in both transport models. Third, regional-scale carbonate-bearing rock layers may be substantial sinks for carbon in ascending fluids if sufficient porosity and permeability can be maintained during reaction. Finally, typical models of retrograde reactions predict that porosity ultimately becomes clogged and, thus, fluid flow is hindered. However, porosity wave transport provides a potential mechanism for creating new porosity to sustain fluid flow during retrogression.

Re-evaluation of Infiltration-driven Regional Metamorphism in Northern New England: New Transport Models with Solid Solution and Cross-layer Equilibration of Fluid Composition

The spatial distribution of reactants and products of infiltration-driven decarbonation reactions can be a record of the geometry and amount of reactive fluid flow during regional metamorphism. In the past, these distributions have been interpreted assuming (1) minerals are fixed in composition and (2) single layers are chemically isolated. Because neither assumption is normally valid, new transport models were developed that specifically predict the spatial distribution of reactants and products of the well-studied biotite-forming reaction in marls from northern New England. The models consider (1) isothermal and isobaric (iso-P–T) flow, (2) horizontal flow in the direction of increasing T (up-T flow), and (3) vertical, upward flow in the direction of decreasing P and T (down-P–T flow). All models assume a medium composed of many thin layers that differ in the amounts and compositions of mineral solid solutions prior to reaction, that fluid flow is parallel to layering, and that fluid composition is homogenized across layering over a distance much greater than layer thickness. All models reproduce (1) the amounts of mineral reactants and products at outcrops in Maine and Vermont where there are extensive data, and (2) regional-scale observations of mineral reactants only at low grades and a region several kilometers wide at higher grade where mineral reactants and products coexist. Models of up-T flow and down-P–T flow are preferred because they additionally predict spatially widespread complete reaction at the highest grades. Results show that when reactants and products are solid solutions, down-P–T flow predicted by simple hydrodynamic models of regional metamorphic fluid flow is fully consistent with observed widespread spatial distributions of mineral reactants and products in metamorphic terrains. Results further demonstrate that layer-scale variations in reaction progress are better explained in terms of layer-by-layer variations in initial mineral abundances and compositions coupled with homogenization of fluid composition across layers than by channelized fluid flow focused into layers with elevated reaction progress. All models predict minimum time-integrated fluid fluxes of ∼103 mol fluid cm−2 rock, ∼1–2 orders of magnitude less than what has been estimated assuming minerals are fixed in composition.

The effects of solid solution and diffusion on infiltration-driven metamorphism with application to metaperidotite, Val d’Efra, Central Swiss Alps

The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (Xi) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between Xi and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξss ≤ ξmax) that depends on input fluid composition (ξmax is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with Xi homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξss in all domains except upstream from the slowest moving front where ξ = ξss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξmax) are best reproduced by down-temperature infiltration of a disequilibrium fluid with \( X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 \) into a multi-domain medium with uniform \( X_{{{\text{CO}}_{2} }} \) at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm2 rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature.

Fluid migration above a subducted slab — Thermodynamic and trace element modelling of fluid–rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)

The amount and composition of subduction zone fluids and the effect of fluid–rock interaction at a slab–mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Sr and Pb distribution during mineral growth, -breakdown and modification induced by fluid–rock interaction. Our results show that Li and B budgets are fluid-controlled, thus acting as tracers for fluid–rock interaction processes, whereas Sr and Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid–rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4 ∙ 10 2 m 3 m − 2 in the weakly deformed rocks and 1–8 ∙ 10 3 m 3 m − 2 in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments.

