Constraints on the petrologic structure of the subduction zone
slab-mantle interface from Franciscan Complex exotic
ultramafic blocks
Robert L. King†
Matthew J. Kohn‡
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA
John M. Eiler§
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
ABSTRACT
Ultramafic blocks within mud-matrix
mélange of the Franciscan Complex, California, preserve a series of metasomatic
mineral zones generated by infiltration of
Si-rich hydrous fluids during subduction.
We describe the petrology and geochemistry of the metasomatic zones and compare
them to current model predictions for the
metasomatism of the mantle wedge by subduction zone fluids. Fluid flow affected a
Cr-spinel lherzolite protolith to form first
serpentinite, then a talc-dominated rock,
and finally an amphibole-rich assemblage.
A diverse suite of accessory minerals in the
amphibole-rich zone (titanite 1 clinozoisite
1 zircon 1 apatite) suggests that the trace
element signature of subduction zone fluids
may be fractionated in this zone. Oxygen
isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic
fluids during serpentinization and that subsequent reactions occurred in equilibrium
with these fluids in a temperature range of
450–500 8C. Whole-rock geochemistry indicates mobility of many elements into and
out of the blocks during metasomatism, including elements such as Ti which are currently considered to have low solubilities in
such fluids.
Taken as a whole, the blocks appear to
preserve the metasomatic structure of the
†
Present address: Department of Earth and Environmental Sciences, Lehigh University, 31 Williams
Drive, Bethlehem, Pennsylvania 18015, USA; email: [email protected].
‡
E-mail: [email protected].
§
E-mail: [email protected]
slab-mantle interface in subduction zones
and imply that the chemistry of slab-derived
fluids is modified as they pass through these
metasomatic zones in the mantle wedge.
Our results suggest that the primary composition of subduction zone fluids is not
likely reflected by arc magmas. Instead, we
propose that arc magmas are derived from
regions of the mantle fluxed by fluids residual to the metasomatic processes we
observe.
Keywords: fluids, Franciscan Complex,
mantle, metasomatism, subduction zones,
ultramafic rocks.
INTRODUCTION
Models of arc volcanism commonly call for
the addition of a fluid phase containing fluidmobile elements derived from metamorphic
reactions in the subducting lithosphere to the
sub-arc mantle; these fluids promote partial
melting of the mantle and influence the
composition of the resultant magma (i.e.,
the incompatible-radiogenic-siliceous fluids
of Gill, 1981; see update by Davidson, 1996).
These models are designed to explain distinctive features of the geochemistry of arc magmas (i.e., enriched Sr, Ba; depleted Nb, Ta).
Compared to other variables that contribute to
the overall process—for example, the effects
of composition of subducted materials and the
evolution of derived liquids in the mantle—
the metasomatic processes that occur along
the slab-mantle interface are poorly understood. While this is primarily a function of the
limited number of samples available, it is difficult to evaluate models for magma formation
in subduction zones without an understanding
of the metasomatic and geochemical processes
occurring in the mantle prior to magma
formation.
While trace elements have been the focus
of much research due to their sensitivity to
petrogenetic processes and their use in modeling (i.e., Gill, 1981; Davidson, 1996; Ayers,
1998), major elements—such as, Si, Al, Ca,
Na—likely represent the bulk of the solute
carried by fluids derived from subducting sediments and oceanic crust (e.g., Manning,
1998; Manning et al., 2001). Experiments
constraining the solubility of major elements
at appropriate pressures and temperatures are
incomplete (Manning, 1994; Newton and
Manning, 2000; Manning et al., 2001) and
limit models of the sequence of metasomatic
reactions between mantle peridotite and infiltrating fluid to simple systems such as MgOSiO2-H2O (Manning, 1995, 1997). Accordingly, the current conception of metasomatic
reactions along the slab-mantle interface has
been limited to serpentinization and Si metasomatism of peridotite. The occurrence of
forearc serpentinite seamounts in the Mariana
subduction system (e.g., Fryer et al., 1995)
and seismic observations of serpentinized
forearc mantle within active subduction zones
(Kamiya and Kobayashi, 2000; Zhao et al.,
2001; Bostock et al., 2002) are good evidence
that extensive metasomatic alteration of the
mantle wedge occurs. However, on-land exposures of peridotites metasomatized by
subduction-zone fluids amenable to detailed
analytical study are rare (e.g., Peacock, 1987).
