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Fluid flow
Chinner further suggested that in contact metamorphic aureoles it was more common
for the fluid phase to overwhelm buffering capacity of mineral assemblages.
Fluid flow in aureoles remains topical because of uncertainty about direction
of fluid flow (Ferry and co-authors, Contributions to Mineralogy and Petrology,
v. 142, p. 679-699). Also uncertain is whether the commonly assumed uniform
and unidirectional fluid flow is correct or whether fluid flow depends strongly
on time and space (Cui and co-authors, Geological Society of America Bulletin,
v. 114, p. 869-882).
In an innovative paper, Boswell A. Wing and John M. Ferry (Geology, v.
30, p. 639-642) argued that first-order control on gross geometry of peak metamorphic
fluid flow during medium-pressure metamorphism is regional structure, and that
cross-layer transport is secondary to terrane-scale fluid flow in driving prograde
decarbonation reactions. Of particular importance are timing of fluid flow and
source(s) of fluid. By studying oxygen-18 and Y zoning in garnet, A. Skelton
and co-authors (Journal of Metamorphic Geology, v. 20, p. 457-466) argued for
two pulses of metamorphic fluid flow, the first during deformation and a second
following deformation. They suggested that fluid was sourced in neighboring
calcareous pelites, thus requiring cross-layer transport.
Dehydration of hydrothermally altered oceanic crust at the top of subducting
lithosphere is an important source of fluids. This fluid may be tracked because
it will add silica and alkalis to metasedimentary rocks of an overlying accretionary
prism. Such an example has been documented from the Otago Schist, New Zealand,
by Christopher M. Breeding and Jay J. Ague (Geology, v. 30, p. 499-502),
where increasing metamorphic grade from prehnite-pumpellyite facies to greenschist
facies is accompanied by 10 to 30 percent volume quartz veins. These authors
conclude that transfer of silica from subducting slabs into accretionary prisms
is a plausible mechanism to increase silica content of continental crust beyond
that generated by magmatic differentiation in arcs.
Fluids in UHPM
Concentration of energy at the extremes of metamorphism has led to interesting
discoveries concerning fluids attending UHPM. For example, Yilin Xiao and co-authors
(Journal of Petrology, v. 43, p. 1505-1527) have documented a difference
in fluid and metamorphic evolution between the South Dabie Terrane and the North
Dabie Complex, China. Fluid inclusions in rocks from South Dabie are mainly
aqueous with varying salinities in rocks with low oxygen-18 values, indicating
meteoric (atmospheric, not metamorphic) water-rock interactions before UHPM.
In contrast, rocks from North Dabie have "normal" oxygen-18 values
that preclude meteoric water-rock interaction. This conclusion is consistent
with excellent results from seismic reflection profiling across the Dabie Shan
(Xue-Cheng and co-authors, Geology, v. 31, p. 435-438), in which South
Dabie is clearly identified as the subducted unit, whereas North Dabie represents
the facing plate.
In a spectacular example of the role of "water" in deformation of
minerals under UHPM, Wen Su and co-authors (Geology, v. 30, p. 611-614)
report the presence of hydrous species (clusters of water molecules and hydroxyl
ions) in elongate garnet from continental crust. Apparently, hydrolytic weakening
enabled deformation of garnet, a feature that suggests that subducted continental
crust may carry water into the mantle.
Fluids in granulite metamorphism
One controversy in granulite facies metamorphism concerns whether the "dry"
mineral assemblages are residue after extraction of melt or whether such assemblages
are consequent upon infiltration metasomatism by carbonic fluids. Unfortunately,
both models predict a preponderance of carbonic fluid inclusions associated
with peak metamorphic conditions. Writing in the May 2002 Journal of Petrology
(v. 43, p. 769-799), Daniel E. Harlov and Hans-Jürgen Förster suggest
an alternative to melting for producing the anhydrous (pyroxene-bearing) mineral
assemblages in rocks of the granulite facies (high-temperature crustal metamorphism).
They argue that fluid metasomatism - mass transfer by advective fluid flow resulting
in change in bulk-rock composition - can explain grain boundary microstructures
in mafic granulites of the Ivrea-Verbano Zone in Italy. These authors argue
for propagation of a fluid front, with an initial fluid dominated by carbon
dioxide becoming diluted with water as it moves up the rock column. However,
the evidence Harlov and Förster adduce in favor of metasomatism potentially
can be explained by a small amount of partial melting (a few percent by volume),
consistent with more extensive melting (20 to 40 percent by volume) of metasedimentary
rocks to create the associated stronalites (melt-depleted granulites, named
after their occurrence in Val Strona). Very high-density carbonic fluid inclusions
captured during maximum pressure-temperature conditions were identified from
ultra-high temperature metamorphic — UHTM: metamorphism of crustal rocks
at temperatures greater than 900 C {emdash} terranes in Antarctica's Napier
Complex (Toshiaki Tsunogae and co-authors, Contributions to Mineralogy and
Petrology, 2002, v. 143, p. 279-299); southern India (M. Santosh and Toshiaki
Tsunogae, Journal of Geology, 2003, v. 111, p. 1-16); and the Eastern
Ghats (Sushmita Sarkar and co-authors, Geology, January 2003, v. 31,
p. 51-54). It is unambiguous that the very high-density carbon dioxide fluid
inclusions provide a strong case for the presence of carbon dioxide-rich fluids
during ultra-high temperature metamorphism, but it remains unclear whether this
outcome requires carbon dioxide infiltration.
Melting
One issue in all experimental petrology involving natural samples at extreme
metamorphic conditions is the question of what constitutes an appropriate starting
material. Rajeev Nair and Thomas Chacko (Journal of Petrology, v. 43,
p. 2121-2142) argue it is important to select natural samples from granulite
facies rocks immediately below the solidus in order that the phase compositions
correctly represent those stable prior to melting. Under these circumstances,
melting due to biotite breakdown in rocks of semi-pelitic composition is delayed
until approximately 875 degrees Celsius, due to higher titanium and fluorine
in biotite in the starting materials, which stabilize the materials to higher
temperatures. These authors suggest that temperatures of at least 875 to 1025
degrees Celsius are required to stabilize orthopyroxene under volatile phase
absent conditions at lower-crustal depths, which is consistent with recently
recalibrated temperature estimates of granulite facies metamorphism by David
R. Pattison and co-authors (Journal of Petrology, v. 44, p. 867-900).
Melting and the distribution of residual melt remain topical. Examples include
controls on distribution of small amounts of melt (a few percent by volume)
in contact aureoles (Nathalie Marchildon and Michael Brown, Journal of Metamorphic
Geology, May 2002, v. 20, p. 381-396); in granulites by Souad Guernina and
Edward W. Sawyer (Journal of Metamorphic Geology, February 2003, v. 21,
p. 181-201); and in experiments by C.W. Holyoke and Tracy Rushmer (Journal
of Metamorphic Geology, June 2002, v. 20, p. 493-512). The distribution
of melt is important, because it relates to retrogression by reaction between
peritectic phases formed at the metamorphic peak and melt during cooling (Richard
W. White and Roger Powell, Journal of Metamorphic Geology, September
2002, v. 20, p. 621-632). Such a process may be responsible for limited occurrence
of UHTM mineral assemblages within "common" (apparent peak metamorphic
temperature below 900 degrees Celsius) granulite terranes (Renato Moraes and
co-authors, Journal of Petrology, September 2002, v. 43, p. 1673-1705).
Also, studies of "melt" inclusions, in which the melt is represented
by an appropriate crystallized mineral assemblage, are likely to be a more important
part of future research (see figure).
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