Extreme channelization of fluid and the problem of element mobility during Barrovian metamorphism

Geochemical profiles across chemically altered zones (selvages) surrounding amphibolite facies quartz+kyanite veins were investigated to determine major, minor, and trace element mass transfer and volume strain due to channelized fluid flow. The three profiles are perpendicular to two veins cutting the Wepawaug Schist, Connecticut, U.S.A., which underwent Barrovian-style metamorphism during the Acadian orogeny. Selvages are highly aluminous with considerably more kyanite, staurolite, and garnet, and less quartz, plagioclase, and mica, than surrounding wallrocks. Kyanite crystals increase in size toward veins, and reach several centimeters in length within veins. Mass balance analysis indicates 50% silica loss and 34% volume loss from selvages, on average. Silica, transferred locally from selvages to veins, accounts for 40 to 80% of the vein silica; the remainder must have been precipitated from fluids that flowed through the veins during regional devolatilization. Fluid flow also transported elements into the rock mass, which were concentrated in the selvages. Gains of Fe, Mn, Y, and HREE were due to the growth of selvage garnet; Fe, Zn, and Li were sequestered into staurolite. Kyanite, staurolite, and garnet growth resulted in Al mass gains. Destruction of mica (particularly muscovite) and of plagioclase in the selvages resulted in losses of K, Na, Ba, Pb, Sn, and volatiles and losses of Na, Sr, and Eu, respectively. Thin Na and Sr enrichment zones associated with increased modal plagioclase are found along the margins between selvages and less altered wallrock and may represent either chemical self-organization produced during diffusive mass transfer and reaction, or are relics from a possible earlier period of Na enrichment in the selvages. Simple, two-dimensional numerical modeling of flow in a fractured porous medium indicates that fluxes vary significantly over short distances (<1 m) adjacent to veins. Fluids, channelized into the high-permeability fracture conduits, carry the bulk of the fluid flow; mass transfer to and from selvages adjacent to the conduits occurred by some combination of diffusion and flow. In contrast, areas distal to the conduits are impoverished in fluid and undergo much more limited infiltration. As a consequence, different workers can come to vastly different conclusions about the magnitude of fluid fluxes and element transfer depending on the part of a fractured outcrop studied. This extreme spatial variability due to channelization can largely explain contrasting views that have arisen in the literature regarding the nature and intensity of non-volatile element mass transfer during Barrovian metamorphism. Determining “average” time-integrated fluid fluxes and levels of element transport across outcrops remains as important research challenges due to the spatial variability of flow.

Modeling metamorphism in collisional orogens intruded by magmas: II. Fluid flow and implications for Barrovian and Buchan metamorphism, Scotland

This paper presents the results of two-dimensional numerical modeling of crustal fluid flow in a collisional overthrust setting during emplacement of syn-metamorphic mafic sills. An important feature of the fluid flow in the presence of magmatic intrusions is the long-lived prograde up-temperature (up-T) flow of fluid. This upward flow is driven by dehydration reactions in the rocks underlying the intrusions and upward is the consequence of the inverted thermal profile that appears within the depth range of magmatism. If magma is intruded at mid-crustal depths following crustal collision, the up-T flow regime may persist for as much as 5 Myr. Such long-lived up-T fluid flow is not observed in models without magmatism. Strong retrograde reactions in rocks surrounding intrusions may give rise to downward up-temperature flow into the heated but rapidly cooling areas of the crust during exhumation, but the fluid fluxes in these areas are typically small. The total time-integrated fluid fluxes produced by magmatism and crustal overthickening in the middle and deep crust are substantial, in some cases reaching 3,000 m3 m−2 over 35 Myr. Early magmatic events may give rise to fluid fluxes exceeding 1,000 m3 m−2 in only 1 Myr. The strong dehydration that accompanies the emplacement of hot magma into the relatively low-temperature wall rock leads to hydrofracturing of large volumes of crustal rocks surrounding intrusions. The fractured rock region may extend over tens of kilometers from the zone of magma emplacement. More hydrofracturing is predicted for the middle and deep crust than for the shallow crust because of the exponential decrease of crustal permeability with depth that is assumed in our models. Endothermic dehydration, exothermic retrograde hydration, and the latent heat of wall rock fusion can all influence thermal budgets. In the final part of the paper we integrate the results of the fluid flow modeling presented here, the reconstructions of thermal evolution presented in Part I, and field-based data for the type locality of Barrovian and Buchan metamorphism in Scotland. Many key phenomena predicted by our numerical models are observed in the Scottish Dalradian, including the very short thermal peaks attained over a relatively narrow time span across a broad range of metamorphic zones; penecontemporaneous Barrovian, Buchan, and granulite facies metamorphism; steep Metamorphic Field Temperature Gradients (MFTGs) in the vicinity of intrusions; and petrological evidence consistent with up-temperature fluid flow. Our results suggest that some controversial aspects of regional metamorphism in the Scottish Dalradian, including heat transfer processes and the steep MFTGs, may be plausibly explained by the effects of plutonism.