Here we describe metasomatized ultramafic
blocks in the Franciscan Complex of California; we infer that these blocks were infiltrated
GSA Bulletin; September 2003; v. 115; no. 9; p. 1097–1109; 9 figures; 3 tables; Data Repository item 2003120.
For permission to copy, contact [email protected]
q 2003 Geological Society of America
1097
KING et al.
by hydrous fluids during active subduction in
the Franciscan paleo-subduction zone. We present the mineralogy, mineral chemistry,
whole-rock geochemistry, and oxygen isotope
geochemistry of the metasomatic zones produced by fluid-rock interaction in these rocks.
Metasomatic reactions preserved in these ultramafic blocks are similar to those proposed
by Manning (1995, 1997) but also include reactions not considered by those models. Our
results present further evidence that the chemical (including trace element) composition of
fluids leaving this metamorphic system is
modified substantially by metasomatic reactions occurring in the mantle wedge. If so,
these reactions impact general models of geochemical cycling in convergent margins.
BACKGROUND
Subduction-Zone Metasomatism
Studies of primitive arc volcanic rocks have
long recognized that magma formation along
convergent margins is most often a result of
fluid-fluxed melting of the mantle. The trace
element geochemistry of arc lavas provides one
line of evidence for the role of slab-derived,
aqueous fluids in their origin. In particular,
these lavas are typically characterized by enrichments in fluid-soluble large ion lithophile
elements (e.g., Ba) and depletions in fluidinsoluble high field-strength elements (e.g.,
Nb), when compared to similarly evolved
mid-oceanic ridge basalts (Gill, 1981; Davidson, 1996). Such incompatible-element patterns are generally interpreted as the result of
large ion lithophile element-rich fluids imposed on mid-oceanic ridge basalt-like mantle
that has previously experienced depletion of
incompatible elements by extensive partial
melting (Davidson, 1996). Solubility experiments have demonstrated the greater partitioning of large ion lithophile elements as compared to high field-strength elements into
subduction-zone fluids (You et al., 1996),
while other studies have suggested the high
field-strength element content of fluids is low
due to Ti-bearing phases in the slab or mantle
(e.g., Brenan et al., 1994), which sequester
high field-strength elements into solid phases.
Geochemical and petrologic research of exposed subduction complexes complement
studies of arc volcanic rocks because losses of
fluid-mobile trace elements from the slab can
be linked to the prograde metamorphic evolution, and therefore the thermal structure, of
the subducting crust (e.g., Peacock, 1993a;
Bebout et al., 1999). From studies of the Catalina Schist, Bebout et al. (1999) suggest sub-
1098
stantial losses (75–90%) of fluid-mobile trace
elements in more strongly metamorphosed
sections of the down-going slab (amphibolite
and epidote-amphibolite facies) but only modest removal of these elements in lower temperature lawsonite blueschists (;25%). Stable
isotopic compositions of mélange zones in the
Catalina Schist are homogenized over several
kilometers, suggesting that fluid-hosted mass
transfer involves interconnection and equilibration between fluids and the metamorphic
rocks they pervade on scales of kilometers
during fluid flow (Bebout, 1991; Bebout and
Barton, 1993).
