Elements v. x, issue y, month 2010 (corrected 150310)
PERSPECTIVES
SOME FUTURE RESEARCH PERSPECTIVES ON
METAMORPHIC PROCESSES AND METAMORPHIC FLUIDS
Alan Bruce Thompson*, ETH Zurich
Metamorphic aqueous fluids makes things happen inside the Earth: they
considerably speed up heat and mass transfer, they induce weakness and
instabilities in rock masses, they are instrumental in localising deformation enabling
tectonic response to plate motion, they markedly lower the melting temperature of
silicate rocks, they noticeably lower the viscosities of silicate magmas, and they
transport large quantities of dissolved materials in selected geological environments.
In the future we need to determine what makes deep crustal fluids go where they go
– in what quantity, with what speed? with what chemical effect? and with what
consequence for mineralization? for rock deformation?
Metamorphic petrology has developed much in the last twenty years. From
laboratory and thermodynamic calibrations, the mineral assemblages in metamorphic
rocks can be used to determine pressures (P) and temperatures (T) of equilibration. Some
rocks containing chemically zoned minerals, or overgrown mineral assemblages, can
reveal segments of PTt(time) paths of the rocks burial/exhumation history. These PTt
paths can be related to structural features and tectonic indicators to estimate geodynamic
evolution in distinct plate tectonic settings. Convergent plate settings include mountain
building (orogenesis) above subduction zones or upon continental collision. Divergent
plate settings include rifting in arcs, continental interiors or at mid ocean ridges. Careful
comparison of deduced PTt paths with rheologically constrained geodynamic models is a
developing field in each tectonic setting. This rich legacy of petrological methods is
central to our understanding of plate-scale tectonic processes. The same methods will be
vital to understanding nature when obtaining riches (fluids, minerals, heat) from the
Earth’s crust There are now good criteria for indicating when such resources can be
obtained and when they cannot.
Heat sources for crustal metamorphism
Metamorphism, the mineralogical transformation of rocks, occurs mainly by
transposing rocks closer to a heat source - either by burial to depth where temperatures
are higher, or next to a magmatic intrusion, or by concentrating radiogenic heat sources,
or by locally converting work to heat during deformation. The latter two can refer to heat
generation within the crust, the former two reflect the mantle as the ultimate heat source
for crustal metamorphism. From heat balance considerations we are beginning to
understand the extent of involvement of mantle heat in crustal metamorphism (oceanic
and continental). Quantified heat balance required to induce particular metamorphic
transformations has been determined from comparison of measured heat of specific
metamorphic reactions compared to likely natural sources of heat (e.g. a selected
magmatic intrusion) in selected terrains.
Multiepisodic metamorphism of the lower crust
As rock units from near the surface become tectonically buried, they transform
their mineral assemblages in response to higher temperatures in the new surroundings at
depth. Such metamorphic rock units are later returned to the surface by subsequent
processes of tectonic exhumation. In many cases burial and exhumation do not occur in
the same tectonic episode, sometimes being separated by tens of millions of years (10’s
Ma). While multiplicity is suggested for many episodes of crustal tectonics, there is a sad
paucity of geochronological data with which to evaluate the geodynamic evolution of
most orogenic zones. Rates of burial/exhumation can proceed at plate velocities (1mm to
10 cm yr-1) or at much slower erosional velocities (0.01 to 0.5 mmyr-1). When correlated
with available radiogenic geochronology it is clear that regional deformation proceeds in
spurts of activity with pauses in between (episodic, punctuated). A region may suffer
migrating orogenesis over 100’s of Ma, but individual episodes of tectonic activity within
may be on a Ma timescale. The lack of age data (absolute or relative) means that we
cannot say under which circumstances tectonic motion is continuous and controlled by
large scale remote forces, and when it is perturbed by local adjustment to the changes in
P (or depth), T, stress or fluid influx. We must consider multiple sequential separate
episodes of deep crustal rock heating (which become recorded by new metamorphic
minerals) and related fluid release. Slow fluid migration results in long-term mid crustal
fluid storage. Stored metamorphic fluids become involved in regional (exhumation) or
local (faulting, earthquakes) deformation only in a much later tectonic episode.