The effect of fluid salinity on infiltration-driven contact metamorphism of carbonate rocks

Calculated phase equilibria involving minerals and H2O–CO2–NaCl fluid lead to predictions of how infiltration of rock by H2O–NaCl fluids with XNaCl in the range 0–0.3 (0–58 wt% NaCl) drives the reactions calcite + quartz = wollastonite + CO2 and dolomite = periclase + calcite + CO2. Calculations focus on metamorphism in four aureoles that together are representative of the normal P–T conditions and processes of infiltration-driven contact metamorphic reactions. The effect of salinity on the spatial extent of oxygen isotope alteration was also computed. The time-integrated input fluid flux (q°) that displaces the mineral reaction front an increment of distance along the flow path always increases with increasing XNaCl. For input fluids with salinity up to approximately five times that of seawater (XNaCl ≤ 0.05), values of q° required to explain the spatial extent of decarbonation reaction are no more than 1.1–1.5 times that computed assuming the input fluid was pure H2O. For more saline fluids, values of q° may be up to 1.4–7.9 times that for pure H2O. Except for reaction in the presence of halite and vapor (V), infiltration of H2O–NaCl fluids expands the region of oxygen isotope alteration relative to the size of the region of mineral reaction. The expansion is significant only for saline fluids with XNaCl ≥ ~0.1. Immiscible fluid phase separation and differential loss of the liquid (L) or V phase from the mineral reaction site increases the amount of reactive fluid required to advance the mineral reaction front compared to conditions under which equilibration of minerals and fluid is attained with no loss of L or V. Decarbonation reactions driven by infiltration of fluids with even modest seawater-like salinity can explain the occurrence of salt-saturated fluid and solid halide inclusions in contact metamorphosed carbonate rocks.

Formation of replacement dolomite in the Latemar carbonate buildup, Dolomites, northern Italy: Part 2. Origin of the dolomitizing fluid and the amount and duration of fluid flow

Replacement dolomite in the Latemar carbonate buildup developed when limestone was infiltrated by reactive fluid. Minor-element, trace-element, and oxygen and carbon isotope compositions of dolomite and precursor limestone constrain the origin of the fluid and fundamental aspects of the flow. Inferred salinity (similar to seawater); temperature (45°–85°C); ⁸⁷Sr/⁸⁶Sr (0.7076–0.7079); Ca/Mg (&lt;1.4); and Fe, Mn, and Zn content of the dolomitizing fluid are consistent with a fluid similar to modern diffuse effluent. Modern diffuse effluent itself is approximately a mixture of seawater with up to ∼25 percent high-temperature mid-ocean ridge hydrothermal vent fluid. Time-integrated fluid flux was in the range (2–4) · 10⁶ mol fluid/cm² rock or (4–7) · 10⁷ cm³ fluid/cm²rock. Estimation of time-integrated flux leads to an internally consistent framework for the appropriate interpretation of the oxygen, strontium, and carbon isotope compositions of replacement dolomite. The oxygen and strontium isotope compositions reflect equilibration with dolomitizing fluid and provide a chemical fingerprint of the fluid. The carbon isotope composition of dolomite, however, was simply inherited directly from the precursor limestone in nearly all cases. A quantitative evaluation of the minor- and trace-element budget of dolomitization verifies that a fluid like modern diffuse effluent, but not unmodified seawater, could supply sufficient Fe, Mn, and Zn to enrich dolomite in these elements compared to limestone. If the flux of dolomitizing fluid was similar to that of modern diffuse effluent, ∼0.02 cm³/cm² · s, the duration of fluid flow and mineral-fluid reaction was short, ∼100 years. The total duration of dolomitization, however, could have been much longer if fluid flow was episodic, as in modern seafloor hydrothermal systems, depending on the time elapsed between episodes of flow. Conversion of limestone to dolomite likely occurred by a mechanism intermediate between the end-member cases of replacement at constant oxygen and carbon and replacement at constant volume.

