Silica Metasomatism Research Articles

Metasomatism and Slow Slip: Talc Production Along the Flat Subduction Plate Interface Beneath Mexico (Guerrero)

AbstractTalc‐rich rocks are common in exhumed subduction zone terranes and may explain geophysical observations of the subduction zone interface, particularly beneath Guerrero, Mexico, where the Cocos plate subducts horizontally beneath North America and episodic tremor and slow slip (ETS) occurs. We present petrologic models exploring (a) the degree of silica metasomatism required to produce talc in serpentinized peridotites at the pressure‐temperature conditions of the plate interface beneath Guerrero and (b) the amount of silica‐bearing water produced by rocks from the subducting Cocos plate and the location of fluid pulses. We estimate the volumes of talc produced by the advection of silica‐rich fluids into serpentinized peridotites at the plate interface over the history of the flat‐slab system. In the ETS‐hosting region, serpentinites must achieve ∼43 wt. % SiO2 to stabilize talc, but minor additions of silica beyond this produce large volumes of talc. Our models of Cocos plate dehydration predict that water flux into the interface averages 3.9 × 104 kg m−2 Myr−1 but suggest that only where subducting basalts undergo major dehydration reactions will sufficient amounts of silica‐rich fluids be produced to drive significant metasomatism. We suggest that talc produced by advective transport of aqueous silica alone cannot account for geophysical interpretations of km‐thick zones of talc‐rich rocks beneath Guerrero, although silica‐bearing fluids that migrate along the plate interface may promote broader metasomatism. Regions of predicted talc production do, however, overlap with the spatial occurrence of ETS, consistent with models of slow slip based on the frictional deformation of metasomatic lithologies.

Link between melt-impregnation and metamorphism of Atlantis Massif peridotite (IODP Expedition 357)

IODP Expedition 357 drilled 17 shallow sites scattered over ~ 10 km in the west to east spreading direction across the Atlantis Massif oceanic core complex (OCC, MAR, 30 ºN). Mantle exposed in the footwall of the Atlantic Massif OCC is nearly wholly serpentinized (80–100%) harzburgite and subordinate dunite. A recent whole-rock chemistry study by Whattam et al. (Chemical Geology 594. 10.1016/j.chemgeo.2021.120681, 2022) subdivides Atlantis Massif peridotites into: Type I fluid–rock dominated serpentinite, which exhibits almost nil evidence of melt-impregnation or silica metasomatism; Type II melt–rock dominated, mafic melt-impregnated serpentinite; and Type III melt–rock dominated Si-metasomatized serpentinite. In this study, on the basis of EPMA, three kinds of Cr–spinel are distinguished in Expedition 357 serpentinite: (I) primary, unmetamorphosed mantle array, (II) low-Ti metamorphosed, and (III) high-Ti melt reacted. All Cr–spinel of western site Type I serpentinite is unmetamorphosed (n = 34) and comprises 68% of all unmetamorphosed Cr–spinel. Metamorphosed Cr–spinel (n = 100) are the most abundant and occur in the central and eastern site Type II and Type III serpentinite, whereas melt-reacted Cr–spinel and chromite are restricted to the central sites and occur predominantly in serpentinized dunite. Estimates of the degree of melt extraction of Type I serpentinite using F = 10ln(spinel Cr#) + 24 are ~ 9–17%. Fugacity calculations of primary, unmetamorphosed Cr–spinel yield Δlog(fO2)FMQ of − 1.7 to + 1.0 and calculations using olivine–spinel Mg–Fe exchange thermometry yield a mean closure temperature of 808 ± 39 °C. Mafic melt-impregnation resulted in Cr–spinel with anomalously high TiO2 of 0.27–0.68 wt.% and production of Ti-rich chromite (up to 1.23 wt.% TiO2). Greenschist facies metamorphism (< 500 °C) resulted in Mg–Fe2+ exchange between Cr–spinel and forsterite and anomalously low Cr–spinel Mg#; higher degrees of amphibolite facies metamorphism (~ 500–700 °C) also resulted in anomalously high Cr# due to Al–Cr exchange. As has been previously established, significant Al loss from chromite cores above 550 °C is the result of equilibration with fluids in equilibrium with chlorite, which may be valid for our samples. On the basis of Cr–spinel vs. whole-rock compositions, a clear relationship exists between melt-impregnation and metamorphism of central and eastern serpentinite, which we postulate to be the result of heat associated with magma injection and subsequent localized contact metamorphism. To our knowledge, such a relation between mafic melt-impregnation of peridotite and metamorphism (of peridotite) has not previously been established in general and specifically for the Atlantis Massif peridotite. Closure temperatures of 440–731 °C of metamorphosed Cr–spinel approximate greenschist to amphibolite facies metamorphic conditions.