Types of metamorphic fluids
Deep metamorphic fluids are released beneath the continents through
devolatilization reactions in response to tectonic transport towards a heat supply
(products of prograde metamorphism). On the other hand metamorphic fluids on the
ocean floor and in the continental crust are the cause of retrograde metamorphism, when
drier hot rocks become infiltrated (making hydrated and carbonated minerals) next to
fractures and in shear zones. Fluid production and fluid migration are very irregular
events in the deeper Earth’s crust. Competition among the rates of these various
processes generates the diversity seen in the patterns of microtexture and mineralogy in
metamorphic rocks. We must never forget that exhumed deep crustal metamorphic rocks
contain unstable mineral assemblages, which are very sensitive to destruction by fluids,
especially where access is aided by penetrative deformation. We will discuss next the fate
and effects of the fluids generated during metamorphism (more than rocks left behind).
We consider that fluid-rock interaction at any PT involves “metamorphic” fluids,
including shallow metamorphic fluids that have equilibrated with the atmosphere
(meteoric, as in weathering). Stable isotope studies permit distinction of the loss of
former meteoric signatures during fluid-rock interaction. Much of our understanding of
redox reactions (and corrosion/erosion) between groundwaters, minerals and metals is
being gained by geochemical observations - very important for construction, mineral
processing, and deep engineering.
Some metamorphic fluids, trapped in fluid inclusions, have geochemical
signatures of equilibration with higher-grade metamorphic, mantle, or magmatic mineral
assemblages. Mineral assemblages that have not been reset by retrograde reactions in the
upper crust, indicate that fluid infiltration is rarely pervasive but rather focussed in spaced
channelways. Related isotope studies indicate that retrograde fluid flow is usually
focussed in regions of internal structural weakness, or in zones of external tectonic
disturbance (rather than pervasively through all rock types). Particular elemental changes
during metamorphism have been quantified from studies of mass-balance between
mineral assemblages involved in specific rock transformation. These studies show which
elements are sensitive to fluid migration near crystalline rock reservoirs and repositories.
Ion microprobes able to analyse one micron sized spots in rock slices (eg, Gordon et al.
2009), are now providing necessary isotopic data relevant to the age, duration and
intensity of fluid transport processes.
Chemically concentrated metamorphic fluids
Heating of buried rock units causes the volatile species in minerals (OH-, CO32-,
2H2O, S , SO42-, NH4-,) to be released as a metamorphic fluid phase. Metamorphic fluids
are predominantly H2O, and CO2 is the prevalent carbon species (CO, CH4 are
insignificant). CH4 is an important component of some ocean floor systems, indicating
biological input or Fe3+ - Fe2+ redox control. Reduced nitrogen and sulfur gases (NH3,
H2S, S2) are the stable species in metamorphic fluids when graphite or diamond is
present. The fluids contain minor amounts of charged species, including Cl-, that bind
with metal and other cations. Such fluids can become quite concentrated solutions at
depth (e.g., Newton and Manning, 2010) especially close to the temperatures where
crustal rocks melt (e.g., Hack et al, 2007). New work on understanding how metal and
silicate solubility changes in natural fluids in the PTX(composition) gradients of the
Earth, will help to understand the evolution of natural chemical concentration processes
needed for mineral deposits.
Timescales of metamorphism and fluid migration mechanisms
Time-scales for prograde regional and contact metamorphic heating, are mainly
determined by heat conduction from a finite heat source (distance to transposed
asthenospheric mantle or to a magma body). The length- to time-scaling for solid heat
conduction is as length squared proportional to к x time (where к ~10-6 m2 s-1). This
proportionality directly controls the time-scales for fluid production. The rate at which
minerals devolatilize is thus dependent upon the heat supply. In higher temperature
regimes, natural compaction or shearing tend to govern fluid expulsion. Length-scales for
fluid migration depend upon to what extent the fluid flow is focused for a given amount
of fluid released by the devolatilization reactions. Focussing factors can be scaled to areas
of porous media flow compared to a fracture, to obtain scaled fluxes.