Finally, the interaction between slab-derived
fluids and peridotite in the mantle wedge has
been examined by experiments and theoretical
models. Using quartz solubilities at subduction zone pressure-temperature (P-T) conditions (Manning, 1994) and phase equilibria in
the system K2O-Al2O3-SiO2-H2O, Manning
(1996) demonstrated that subducted sediments
will buffer the concentration of aqueous SiO2
at or near quartz saturation in subduction-zone
fluids regardless of sediment mineralogy, i.e.,
even in the absence of quartz. Using these data
and P-T paths estimated for subducted slabs
by Peacock (1993a), Manning (1995, 1997)
modeled the interaction between model peridotite and Si-rich, slab-derived fluids in the
system MgO-SiO2-H2O. According to this
model, the concentration of aqueous SiO2 in
the initial stages of the metasomatism of a
model peridotite is initially buffered by the reaction of brucite to antigorite, where the equilibrium
48Mg(OH)2 1 34SiO2(aq)
5 Mg48Si34O85 (OH)62 1 17H2O
(1)
removes silica from the fluid by producing antigorite from brucite until the composition of
the resulting serpentinite reaches 100% antigorite. This reaction considers the buffering of
SiO2 in the metaperidotite in the context of the
brucite-antigorite reaction in that both of these
phases are stable during metasomatism,
whereas the minerals of the peridotite protolith (forsterite and enstatite in the models of
Manning, 1995, 1997) are not. The initial relative proportions of brucite and antigorite,
without considering the addition of Si from
fluids, are dictated by the initial proportions
of forsterite and enstatite. A more forsterite-rich
peridotite will yield a greater initial proportion
of brucite during hydration. With continued
Si-saturated fluid flow in a metaperidotite, the
proportion of brucite will steadily decline due
to the high Si contents of the fluids favoring
the products of Reaction 1 (i.e., antigorite).
Once brucite has completely reacted to
form antigorite and Reaction 1 is no longer
capable of buffering the concentration of
aqueous SiO2 in the metaperidotite, new equilibrium conditions for the concentration of
aqueous SiO2 are defined by the production of
talc from antigorite by
Mg48Si34O85 (OH)62 1 30SiO2(aq)
5 16Mg3Si4O10 (OH)2 1 15H2O
(2)
and SiO2 metasomatism of the metaperidotite
begins in earnest. This second buffering reaction is only stabilized once Reaction 1 has
been completed. The antigorite-talc buffering
reaction proceeds until it has gone to completion or fluid flow has ceased through the rock.
Further reactions were not considered by
Manning (1995, 1997) due to limits in the
available solubility data for other chemical
constituents (i.e., Al, Ca) and uncertainties in
the thermodynamic data set (i.e., the effect of
Al on the stability of serpentine-group minerals). While these models are limited in
scope, they provide important constraints on
the sequence and form of metasomatic reactions that probably occur in the mantle wedge.
Geologic Setting and Field Relations
The Franciscan Complex in the study area
(Fig. 1) is principally composed of metagraywacke flow mélange (cf. Cloos, 1984; Cloos
and Shreve, 1988). Metamorphic recrystallization within the study area is limited (e.g.,
white mica 1 chlorite), leaving numerous relict detrital grains of quartz and feldspar. Gilbert (1973) reports a lawsonite isograd in the
area, but lawsonite was not observed in samples of metasediment in this study. Estimates
of peak metamorphic conditions for the mélange matrix in the study area are in the range
of 150–200 8C and 4–6 kbar (Ernst, 1980;
Underwood and Laughland, 2001). Exotic
blocks within the mélange in the area include
metabasic greenstone/schist, bedded chert, and
the ultramafic (metaperidotite) blocks that are
the focus of this study (Hall, 1991; this study).
The three-dimensional shape of the ultramafic
blocks is uncertain in that many of the blocks
are covered by a Quaternary strath terrace so
that the principle exposures are only in sea
cliffs (Fig. 1). The map pattern of the ultramafic exposures suggests one or two large, coherent blocks, and these blocks are probably
ellipsoidal in shape; along some contacts, the
blocks appear to pinch out in the style of a
Geological Society of America Bulletin, September 2003
SUBDUCTION ZONE METASOMATISM OF FRANCISCAN COMPLEX ULTRAMAFIC BLOCKS
Figure 1. Maps of the study area. (A) Simplified map of central coastal California, USA,
shows study area location and points of geographic reference. (B) Coastline of the Cape
San Martin 7.50 topographic quadrangle with local points of reference; dashed box shows
Franciscan Complex region mapped in detail. (C) Field geology of the Franciscan Complex
in the study area, showing metagraywacke mélange matrix (JKfm) and tectonic inclusions
of ultramafic material (JKfu). A disjunctive cleavage in the mélange matrix strikes between N55W to N70W and dips ;508NE. Quaternary cover is alluvium (Qal) and landslide
debris (Qls). (D) Expanded view of the study area, showing sample locations. Arrow between samples numbers 98RKJC-3A and 98RKJC-13A is inclusive for all samples
98RKJC- between these numbers; sampling density is too high to resolve at this scale.