Alteration of abyssal peridotites is a major sink in the W geochemical cycle

Arc lavas and the continental crust exhibit a selective enrichment of W relative to similarly incompatible elements, indicating that subduction zone environments are a tectonic setting where W is mobilized from the subducting slab. Here we present evidence that ultramafic portions of altered oceanic lithosphere are a major sink for W and evaluate how its W budget needs to be included in constraining the global geochemical cycle of W. We report high precision W, HFSE and U-Th data obtained by isotope dilution for altered basement formed at super-slow spreading rates along the Mid-Atlantic Ridge drilled during ODP Leg 209 (between 14°N and 16°N). The oceanic basement consists of abyssal peridotites and associated gabbroic rocks exhumed in a magma starved setting. Hydrothermal alteration styles covered in the sample set are serpentinization, talc alteration, and low-T seawater alteration. The drilled rock portions are among the most depleted in HFSE that have ever been studied.Tungsten concentrations range from 4 ppb in the least altered harzburgites to 500 ppb in strongly serpentinized dunites. Locally, W is significantly enriched relative to U, Th and Ta by factors of up to 100 compared to canonical mantle values, much higher than in mafic portions of altered oceanic crust such as hole 1256D (up to 10). The samples from ODP Leg 209 show strong W enrichment during progressive serpentinization (holes 1268A, 1270 and 1271) and during late-stage oxidative seawater alteration (hole 1270D and 1272A). In contrast, silica metasomatism associated with talc alteration (hole 1268A) is associated with selective W depletion relative to Th and Ta. Trace element modelling indicates that hydrothermal enrichment of W by redistribution is a more likely source than seawater-derived W. Collectively, our data show that altered ultramafic rocks likely constitute an important geochemical reservoir in the global geochemical cycle of W contributing to the enrichment of W found in arc lavas and to the recycling of W into the Earth’s mantle.

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.

Alteration processes recorded by back‐arc mantle peridotites from oceanic core complexes, Shikoku Basin, Philippine Sea

AbstractWe determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non‐transform offset massif, which are located within the Shikoku back‐arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high‐temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.

Multiple Kinetic Parameterization in a Reactive Transport Model Using the Exchange Monte Carlo Method

Water–rock interaction in surface and subsurface environments occurs in complex multicomponent systems and involves several reactions, including element transfer. Such kinetic information is obtained by fitting a forward model into the temporal evolution of solution chemistry or the spatial pattern recorded in the rock samples, although geochemical and petrological data are essentially sparse and noisy. Therefore, the optimization of kinetic parameters sometimes fails to converge toward the global minimum due to being trapped in a local minimum. In this study, we simultaneously present a novel framework to estimate multiple reaction-rate constants and the diffusivity of aqueous species from the mineral distribution pattern in a rock by using the reactive transport model coupled with the exchange Monte Carlo method. Our approach can estimate both the maximum likelihood and error of each parameter. We applied the method to the synthetic data, which were produced using a model for silica metasomatism and hydration in the olivine–quartz–H2O system. We tested the robustness and accuracy of our method over a wide range of noise intensities. This methodology can be widely applied to kinetic analyses of various kinds of water–rock interactions.

Competitive hydration and dehydration at olivine–quartz boundary revealed by hydrothermal experiments: Implications for silica metasomatism at the crust–mantle boundary

Serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust. Given that such a fluid has a high silica activity compared with mantle peridotite, it is thought that serpentinization and silica metasomatism occur simultaneously at the crust–mantle boundary. In this study, we conducted hydrothermal experiments in the olivine (Ol)–quartz (Qtz)–H2O system at 250 °C and vapor-saturated pressure under highly alkaline conditions (NaOHaq, pH = 13.8 at 25 °C) to clarify the mechanism of silica metasomatism at the crust–mantle boundary. Composite powders consisting of a Qtz layer and an Ol layer were set in tube-in-tube vessels. After the experiments, the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the Ol–Qtz boundary.The mineralogy of the reaction products in the Ol-hosted region changed with increasing distance from the Ol–Qtz boundary, from smectite + serpentine (Smc zone) to serpentine + brucite + magnetite (Brc zone). Olivine hydration proceeded in both zones, but the total H2O content in the products was greater in the Brc zone than in the Smc zone. Mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone. In contrast, the Smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions. In the Smc zone, smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration, and around the boundary of the Smc and Brc zones, serpentine formation occurred by olivine hydration and brucite dehydration. The relative extent of hydration and dehydration reactions controlled the along-tube variation in the rate of H2O production/consumption and the rate of volume increase. Our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure, possibly affecting the mechanical behavior of the crust–mantle boundary.