The lower density of metamorphic fluid (H2O, ca. 1 g cm-3) compared to rock (ca.
3 g cm-3) means that fluids tend to migrate upwards when they can. The rate of which
metamorphic fluids migrate depends on whether the flow is dispersed or focused. Porous
flow along grain boundaries is more typical for hotter (or more ductile) rheological
behaviour, whereas fracture flow is expected in the brittle rock rheology PT-range.
Hydrofracturing in response to rapid fluid release and fast strain rate deformation may
occur in rocks even in the nominally ductile PT-range.
The diverse factors controlling fluid flow can be quantified in forward models of
metamorphism but are not easily recoverable from geological observations - which are
inverse or “backwards oriented”. This reflects mainly the difficulty of obtaining three
dimensional field coverage of actual metamorphic fluid channelways even from a mined
ore deposit. It is not simple to know how much of specific fluid flow indicators belong to
a particular episode which is to be identified with a given heat source. To attempt to
quantify crustal-scale fluid migration thus requires new geochemical work using ratios of
elements characteristic of the various fluid/melt/mineral partition characteristics. Massof-fluid fluxes so obtained with ion microprobes, help determine the real details of
natural transport processes, and estimates of their magnitude and duration A vital aspect
still to be resolved on all scales is how efficient is fluid recirculation within the
lithosphere? In other words, when is fluid migration single- rather than multiple-pass
(only upwards advection or more circular as in convection, e.g., Walther and Wood
1986)?
Exhumation of metamorphic rocks
Many crystalline rocks metamorphosed at depth stay there constituting the lower
continental crust, if not exhumed by upwards oriented deformation processes. These
rocks retain their higher grade metamorphic mineral assemblages and segregated ore
deposits at depths inaccessible to our mining. Fuller understanding of the tectonic engines
which lead to the exhumation of slices of metamorphosed lower crust, would certainly
aid our prospecting initiatives in future years.
Fluids go where they can travel fastest for a given pressure gradient, but become
consumed where they can make volatilised minerals or hydrous melts, or enter magmas.
In brittle rocks, fluids will tend to follow existing lithological/structural heterogeneities
(as it is more difficult to make faults from scratch in homogeneous media). Thereafter the
first heterogeneities tend to be continually reactivated and therefore localised in same
place. Thus we would expect much continuous overprinting of isotopic signature,
requiring careful separation of samples before measurement. Detailing the processes that
lead to focussed rock weakening is also fundamental to understanding how deep
metamorphic rocks become exhumed. Focused rock weakening is a vital part of the
metamorphic cycle for mineralisation and for exhumation, and presumably involves the
fluids released by deeper metamorphic reactions.
Heat advection by metamorphic fluid
The predominant mechanism of heat transfer during regional metamorphism is by
conduction through minerals in rocks rather than by fluid advection through the porosity,
simply because deep crustal fluid flow is naturally restricted. Fluid flow exceeding 10 -10
m3m-2s-1 is needed to permit any heat advection (eg. Bickle and McKenzie, 1987) and
would be very rare in metamorphic settings (e.g., Brady 1988). Natural advective heating
by rising fluids would be a faster mode of heat transfer than by conduction, but is
restricted both in space and time. It is restricted geologically to natural convection
(buoyant upflow and downflow) in hydrothermal - geothermal systems clearly related to
magma intrusion at quite shallow depths.
More themes for future research
With the chemical and mineralogical background of metamorphic petrology well
established, the next decade will see advances in scaled up (macroscopic) geophysical
interpretations and field considerations, from regional to outcrop or hand-specimen scale.
