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School of Earth & Environment FACULTY OF ENVIRONMENT

The Chemical Composition of Metasomatic Fluids in the Crust

Bruce Yardley

School of Earth and Environment, University of Leeds, UK

Written during the tenure of a Humboldt Award from the AvH Foundation Metasomatism: Some Basics

Classically, Korzhinski divided metasomatism into Infiltration Metasomatism and Diffusion Metasomatism, according to the inferred transport mechanism for dissolved components Implicitly, this talk is concerned almost entirely with Infiltration Metasomatism In Infiltration Metasomatism, some components are added or removed in solution (mobile components) while others are derived from the host rock and do not move (immobile components) The Korzhinski Phase Rule states that the number of phases in a metasomatic rock is related to the number of immobile components, not the total number of components: metasomatic rocks have fewer phases

Examples of flow systems that can trigger metasomatism

Fluid that is in equilibrium with one type of rock flows into a different rock type. Fluid moves through a uniform rock type to regions at different temperatures A fluid exsolves from a parent fluid or melt and moves independently

What sorts of fluids cause metasomatism?

In principle, any fluid that migrates into a rock volume where it is not at equilibrium will react with its new host until equilibrium is established. Whether or not this results in a significant change to the composition of the rock (i.e. causes metasomatism) will depend on the quantity of fluid and its dissolved load. •Concentrated fluids have the potential to cause more metasomatism than dilute ones. •Systems that experience large fluid fluxes are most likely to undergo metasomatism

Controls on Fluid Compositions

Buffered Fluids: Saturated solutions of the rock that hosts them Mass Limited Fluids (Houston et al., 2011, Marine & Petroleum Geology): Concentrated fluids whose dissolved load dominates the material available for exchange in the host rock, so that the buffering capacity of the host is exceeded, usually brines in highly porous rocks rich in quartz. Kinetically Limited Fluids: Fluids whose composition is dominated by fast-reacting minerals in the host but does not come to equilibrium with the full assemblage

The Dominant Metasomatic System through Earth History

Sea floor hydrothermal circulation In some sedimentary basins, including many oilfields, sandstones have been heavily metasomatised ; for example, very few North Sea sandstones contain intermediate plagioclase, there is albite and K-feldspar. The Dominant Sources of Metasomatic Fluid

In addition to seawater, brines that form during evaporation and sink into the sediment are a potent metasomatic fluid Deep-Penetrating Basinal Fluids

Basinal fluids commonly penetrate the crystalline rocks of their basement. In this example, from the Precambrian Modum complex of west Norway, the fluid inclusions which fluoresce in UV light are of hydrocarbons derived from the overlying Caledonian foreland basin and derived from the Alum Shale.

Munz et al., 1995, GCA, 2002, Geofluids Gleeson et al., 2003, Geofluids Deep-Penetrating Basinal Fluids An example from the Massif Central (Susannah Bruce, unpubl. PhD thesis): Fluid inclusions in veins from both Jurassic sediments of the cover and underlying Palaeozoic metasediments lie along the same evaporation trend for Br-Cl.

Igneous Fluids

A classic example of metasomatism in an igneous setting is the formation of greisen from granite, with the quartz-muscovite – rich greisen flanking a central vein with tourmaline.

Cligga Head, Cornwall Skarns are one of the most important classes of metasomatic rocks and can be developed in an igneous or a purely metamorphic setting. Permeability and Fluid Flux

Metasomatism is dependent on focussed fluid flow, and this requires permeable pathways to be sustained for long enough for sufficient fluid flow to take place to produce metasomatic effects. •Permeability may be linked to fractures and these can be dynamically regenerated by tectonic or igneous activity. •Some sedimentary rocks may have an initial intrinsic permeability - likely to reduce as a result of metasomatic mineral growth. •There are mineral reactions that result in a reduction of solid volume, and hence can produce porosity if they proceed faster than the porosity collapses by creep. Large fluid fluxes most likely arise in near-hydrostatically pressured systems (convection, gravity-driven flow) because fluid is recycled. These can drive metasomatism with quite dilute fluids. Overpressured fluids from magma crystallisation or metamorphic reactions are most likely to cause metasomatism if they are concentrated solutions and released rapidly so that flow is focussed.

Rates of Fluid Production

Dehydration reactions are strongly endothermic – how fast they give off water depends on how fast they are heated, just like boiling water on a stove. But rocks are very slow to change temperature, and so even in contact metamorphism at mid-crustal depths, heating rates are likely less than 10 degrees per thousand years. Rates of Barrovian regional metamorphism appear to be around 8 – 15oC/Ma based on geochronology. Extensive dehydration will result in slower heating rates. This means that even narrow temperature windows, such as at the greenschist – amphibolite facies transition, will take 5 – 10 Ma to pass through. These times are very long compared to time scales of deformation, so long term coherent focussed flow is unlikely. Reaction-enhanced permeability Nearly pure diopside cores formed from dolomite + quartz, creating porosity which allowed overpressured water to infiltrate. This caused initial corrosion but then overgrowth by Fe-bearing diopside, with the Fe introduced in solution in the infiltrating fluid.