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P–T–fluid regime in parts of the Pan-African mobile belt of South India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite + clinopyroxene (mg# = 71–73) + grandite (16–21 mol% Adr) + quartz ± calcite, Type II: grandite (25–29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36–38 mol% Adr) + clinopyroxene (mg# = 55–65) + epidote + scapolite + calcite + quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO2 → wollastonite + V, 2. calcite + clinopyroxene + O2 → grandite + SiO2 + V, 3. scapolite + calcite + quartz + clinopyroxene + O2 → grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O2 → grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2 11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.

Comparative Deformation Behavior of Minerals in Serpentinized Ultramafic Rock: Application to the Slab-Mantle Interface in Subduction Zones

The layer-structure minerals serpentine, brucite, and talc are postulated to form in the mantle wedge above a subducting slab as a result of progressive hydration and silica metasomatism. Tectonic mixing at the slab-mantle interface generates serpentinite mélanges that contain blocks of high-pressure (HP) or ultrahigh-pressure (UHP) metamorphic rock derived from the subducting slab. Such serpentinite mélanges may provide a means of exhumation of HP/UHP metamorphic rocks, and may define the lower limit of locked regions on the subduction interface that fail in large earthquakes. We review recently obtained frictional strength data for brucite and talc over the temperature range 25-400°C at 100 MPa effective normal stress and compare them with new data for antigorite. These minerals respond to heating in different ways, causing their frictional strengths to diverge. Water-saturated antigorite strength increases toward the fixed dry value of μ ≈ 0.75-0.80 with heating: μ ≈ 0.50 at 25°C and μ > 0.60 at 400°C. The difference in μ between dry and watersaturated talc gouge also decreases with increasing temperature, but both the dry and watersaturated values of μ are lower at elevated temperatures. For dry talc, μ decreases from 0.35 to 0.25 between 25° and 300°C, whereas for water-saturated talc, μ is approximately 0.20 at 25°C and 0.10-0.15 at elevated temperatures. Weakening of the interlayer bond of talc with heating may be responsible for the overall reduction in its frictional strength. The strength of dry brucite also is fixed at μ = 0.45-0.50, but the water-saturated value of μ decreases from ≈0.30 at 25°C to 0.20-0.25 at 200°-400°C. The water-saturated brucite gouge has extensively recrystallized along the shear surfaces, and its weakening may be attributable to solution-transfer processes. Because the serpentine minerals become stronger at elevated temperatures, to achieve low frictional strength in a serpentinized mantle wedge or serpentine-rich mélange would require some other cause such as nearly lithostatic fluid pressures or the addition of lower-strength minerals. Increasing abundance of brucite and talc in serpentinite at constant physical conditions would progressively reduce its frictional strength. The concentration of talc along a metasomatic front at the edge of the mantle wedge or in reaction zones surrounding HP/UHP blocks in serpentinite mélange should tend to localize shear in this extremely weak material.