Vein samples in this study are also contained within this arrow. Samples that are 00RKJCare triple digit numbers (e.g., 101).
large boudin. Contact relations between the
ultramafic blocks and the mélange matrix are
generally diffuse and include small (,1 m)
blocks of metaperidotite locally derived from
disaggregation of the larger metaperidotite
body. These relations are interpreted as being
produced by deformation of the metaperidotite
within the Franciscan subduction channel.
Two post-metamorphic faults bound the Sand
Dollar Beach and Jade Cove metaperidotite
blocks. At the extreme north end of Sand Dollar Beach, just north of sample 98RKSD-1A
in Figure 1D, a fault of indeterminate motion
has produced a sharp contact between the metaperidotite and mélange matrix. The total displacement of this fault is unknown. At the ex-
treme southern end of the Jade Cove block,
just south of sample 00RKJC–102 in Figure
1D, a small thrust fault displaces the Jade
Cove block approximately #5 m over the mélange matrix. Petrographic examination of mélange matrix throughout the field area is extremely similar; there is no discernible
difference in metamorphic reconstitution or
initial sedimentary composition. We interpret
the displacement of the metaperidotite blocks
along these two post-metamorphic faults as
small and suggest the current disposition of
the metaperidotite blocks in the Franciscan
Complex is primarily the result of subduction
processes in Franciscan time. We believe the
peridotite blocks likely originated from the
basal section of the Coast Range ophiolite, as
this scenario is the most tectonically feasible;
that is, either extensional deformation due to
oversteepening of the Franciscan accretionary
system (Platt, 1986; Harms et al., 1992) or
tectonic erosion of the hanging wall (Cloos
and Shreve, 1988) could have introduced
mantle material into the accretionary complex.
Regardless of how peridotite was ultimately
incorporated into the Franciscan mélange, deformation and metasomatism of the peridotite
blocks were synchronous. The ultramafic
blocks are highly sheared and faulted within
the mélange matrix in a style somewhat analogous to porphyroclasts in mylonites, which
involve similar contrasts in relative mechanical strengths of block and matrix. Flow mélange deforms in a manner similar to a viscous
fluid due to high fluid pressures maintained by
material within the subduction channel and
experiences ductile deformation at almost any
temperature expected for the upper 50 km of
a subduction zone (i.e., T # 200 8C; Shreve
and Cloos, 1986; Cloos and Shreve, 1988). In
contrast, peridotite is initially much stronger
mechanically and most likely deformed via
brittle mechanisms as the surrounding matrix
was sheared by subduction. Pervasive simple
shear is recorded within the mélange matrix,
whereas the ultramafic blocks contain only a
disjunctive cleavage that is largely a product
of fracturing, veining, and internal rotation of
coherent phacoids.
The most important result of this brittle deformation for our study is the increase it
caused in porosity and permeability within the
ultramafic blocks, thus providing avenues for
the infiltration of fluids from the metasedimentary mélange matrix. The chemical gradient between anhydrous, SiO2-undersaturated
peridotite and SiO2-saturated aqueous fluids
buffered by the mélange matrix (Cloos and
Shreve, 1988; Manning, 1996) drove serpentinization and SiO2 metasomatism of the ul-
Geological Society of America Bulletin, September 2003
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KING et al.
tramafic blocks as they approached equilibrium with fluids.
Several episodes of regional postmetamorphic brittle faulting occurred near the
study area, but they do not affect the metasomatic history preserved within the ultramafic blocks. Emplacement of the Southern
California Allochthon occurred in Paleocene
time (65–55 Ma; Hall, 1991), and brittle deformation associated with the San Andreassystem of faults commenced in the Oligocene
(ca. 30 Ma). The Southern California Allochthon originated from the present CaliforniaArizona border area and was thrust northwestward along the Sur thrust (Hall, 1991).