Scaling down, several detailed regions of microscopic chemical and isotopic study will
be related to microstructures to establish diagnostics for time-dependent stress relaxation
and mass transport. I have listed, some other areas of fluid-rock interaction research that I
would certainly like to pursue, in decreasing scale of observation:1) quantifying the subducted flux of volatiles into deep earth compared to deep mineral
storage of primordial volatiles,
2) mechanisms of access of H2O and CO2 from subducted slab into mantle wedge
magmas, then from fractionated magma into the near -surface hydrothermal system,
3) specifics of role of water access in large-scale rock weakening leading to major
tectonic processes (e.g., earthquake sources, crustal scale shear zones, thinning of
lithosphere, promotion of subduction, aiding of magma intrusion).
4) mechanisms of transition from porous media flow in fluid source regions, to focussed
flow in fractures or shear zones,
5) role of growth of hydrous/carbonate minerals in promoting rock failure. Such study is
related to the CO2 -sequestration problem (see Elements v. 4, issue 5, October 2008),
6) the relative importance of oxidised versus reduced species of C, S and N in magmatic
and hydrothermal processes in different fO2 ranges, migrating along natural gradients in
pressure (driving fluid motion) and temperature (away from natural heat sources),
7) when do metamorphic fluids have catastrophic impact upon crustal heat transport or
release of volatiles to crust (causing deep earthquakes?) or atmosphere (changing its
composition?), or are such examples seldom?
8) depth of phase separation and relation to chemistry of fluids in ocean floor and landbased hydrothermal systems,
9) actual depth-temperature paths around natural critical points in liquid-vapour systems
in hydrothermal systems and in fluid-melt systems (e.g., in subduction zones, Hack et al,
2007). Such study is also central to many chemical engineering projects for selective
extraction or industrial concentration of particular elements in aqueous or carbonic fluids,
10) development of robotics in deep mining (which like space travel is not suited to the
human body),
11) ways of determining the relative roles of sulfur and chlorine for metal transport in
combined volatile systems, e.g., Seward and Barnes (1997),
12) mechanisms of proton weakening of mineral atomic bonding.
I am grateful to James Connolly, Haakon Austrheim, Bjorn Jamtveit and Susan Stipp for
comments.
REFERENCES
Bickle MJ and McKenzie DP (1987)
The transport of heat and matter by fluids during metamorphism
Contributions to Mineralogy and Petrology 95:384-392
Brady J B (1988)
The role of volatiles in the thermal history of metamorphic terranes
Journal of Petrology 29: 1187-1213
Gordon SM, Grove M, Whitney DL, Schmitt AK and Teyssier C (2009)
Fluid-rock interaction in orogenic crust tracked by zircon depth profiling
Geology 37: 735-738
Hack AC, Thompson AB and Aerts M (2007)
Phase Relations Involving Hydrous Silicate Melts, Aqueous Fluids, and Minerals
Reviews in Mineralogy and Geochemistry 65: 129-185
Newton, RC, and Manning, CE (2010) Role of saline fluids in deep-crustal and uppermantle metasomatism: insights from experimental studies. Geofluids 9: 1–15
Seward, TM and Barnes, HL (1997) Metal Transport by Hydrothermal Ore Fluids. In:
Barnes, HL (ed) Geochemistry of hydrothermal ore deposits, Wiley New York, Chapter
9: 972 pp.
Wood B J and Walther J V (1986) Fluid flow during metamorphism and its implications
for fluid-rock ratios. In: Walther, J. V. & Wood, B. J. (eds) Fluid-Rock Interactions
during Metamorphism, Springer, New York: 89–108
Footnote: *Alan B. Thompson is Professor of Petrology at ETH Zurich and at the
University of Zurich, Switzerland. His research considers physical and chemical
evolution of the Earths lithosphere, particularly the role of aqueous fluid and magma
evolution in mass and heat transfer and tectonic processes.