THE MELILITE (Gh50) SKARNS OF ORAVITA, BANAT, ROMANIA: TRANSITION TO GEHLENITE (Gh85) AND TO VESUVIANITE

Almost monomineralic Mg-rich gehlenite (similar toGh(50)Ak(46)Na-mel(4)) skarns occur in a very restricted area along the contact of a diorite intrusion at Oravita, Banat, in Romania, elsewhere characterized by more typical vesuvianite-garnet skarns. In the vein-like body of apparently unaltered gehlenite, the textural relations of the associated minerals (interstitial granditic garnet and, locally, monticellite, rare cases of exsolution of magnetite in the core zone of melilite grains) suggest that the original composition of the gehlenite may have been different, richer in Si, Mg, Fe (and perhaps Na), in accordance with the fact that skarns are the only terrestrial type of occurrence of gehlenite-dominant melilite. The same minerals, monticellite and a granditic garnet, appear in the retrograde evolution of the Mg-rich gehlenite toward compositions richer in Al, the successive stages of which are clearly displayed, along with the final transformation to vesuvianite. These changes include (1) the local development of small rounded patches with Al-rich compositions (Gh(60) to Gh(85)), accompanied by monticellite, spurrite (or tilleyite, afwillite, kilchoanite) and, at a later stage, a granditic garnet, and (2) the complete transformation of gehlenite to vesuvianite, associated with minor clintonite (+ monticellite and probably ellestadite), usually along a sharp front typically rimmed by a 0.5-mm-wide zone of Al-rich gehlenite (Gh(85)). The gehlenite rim, as well as the gehlenite in the local modifications, show a remarkable correlation between the akermanite and Na-melilite contents, probably of crystal-chemical origin. The local modifications (1) are interpreted as nearly closed-system (except for Na and Fe) retrograde reactions at moderate temperature (500-600degreesC), controlled by the localized presence of small amounts of fluid at low pressure, mainly involving the transformation of the akermanite component to monticellite. The silica released by this transformation resulted in the formation of spurrite (among others) and of the Na-melilite component at first, and garnet later. The subsequent transformation (2) of geldenite to vesuvianite and clintonite, which involved silica metasomatism, may result from the more pervasive infiltration of the same fluid, at higher pressure, probably related to the development of the garnet-vesuvianite skarns elsewhere along the intrusive contacts.

Geochemistry and oxygen isotopic composition of the Canton Saint-Onge wollastonite deposit, central Grenville Province, Canada

The geology and geochemistry of the Canton Saint-Onge wollastonite deposit indicate that it is a skarn formed by silica metasomatism of dolomite-rich rocks. Oxygen isotopic compositions of the skarn rocks and the nearby plutons show that the fluids responsible for the metasomatism were not meteoric, but were probably associated with the Du Bras granitic pluton. However, the granite is not in contact with the wollastonite rocks at the present level of exposure, hence the fluids must have been released at greater depths. Wollastonite-rich parts of the skarn rocks are isotopically lighter than diopside-rich rocks, suggesting that wollastonite formed in regions of higher fluid flux.

Millimetre-scale variation in metamorphic permeability of marbles during transient fluid flow: an example from the Reynolds Range, central Australia

The Upper Calc-Silicate Unit of the Reynolds Range, central Australia, contains marbles with centimetre-wide layers of metamorphosed marls. Wollastonite-bearing marbles record fluid infiltration by water-rich fluids (XCO2=0.1–0.3) at 650–700 °C and 300–400 MPa during cooling from granulite-facies metamorphism at ∼1.6 Ga. The pattern of fluid flow recorded by stable isotope ratios of calcite, the distribution of wollastonite, and calcite luminescence is heterogeneous on a millimetre-scale. Calcite from decimetre-size hand specimens has δ18O values and δ13C values that vary by up to 10 and 12‰, respectively. The lowest δ18O and δ13C values are centred on millimetre-wide grandite-rich skarn zones, whereas higher values characterise relict calcite that co-exists with quartz. The variation in δ18O values together with related δ13C variations and the abundance of wollastonite may be used to determine the distribution of zones of higher and lower permeability. Skarn zones and regions of low δ18O values at marl–marble contacts mark channels of fluid flow. Fluid flow at the contacts was promoted by the rheological contrast and by the juxtaposition of calcite (in the marble) and quartz (in the marl) that could form abundant wollastonite via the reaction: calcite + quartz=wollastonite + CO2. This reaction results in a large negative volume change, and fluid flow may have been concentrated in a small number of channels once they were established, provided that fluid pressures were sufficient to overcome compaction. Some isolated skarn zones and low δ18O regions within the marbles may be located at the intersections of fractures that would have also been zones of high permeability. The formation of large volumes (>15–20%) of wollastonite within the marble layers requires that silica metasomatism via the reaction calcite + SiO2(aq)=wollastonite + CO2 has occurred, and the grandite skarns record metasomatic addition of Fe. These metasomatic reactions have positive volume changes that would have sealed existing fluid flow channels, allowing new channels to develop elsewhere in the rock. Preservation of steep stable isotope gradients suggests that fluid flow occurred over only a few thousand years after which the rocks were fluid absent.

