Download PERSPECTIVES ON METAMORPHIC FLUIDS

Comments on ABT’s perspectives by Jamie Connolly and Haakon Austrheim:
Elements v. x, issue y, month 2009 (171209)
PERSPECTIVES
FUTURE RESEARCH PERSPECTIVES ON METAMORPHIC FLUIDS
Alan Bruce Thompson*, Professor, ETH Zurich
*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.
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 such magmas, and they
transport large quantities of dissolved materials in selected geological environments.
Metamorphic fluids, mainly H2O and CO2, are the products of prograde
metamorphism released through devolatilisation reactions in response to an elevated heat
supply beneath the continents. On the other hand metamorphic fluids are the cause of
retrograde metamorphism on the ocean floor and in the continental crust, when drier hot
rocks become hydrated and carbonated next to fractures and in shear zones. Sometimes
[this “sometimes” is a bit misleading, for fluids generated by dehydration and
decarbonation, reduced carbon species {CO, CH4} are insignificant, whereas S and N are
present almost invariably as reduced species {NH3, H2S, S2}] reduced carbon, nitrogen
and sulfur gases are the stable species in metamorphic fluids, usually when
graphite/diamond is present.
It is possible to distinguish on the basis of stable isotopes, metamorphic processes
involving fluids that have equilibrated with the atmosphere (meteoric, as in weathering)
from fluids that through fluid-rock interaction have lost former meteoric signatures. Other
metamorphic fluids have signatures of equilibration with higher-grade metamorphic,
mantle, or magmatic mineral assemblages. Elemental changes during metamorphism
have been quantified from studies of mass-balance between mineral assemblages
involved in specific rock transformation. Related isotope studies indicate that retrograde
fluid flow is usually focussed in regions of structural weakness rather than pervasive
through all rock types.
The heat balance required to induce a particular metamorphic transformation has
been determined from comparison of heat of specific metamorphic reactions compared to
likely sources of heat (e.g. a selected magmatic intrusion) and mechanisms of heat
transfer (conduction versus advection, e.g., Bickle and McKenzie 1987, Brady 1988). It
has been possible to quantify several aspects of orogenic processes (tectonically related
metamorphism, magmatism and deformation) particularly the relative thermal roles of
crust and mantle in the various metamorphic processes. Further progress is to be expected
using deduced energy balance in large-scale studies of plate tectonics, as in detailed
aspects referred to in the papers in this volume. The length-scales and time-scales of the
various metamorphic processes, fluid present to fluid absent, are significantly different
and it is study of these variables that will provide the most important future inputs.
Time-scales for prograde metamorphic heating, regional and contact, 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  x time (where  ~10>-6 m2 s-1). This
directly controls the time-scales for fluid production. Length-scales for fluid production
[it is not clear what is meant by the length scale for fluid production] depend upon to
what extent the fluid flow is focused for a given amount of fluid released by the
devolatilisation reactions [focusing has little relevance to fluid production, but it does
control the efficacy of advective heat and mass transport by fluids]. Focussing factors can
be scaled to areas of porous media flow compared to a fracture, to obtain scaled fluxes.
These factors can be quantified in forward models of metamorphism but are not easily
recoverable from inverse geological observations. This reflects mainly the difficulty of
obtaining three dimensional field coverage of actual metamorphic fluid channelways, and
of knowing how much of specific fluid flow indicators belong to the particular episode 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. Mass-of-fluid fluxes so obtained help
determine transport processes, and estimates of their magnitude and duration. Ion
microprobes (eg, Gordon et al. 2009) are providing necessary isotopic data relevant to the
age, duration and intensity of fluid transport processes. A vital aspect still to be resolved
is how efficient is fluid recirculation within the lithosphere, or to what extent is fluid
migration single- rather than multiple-pass (e.g., Walther and Wood 1986)?
Fluids go where they can travel fast [oddly this is not a bad generalization, but it’s
technically not true, in Darcyian flow fluids flow toward regions of low pieziometric
head], but become rapidly immobilised [they are consumed] where they can make
volatilised minerals, or enter magmas. Fluids will tend to follow existing
lithological/structural heterogeneities as it is more difficult to make faults from scratch in
homogeneous media [this is a chicken and egg story, that fluids are localized by
structures is the seismological consensus view and is certainly well justified in the
seismogenic crust, it becomes harder to explain in high-T regimes where compaction is
efficient]. 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.
Some perspectives on other areas requiring attention:
1) mechanisms of proton weakening of mineral atomic bonding,
2) 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),
3) mechanisms of transition from porous media flow to focussed flow in fractures or
shear zones,
4) depth of phase separation and relation to chemistry of fluids in ocean floor and landbased hydrothermal systems,
5) ways of determining the relative roles in sulfur and chlorine in combined volatile
systems, (e.g., Seward and Barnes (1997),
6) the relative importance of oxidised versus reduced species of C, S and N in magmatic
and hydrothermal processes, migrating along natural gradients in pressure (driving fluid
motion) and temperature (away from natural heat sources),
7) 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 concentration of particular elements in aqueous or carbonic fluids,
8) mechanisms of access of H2O and CO2 from subducted slab into mantle wedge
magmas, then from magma into the hydrothermal system,
9) subducted flux of volatiles into deep earth compared to primordial volatiles,
how far do subducted volatiles penetrate the whole mantle? Do mantle plumes catch
subducted volatiles (more H2O than CO2?) or tap primordial volatiles (more CO2 than
H2O?),
10) do metamorphic fluids ever have catastrophic impact upon crustal heat transport or
release of volatiles to crust or atmosphere, or are such examples seldom?
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
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
Comments by Austrheim:
Comments on “Future research perspectives on metamorphic fluid”
A,B.Thompson
I had expected a bit more “Fire-work” and a slightly different approach dealing more
with metamorphism in general not only the fluid aspect.
I miss a short intro to the highlights, new trends and methods in recent research in
metamorphic petrology, including geodynamic modelling, geochronology, etc. Such an
intro is naturally followed with an evaluation of what is prosperous in present day
research and what are clear detours, ending with advices on how to proceed. The need
for such advice and correction are obvious and can be illustrated with a glance at UHP
research, the apparently very successful branch of metamorphic petrology of today. The
symposium volume, 25 years of research in UHP, now online in EJM contains three
papers dealing with the UHP of WGR. From the same region one paper suggest
superdeep subduction, the second call for overpressure of several Gpa and the third claim
extensive metastability and that only a small fraction of the WGR was reacted during
subduction. At the same time the geodynamic models for collision zones which rely on
these conflicting results becomes ever more colourful and psychedelic.
How do we end up in the mess and how can we rescue metamorphic petrology as a
science and bring young researcher back on track? This is where I feel your advice will
be most valuable.