Although the Southern California Allochthon
directly overlies the Franciscan Complex east
of the study area, deformation was highly localized along the Sur thrust and did not obviously affect the ultramafic blocks or mélange matrix in the study area (Hall, 1991; this
study). The San Andreas fault and related
strike-slip faults—e.g., the San GregorioHosgri fault zone—have created numerous,
subsidiary strike-slip, thrust, and transpressional faults within the study area and
throughout the central California coast ranges
(Hall, 1991). These faults have undoubtedly
translated much of the study area northward
to some extent since ;30 Ma. Some small
fault splays intersect portions of the ultramafic blocks (see Reinen, 2000, for features
of a post-metamorphic, brittle shear zone
within the northern portion of the Sand Dollar
Beach serpentinite block), but these are texturally obvious in the field and were avoided
during sampling.
Previous stable isotopic investigations of
serpentinized ultramafic rocks in the Franciscan Complex and elsewhere (Wenner and Taylor, 1971, 1973, 1974; Magaritz and Taylor,
1976) have generally noted two types of serpentinites with distinct isotopic signatures.
Specifically, antigorite and lizardite-chrysotile
serpentinites have been observed to have distinct oxygen and hydrogen isotope systematics. Antigorite serpentinites generally have restricted ranges (d18O 5 14.7 to 18.7; Wenner
and Taylor, 1974) and are consistent with a
metamorphic origin. Lizardite-chrysotile varieties have much wider ranges, and their d18O
values reflect the influence of local meteoric
waters. This suggests that lizardite-chrysotile
serpentinites are near-surface products, consistent with their serpentine mineralogy. The serpentinites in this study have been verified as
antigorite serpentinites by electron microscopic methods (King, 1999, and see below), while
the distribution of lizardite-chrysotile serpentinite is restricted to within Tertiary to recent
1100
shear zones, i.e., the shear zones studied by
Reinen (2000). Our stable isotopic results are
consistent with the previous studies of serpentinization in the Franciscan Complex, in that
the antigorite serpentinites of this study preserve stable isotopic compositions consistent
with a metamorphic petrogenesis and not a
near-surface, meteoric water-influenced origin.
METHODS
Serpentinization reaction textures were categorized using the nomenclature of O’Hanley
(1996). High-resolution transmission electron
microscopy analyses of serpentine-group minerals in the ultramafic blocks showed the serpentine sub-types to be solely antigorite or a
mixture of predominantly antigorite with minor lizardite (King, 1999). Chrysotile 1 lizardite was only observed in samples that had
been overprinted by post-metamorphic deformation (e.g., Reinen, 2000).
Whole-rock major element data were obtained using the Philips 2400 X-ray fluorescence spectrometer at the University of South
Carolina and inductively coupled plasma
atomic-emission spectroscopy by Actlabs, Inc.
X-ray fluorescence analyses were conducted
on lithium tetraborate-fluxed fusions of ignited powders. All elemental data (X-ray fluorescence and inductively coupled plasma
atomic-emission spectroscopy) include loss on
ignition in the total.1 Both data sets are in
good agreement based upon similar wholerock geochemistry for petrographically similar
samples (King, 2001).
Mineral compositions were measured using
the Cameca SX-50 electron microprobe at the
University of South Carolina. Quantitative
analyses were made using an accelerating
voltage of 15 keV and a Faraday cup current
of 40 nA. Counting times were 40 s on peak
and 20 s on two backgrounds. Oxygen normalizations used for mineral formula recalculations were 23 for amphibole, 28 for chlorite, 6 for clinopyroxene, 12.5 for epidote
group, 4 for olivine, 3 for orthopyroxene, 7
for serpentine group, 11 for talc, and 5 for
titanite. No corrections were made for ferric
iron except for epidote-group minerals, where
all iron was assumed to be ferric.
Oxygen isotope ratios were measured on
carbonates and silicates. Carbonates were analyzed by automated phosphoric acid dissolution and measured as CO2 on the University
1
GSA Data Repository item 2003120, chemical
data, is available on the Web at http://www.
geosociety.org/pubs/ft2003.htm. Requests may also
be sent to [email protected].