Modeling metamorphic fluid flow with reaction-compaction-permeability feedbacks

Existing models of metasomatic flow do not allow for the effect that reaction has on the flow patterns. Instead, it is assumed that the volatiles produced are negligible in volume compared to those infiltrated and that reaction does not modify permeability. This is clearly unlikely to be true for infiltration-driven decarbonation reactions. The rates of porosity creation by reaction and porosity loss by creep have been calculated for a representative volume of calcite –quartz-wollastonite marble, and it is found that, even for a weak calcite matrix, the rate of porosity generation by reaction is likely to outstrip the collapse of porosity, as long as the system is out of equilibrium. We have applied a self-consistent 1D finite-difference model to the reaction of calcite + quartz to wollastonite in a 10m thick marble, in response to influx of H2O rich fluid, with fixed boundary conditions. The model allows us to evaluate the effect of reaction on the porosity structure and fluid pressure variation across the layer, from which local Darcy fluxes can be evaluated. The progress of reaction that we model is constrained by hydrological considerations, with the requisite parameters recalculated as reaction progresses, assuming creep compaction of rock under the stress difference between lithostatic and fluid pressures. We fnd that the volume of fluid realised by decarbonation, driven by influx of H20, is sufficient to create a back-flow, so that further advancement of the reaction front is only possible as a result of diffusion of water against the Darcy flux. The effect of creep driven by differences between fluid pressure and lithostatic pressure is to reduce the permeability of the layer and especially reduce the secondary porosity developed in the zone at and behind the advancing reaction front. We predict that in a 3D situation, the porous zone of reacted marble becomes a conduit for layer-parallel flow, and the secondary porosity is infilled by calc-silicate minerals due to silica metasomatism.

High-temperature Retrogression of Granulite-facies Marbles from the Reynolds Range Group, Central Australia: Phase Equilibria, Isotopic Resetting and Fluid Fluxes

Reynolds Range Group rocks underwent granulite-facies metamorphism (M2) at ∼1.6 Ga ( 5 kbar, 750–800°C) and were subsequently retrogressed in narrow strike-parallel zones at 1.59–1.57 Ga. Within these zones, metacarbonates that initially equilibrated at XCO2 >0.8 during M2 were mineralogically reset by the infiltration of water-rich fluids (XCO2≤0.02–0.3) at 650–700°C and 3–4 kbar. δ18O(Carb) values of the retrogressed metacarbonates were variably reset during fluid infiltration, with the lowest values (10–13‰) suggesting that the fluids that caused retrogression were exsolved from segregated partial melts, themselves derived from the underlying granulite-facies metapelites. Mineralogical and isotopic resetting were locally accompanied by silica metasomatism. The mineralogically reset marbles record time-integrated fluid fluxes of typically 101–104m3/m2. For upward flow of high-temperature fluids through the marbles over a distance of 200 m in 18 Ma, the observed mineralogical and isotopic resetting, and metasomatism require intrinsic permeabilities between 10−22 and 10−19 m2 that vary across strike on a centimetre to metre scale, indicating that fluid flow was strongly channelled.

Fluid flow paths and mechanisms of fluid infiltration in carbonates during contact metamorphism: the Beinn an Dubhaich aureole, Skye

A textural study of marbles from the Beinn an Dubhaich granite contact aureole, Skye, has shown that mass transport by diffusion was probably negligible during the metamorphic event, and that the bulk of the carbonates reacted as a consequence of silica metasomatism, permitting the use of calcsilicates as a tracer for fluid infiltration pathways. Fracture‐controlled infiltration was predominant in undeformed marbles, whereas pervasive infiltration occurred during synmetamorphic ductile deformation. Some calcite marbles contain disseminated unoriented calcsilicate minerals that are associated with neither fractures nor a ductile deformation fabric, consistent with an origin via infiltration of fluid along an interconnected grain‐edge porosity. The inference of limited pervasive infiltration of undeformed carbonates is consistent with predictions based on experimentally determined fluid–solid dihedral angles.