Element mobility and scale of mass transport in the formation of quartz veins during regional metamorphism of the Waits River Formation, east-central Vermont

Veins and adjacent alteration selvages in the Waits River Formation were investigated to determine whether associated mass transfer was due primarily to large-scale advection or small-scale diffusion. Samples of the vein, selvage, and adjacent wall rock were collected from the earliest and most numerous generation of veins in pelite and carbonate hosts from outcrops in the chlorite and the kyanite zones. Bulk compositions of selvages and unaltered wall rocks were compared using a reference frame defined by a combination of Zr, Ti, REEs, and U. Selvages from both outcrops typically exhibit losses of Si, K, Ba, and Rb relative to unaltered wall rock. Kyanite zone selvages show losses in Mg and Cs in addition. Differences in K, Ba, Rb, Cs, and Mg between the vein-selvage system as a whole and adjacent unaltered wall rock are possibly accounted for by development of micas in veins. The addition of Si and Ca to veins is not balanced by removal of Si and Ca from the selvages. Vein-selvage systems contain more Si than wall rock, with an overall addition of ≈40 mg Si/cm3 to the chlorite zone and ≈55 mg Si/cm3 to the kyanite zone. Mass balance of Si at the outcrop scale requires that >90% of the Si in quartz veins was derived externally. Quartz veins studied formed primarily by fluid flow and large-scale advective mass transfer with a relatively minor component of local mass transport by diffusion. The estimated time-integrated fluid flux necessary to produce the observed amount of quartz in veins in an entire outcrop is ≈2-6·106 cm3 fluid/cm2 rock. Mineral inclusions in garnet and fracturing of garnets adjacent to veins indicate that formation of selvages and veins initiated prior to formation of garnet and continued after the end of garnet growth.

Magnitude and geometry of reactive fluid flow from direct inversion of spatial patterns of geochemical alteration

Time-integrated equations governing tracer mass-balance provide a framework for interpreting spatial patterns of mineralogical, elemental, and isotopic alteration in rocks caused by reactive fluid flow, and linear inverse theory allows quantitative estimates of the magnitude and three-dimensional (3D) geometry of fluid flow. We demonstrate the inverse technique with a simple model system that involves fluid advection and fluid-rock oxygen isotope exchange only. Analytic forward models of reactive transport enable patterns of isotope alteration to be calculated exactly, and inversions of the resulting spatial patterns of synthetic isotope composition demonstrate the effectiveness of our approach. For a field application of the inverse method, we consider regional metamorphic rocks from the Waits River Formation, southeastern Vermont, that expose a mineralogical and stable isotopic record of reactive fluid flow over an area of ≈120 km². Spatial patterns of prograde changes in whole-rock CO₂, δ¹⁸O, and δ¹³C allow a direct inversion for the magnitude and 3D geometry of reactive flow at the time of final prograde mineral reaction. Consistency tests suggest that the inverse estimate is primarily controlled by patterns of CO₂ changes, and resolution tests indicate that the smallest features in the inverse flux field that can be reliably detected have an east-west dimension of ≈5 km and a north-south dimension of ≈14 km. The regionally-averaged time-integrated fluid flux vector returned by the inversion has a magnitude of 2.2×10⁴ mol fluid/cm² rock, trends 255° in the horizontal plane, and is directed upward at 32° from the horizontal. The geometry of terrain-scale reactive flow was largely controlled by regional structure. Reactive fluids that entered the study area appear to have been tectonically-driven horizontally from the east at a depth ≈0.5 to 2.0 km beneath the present level of exposure.