Yardley, 2009, Jl Geol Soc

The most common example is the development of regional metamorphic skarns due to reaction of silica-saturated water with dolomitic marble. Once initiated, these reactions create porosity and permeability and so become self-accelerating. What are metasomatic fluids like? How can we tell?

Theory: Many of the fundamental relationships between mineral stability and fluid chemistry were worked out many years ago through activity diagrams, although absolute concentrations, and even absolute activities were often unknown. Theory continues to be an essential tool to interpolate and extrapolate. Experiment: Hydrothermal experiments not only provide the fundamental data to quantify activity diagrams, they also provide absolute solubility data. Over the past 50 years it has become possible to perform relevant experiments over the entire range of crust and upper mantle conditions. Observations of natural systems: The only way we know what systems to study in the laboratory is by understanding what fluids occur in nature. This includes both studying modern crustal fluids sampled by drilling and investigating ancient fluids preserved in fluid inclusions. This is how we know that most crustal fluids are brines for example – something that makes thermodynamic modelling difficult! What are metasomatic fluids like? - Basics

Under crustal conditions, metasomatic fluids are dominated by water, salts (mainly NaCl) and non-polar fluid species (mainly CO2). The presence of salts reduces the solubility of species such as CO2 in water (“salting-out”) Salts also have some impact on silica solubility The composition of metasomatic rocks need have very little in common with the composition of the metasomatic fluid a) 500oC, 50 Mpa b) 800oC, 200 Mpa, c) 500oC, 500 MPa (after Liebscher, 2007)

At very high temperatures

Above the wet solidus for any lithology, water will dissolve into the melt. If a fluid phase coexists with melt above the wet solidus, it will have a water activity <1.

Both CO2-rich and salt rich fluids can fulfill this requirement, and both are known from granulites.

Getting water, salt and CO2 all into a single fluid phase is tougher, but happens under mantle conditions. Crustal Fluid Chemistry: the dissolved load

At low pressures, fluids are generally dominated by cation exchange equilibria between minerals and chloride salts. With increasing pressure, Al and Si become more important fluid components, but normally high mantle pressures must be reached before the dissolved load becomes more rock-like than salt-like. Exceptionally, large concentrations of other ligands such as F in low Ca evolved magmatic fluids can flux high “rock solubility” under crustal conditions. Quartz Solubility in Crustal Fluids

Most crustal fluids are quartz- saturated. Quartz solubility increases with both P and T but under crustal conditions seldom exceeds a few thousand ppm. The solute load is dominated by salts. At mantle pressures (>50km depth), quartz solubility increases and fluids can become more akin to rock solutions. Effects of chloride salts on quartz solubility, from Shmulovich et al., Geofluids, 2006. How salinity determines the metal contents of crustal fluids Plotting Fe against Cl for a wide range of fluids from oilfield waters to fluid inclusions from magmatic ore bodies, there is only a very general positive correlation. In detail however, this correlation is made up of an en-echelon array of parallel trends made up of data for fluids of different temperatures. A plot of the Fe/Cl ratio against 1/T clearly demonstrates that Fe becomes increasingly important with increased temperature. It reaches percent levels in some magmatic brines. Not surprisingly, hydrothermal Iron can be of sedimentary origin, requiring large fluxes of relatively dilute fluid, or form from magmatic fluids that are less abundant but more concentrated. Metamorphic fluids are too sparse and too cold.

1 -0.5 0 -1.5

-1 ) ) -2.5 -2

-3.5

-3

log mol (Zn/Cl log mol (Fe/Cl -4.5 -4

-5 -5.5

-6 -6.5 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 1/T (K) 1/T (K)

-0.5

-1.5 Data on natural samples demonstrates

that the crust is effectively able to ) -2.5 buffer the concentrations of many other

-3.5 transition elements in fluids, relative to

log mol (Mn/Cl chloride. Men, Fe and Zn have the -4.5 most complete data sets; Pb is almost -5.5 certainly comparable

-6.5 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 Yardley 2005, Economic Geology 1/T (K) How Cl influences pH and hence metal-carrying capacity

The stability limits of muscovite 6.5 relative to K-feldspar or kaolinite

6 provide useful brackets for the range of pH likely to be 5.5 encountered in the majority of crustal rocks. This plot shows that

H 5 p muscovite is stable over a range of less than 1.5 pH units at any given 4.5 kaol + musc + pg + qz chlorinity, while the total range of pH values likely to be encountered 4 between highly saline fluids

3.5 equilibrated with kaolinite and very -1 -0.5 0 0.5 dilute fluids equilibrated with K- log Cl (mol) feldspar, is around 2.5 pH units.