Formation of wollastonite‐bearing marbles during late regional metamorphic channelled fluid flow in the Upper Calcsilicate Unit of the Reynolds Range Group, central Australia*

Abstract Granulite facies marbles from the Upper Calcsilicate Unit of the Reynolds Range, central Australia, contain metre‐scale wollastonite‐bearing layers formed by infiltration of water‐rich (XCO2= 0.1–0.3) fluids close to the peak of regional metamorphism at c. 700° C. Within the wollastonite marbles, zones that contain &lt;10% wollastonite alternate on a millimetre scale with zones containing up to 66% wollastonite. Adjacent wollastonite‐free marbles contain up to 11% quartz that is uniformly distributed. This suggests that, although some wollastonite formed by the reaction calcite + quartz = wollastonite + CO2, the wollastonite‐rich zones also underwent silica metasomatism. Time‐integrated fluid fluxes required to cause silica metasomatism are one to two orders of magnitude higher than those required to hydrate the rocks, implying that time‐integrated fluid fluxes varied markedly on a millimetre scale. Interlayered millimetre ‐to centimetre‐thick marls within the wollastonite marbles contain calcite + quartz without wollastonite. These marls were probably not infiltrated by significant volumes of water‐rich fluids, providing further evidence of local fluid channelling. Zones dominated by grandite garnet at the margins of the marl layers and marbles in the wollastonite‐bearing rocks probably formed by Fe metasomatism, and may record even higher fluid fluxes. The fluid flow also reset stable isotope ratios. The wollastonite marbles have average calcite (Cc) δ18O values of 15.4 ± 1.6% that are lower than the average δ18O(Cc) value of wollastonite‐free marbles (c. 17.2 ± 1.2%). δ13C(Cc) values for the wollastonite marbles vary from 0.4% to as low as ‐5.3%, and correlations between δ18O(Cc) and δ13C(Cc) values probably result from the combination of fluid infiltration and devolatilization. Fluids were probably derived from aluminous pegmatites, and the pattern of mineralogical and stable isotope resetting implies that fluid flow was largely parallel to strike.

Channelled fluid infiltration and variation in permeability in Reynolds Range marbles, Australia

Strike-parallel flow of H 2 O-rich fluids through Reynolds Range marbles formed wollastonite-bearing assemblages. Wollastonite abundance varies on a millimetre de from 10 to 66 modal%. The mineralogy of unmetasomatized marbles suggests that the formation of more than c . 19% wollastonite requires silica metasomatism. Comparison with advective transport models suggests that the variable wollastonite contents may be explained by time-integrated fluid fluxes and intrinsic permeability varying by up to two orders of magnitude on a millimetre scale. Fluids were probably focused through the marble within microfractures and the variations in intrinsic permeability may reflect variable fracture density.

Igneous petrogenesis of magnesian metavolcanic rocks from the central Klamath Mountains, northern California

Mafic meta-igneous supracrustal rocks in the east-central part of the western Triassic and Paleozoic belt are interlayered with and appear to predominantly overlie fine-grained clastic and cherty metasedimentary strata. This complex constitutes a lithostratigraphic terrane exposed in the vicinity of Sawyers Bar, California. Basaltic flows, dikes, and sills are informally referred to as the Dog greenstones. Dark-colored, amygdaloidal flow breccias rich in titanium, iron, phosphorus, and light rare-earth elements (LREE9s) constitute the lowest, mildly alkalic members of the mafic flow series, intimately interlayered with fine-grained terrigenous detritus. Element proportions suggest that the dark, Ti + Fe* + P-rich basaltic lavas may have been extruded in an oceanic intraplate setting. Interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep-sea sedimentary deposition. The main mass of the stratigraphically higher extrusive sequence is a light-colored, massive tholeiite series, poorer in Ti + Fe* + P + LREE9s. Hypabyssal rocks belong exclusively to this second igneous suite. Compositions of the pale-colored, overlying basaltic/diabasic mass of the Yellow Dog greenstone section are consistent with eruption in an immature magmatic arc. Dikes presumably comagmatic with terminal-stage Yellow Dog volcanism cut the overlying, more eastern Stuart Fork terrane. The Yellow Dog greenstones contain relict magmatic clinopyroxene and/or pargasitic hornblende. Both melt series include Mg-rich units (ave 17 : MgO = 14.4 ± 1.9 wt. %; Cr = 650 ± 200 ppm; Ni = 270 ± 100 ppm). Sea-floor alteration apparently affected these mafic rocks in the marine environment following eruption, as reflected by δ 18 O, MgO, and CaO/Al 2 O 3 values and by minor alkali and silica metasomatism. Yellow Dog igneous activity was extinguished during mid-Jurassic suturing of the oceanward Sawyers Bar terrane against and beneath the pre-existing landward Stuart Fork blueschist complex of Late Triassic metamorphic age. Subsequently, the terrane amalgam was invaded by Middle/ Late Jurassic granitoids. Minor chemical alteration, including 18 O enrichment, also probably accompanied later regional, and especially contact, metamorphism. Nevertheless, many major-, minor-, and rare-earth-element variations in the Yellow Dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust.