Yardley 2005, Economic Geology Ca-Na relationships in brines – not a simple story

Ca-Na relationships are not simply buffered even for a fluid in equilibrium with Ca- and Na-bearing minerals because the valency differences mean that, all else being equal, more saline fluids will have higher

XCa . This is because the equilibrium constant will contain the term: aCa/aNa2

The spread in XCa is much larger in sedimentary formation waters than in metamorphic or igneous fluids because many of these brines are mass limited Metasomatic Fluid Composition – Generalisations for buffered fluids

Si increases with T and P but is significantly lowered for CO2-rich fluids. Al is less well understood but is more soluble than has been thought and solubility also increases with T and P. Many elements have quite different relative concentrations in chloride fluids from what is seen in rocks. Mg is much less soluble relative to Fe, while Mn is enriched, especially under more oxidising conditions. Absolute concentrations of transition metals relative to Cl increase with T K/Na ratio increases with T Hotter fluids have more exchangeable protons than lower-T fluids – hence they can cause acid leaching in cooler rocks of the same composition as their hotter source.

How Saline are Prograde Fluids? Fluid salinity shows a continuum from sedimentary to metamorphic environments – even deep metamorphic fluids inherit salinity from the sedimentary precursors.

Metamorphic and sedimentary fluids from oceanic and Metamorphic and sedimentary fluids from continental accretionary settings margin settings Yardley & Graham, Geofluids, 2002 The origins of chlorinity in crustal brines Given that metamorphic fluids evolve from formation waters, we can likewise look at them together when discussing where Cl comes from.

Halogen ratios provide a ready indication of the involvement of halite in the formation of brines, and provide a clear means of distinguishing between brines formed by halite dissolution (Br- poor) and residual bittern brines from which halite has been extracted (Br-rich) Metasomatic potential of different brine types

Brines which are redissolved evaporites plot below the alkali- feldspar equilibrium curve and have the potential to cause Na- metasomatism, but bittern brines are K-rich and will drive K- metasomatism Oilfield formation waters which acquired their salinity by dissolving halite beds are high in Na, Cl, low in Ca, Br. The residual bittern brines from which halite precipitated are high in Mg, which is then exchanged for Ca during dolomitisation, and in Br. The composition of brines in sediment is hard to change because they dominate the budget for many elements in a reasonably porous rock (mass-limited).

In part b) we can see that low grade metamorphic brines (associated with emeralds in this case) show evidence of significant interaction with wall rocks (enhanced Ca), even though they still have the distinctive low Br due to halite dissolution. To achieve these compositions, the fluids must have caused Na-metasomatism while leaching Ca. Magmatic Fluids

Magmatic fluid - fluid that exsolves from a melt as it crystallises – is released in relatively small amounts and cannot be recycled, but it may be strongly focussed to the top of an intrusion to give large local fluxes. Furthermore it may be rich in Cl and other distinctive components.

Magmatic Fluids – how do they compare?

In some ways, magmatic fluids are just a logical extension of other crustal fluids, because some features of fluid composition reflect temperature as we have seen earlier: Content of Si increases with T Fe/Cl, Mn/Cl, Zn/Cl increase with T K/Na notably increases with T But there can be differences: Evolved acid magmas that crystallise Na-rich plagioclase yield very low Ca fluids. This permits higher levels of ligands such as F,

PO4 which would otherwise form Ca-minerals. Vapour phase separation at shallow levels leads to a complex range of fluids to evolve, including S-rich fluids.

Mantle Fluids and Fenites

Fenites are silica deficient and have assemblages indicative of high pH – unlike greisens they make no sense in terms of a cooling granite-derived fluid.

Mantle fluids (in the sense of low-density C-O-H fluids) are very poorly understand, but inclusions in diamonds suggest they are very rich in both Cl-salts and CO2.

It is possible that loss of CO2 with decreasing pressure drives the fluids to being silica-undersaturated, despite some cooling. Partitioning of HCl into the separating gas phase will likewise drive the remaining brine to high pH

Conclusions

Any crustal fluid that is constrained to interact with a rock mass with which it is not in equilibrium is likely to cause metasomatism. Nevertheless, metasomatism is a rare event in the evolution of any rock mass. Whether or not metasomatism takes place depends on both the dissolved load and the fluid flux. Release of overpressured fluids of low salinity has little metasomatic potential, whereas recirculation of brines is very likely to result in metasomatism. It is quite possible to make predictions about the metasomatism likely to result from movement of fluid though specific changing conditions if it can be assumed to have been a buffered fluid at the outset, but for many of the most saline sedimentary basin brines this is not the case. Basin brines could cause Na or K metasomatism, regardless of the sense of temperature change, depending on their origin.

Conclusions

CO2 unmixes from brine.