Mineralogy, geochemistry and mineralization of the Ririwai complex, northern Nigeria

The Ririwai complex represents the eroded roots of an alkaline volcano developed as part of a sequential chain of anorogenic centres in early Jurassic times. An outer ring-dyke fracture which formed a volcanic feeder is filled with quartz porphyry, and granite porphyry surrounds and partly encloses a caldera-collapsed volcanic pile into which peralkaline granite and biotite granite have been emplaced. The volcanic rocks are dominantly rhyolitic ignimbrites with minor basalts, showing petrological and geochemical features of magmatic crystallization and subsolidus re-equilibrium. the volcanic feeder intrusions are partly quenched and partly degassed representatives of the original granite magma but petrological and geochemical data testify to the limited interaction of an alkaline residula fluid phase. Th effects of the fluid phase are seen as a series of metasomatic reactions generating peralkaline granites, biotite granites and their mineralization. The interactions between crystal and fluids have been monitored by XRD, XRF, INAA, wet chemical analyses and fluid inclusion studies. Ore mineralogy confirms the paragenetic columbite (pyrochlore)-cassiterite-sphalerite evolution. The metasomatic reactions commence with Na + metasomatism followed by K +, then H + and finally Si 4+. The latter reactions are best displayed in the Ririwai lode where particle track studies have delineated the relative mobility of U and Th. Mineralization in biotite granite can be grouped according to the dominant metasomatic process. Columbite, minor cassiterite and sphalerite can be equated with albitite formation. Potash metasomatism generated columbite, wolframite, cassiterite and sphalerite deposition, which continued into greisen formation as H + metasomatism developed. This was accompanied by molybdenite, chalcopyrite and galena deposition during silica metasomatism. According to isotopic data the source of the ore metals and the granite magma was probably the Pan-African continental crust with contributions from the mantle.

Hydrothermal alteration and mineralization of the alkaline anorogenic ring complexes of Nigeria

The Jurassic alkaline anorogenic granitic ring complexes in central Nigeria are discordant high level intrusions emplaced by piecemeal stoping through a collapsed central block. Biotite granites predominate over fayalite or amphibole bearing variants and are important for their ore deposits. A series of alteration processes, with related mineralization, can be recognized. Early high temperature sodic metasomatism may introduce niobium mineralization, can be recognized. Early high temperature sodic metasomatism may introduce beginning with potash metasomatism affect only the biotite granites. Accessory monazite, zircon, cassiterite, rutile, molybdenite and occasionally wolframite are associated with mica distribution. Subsequent acid metasomatism results in greisenization. The ore assemblage deposited at this stage is dominantly of oxides with early monazite, zircon and ilmenite followed by cassiterite—the major ore, wolframine, sometimes columbite or siderite and always rutile. During silica metasomatism early cassiterite is followed by abundant sphalerite, chalcopyrite, galena and occasionally arsenopyrite or pyrite. Chloritization and argillization are important but more restricted alteration processes. Fluid inclusion studies indicate that there was an initial separation of dense saline brine from the residual melt followed by vapour phase separation, to evolve a low density fluid. Fluids responsible for early metasomatic alteration and pegmatide development are highly saline and may contain glassy fluorosilicate phases. Homogenization temperatures are between 380 and 550°C. Later fissure-filling veins form in the range 300–380°C at salinites up to 15 eq. wt% NaCl, with the latest mineralized quartz veins in the range 200–300°C and salinities <10 eq. wt% NaCl. In addition, low temperature, monophase inclusions occur which are probably associated with late stage argillic alteration. Superimposed on this general pattern of decreasing temperature and salinity are important stages of immiscible phase separation related to CO 2 loss and local increase of salinity due to boiling as pressure is released. Mineralization may take the form of: (i) late magmatic pegmatites, (ii) pervasive metasomatic disseminations, (iii) pre- and postjoint pegmatitic lenses, (iv) quartz rafts, stockworks, sheeted veins and altered wall rock, (v) fissure-filling veins, (vi) irregularly shaped replacement bodies, (vii) quart veins, (viii) ring-dykes, (ix) alluvial and eluvial deposits. The different processes of alteration and associated mineralization characterize different parts of a granite pluton: (i) the roof zone, (ii) marginal zone, (iii) contact zone, (iv) the country rock, (v) the ring-dyke zone.