of South Carolina VG Optima mass spectrometer. Silicates were analyzed by laser fluorination at both the University of South Carolina and the California Institute of
Technology; both systems are similar to that
described by Spicuzza et al. (1998). Oxygen
was liberated using CO2 (near-IR) lasers in a
BrF5 atmosphere and quantitatively reacted to
CO2 via heating with either diamond or graphite converters. Samples were either handpicked using a dissecting microscope or cut
from 1-mm-thick sections using the thin saw
blade method (Kohn et al., 1993; Elsenheimer
and Valley, 1993). Cut samples were checked
for homogeneity under a dissecting microscope, and non-monomineralic samples were
rejected. Oxygen isotope ratios are presented
using delta (d) notation, in which
d18O 5
1
2
O/16Osam 2 18O/16Ostd
3 10 3, (3)
18
O/16Ostd
18
where 18O/16Osam is the oxygen isotope ratio in
the sample, 18O/16Ostd is the ratio of the standard, and units of d18O are parts per thousand
(per mil or ‰) relative to Vienna Standard
Mean Ocean Water (see Data Repository).
RESULTS
Mineralogy and Crystal Chemistry
Samples from the ultramafic blocks were
classified into groups based upon their metasomatic mineralogy (Fig. 2). Group 1 samples
(n 5 7) preserve the initial serpentinization of
relict peridotite phases. The peridotite protolith was a spinel lherzolite; estimates of modal
abundances prior to serpentinization are 75–
80% olivine, 7–10% for both orthopyroxene
and clinopyroxene, and #5% Cr-spinel. Both
pyroxenes display high Al contents, and spinel
compositions are rich in Cr (Data Repository),
suggestive of a mantle and not cumulate origin for the peridotite. Serpentinization of olivine produced mesh cells (see O’Hanley,
1996, for serpentinization texture terminology) as serpentinization proceeded along cracks
and grain boundaries. The serpentinization of
olivine proceeded more rapidly than that of
pyroxene, in which serpentinization along
cleavage planes produced bastites (i.e., serpentine pseudomorphs after pyroxene). Magnetite was ubiquitous during serpentinization,
and modal abundances increased with the degree of serpentinization. Cr-spinels are surrounded by chlorite coronas, which can be
rich in Cr (up to 2 cations/28 oxygens). Neoblastic diopside also formed during this stage
of serpentinization; these diopsides are textur-
Geological Society of America Bulletin, September 2003
SUBDUCTION ZONE METASOMATISM OF FRANCISCAN COMPLEX ULTRAMAFIC BLOCKS
Figure 2. Summary of mineralogic compositions produced in Franciscan Complex ultramafic blocks due to metasomatism by subduction-zone fluids and schematic sketch of
mineral zones within an idealized ultramafic block. Localities (see Figure 1) are Sand
Dollar Beach (SDB) and Jade Cove (JC). Mineral abbreviations are: Cpx, clinopyroxene;
Opx, orthopyroxene.
ally and chemically distinct from relict clinopyroxenes (see Data Repository). Brucite was
never observed in any Group 1 sample.
Group 2 was defined by the complete serpentinization of peridotite (n 5 15). Serpentine
mesh cells were replaced by an interpenetrating
fabric, which is common in other antigorite serpentinites where mesh cells were recrystallized
during metamorphism (O’Hanley, 1996). Serpentine comprises ;85–90% of the mineral
mode in Group 2. Modal abundances of magnetite increase up to 10–12%, and neoblastic diopside is not present. Relict Cr-spinels persisted
at this stage of metamorphism, surrounded by
their chlorite coronas. As with Group 1, brucite
is not present in any sample from Group 2.
The replacement of serpentine by talc defines Group 3 samples (n 5 4). In one sample
where serpentine and talc are present together
(00RKJC–102), talc is enriched in Mg relative
to coexisting serpentine (serpentine Mg/
[Mg1Fe] 5 0.82–0.84, talc Mg/[Mg1Fe] 5
0.92–0.93). In all other Group 3 samples, serpentine has completely reacted to talc, with
modal abundances of talc;85–90%. Magnetite is absent in Group 3 rocks. Cr-spinels and
chlorite coronas are present, but within Group
3 chlorite also occurs as discrete grains.
Group 4 rocks (n 5 3) contain tremolite
(Mg/[Mg1Fe] 5 0.85–0.88) rather than talc
as the dominant mineral (80–85% of the
mode) in the altered ultramafic blocks. Group
4 rocks are distinct from Groups 1–3 in that
they contain a diverse assemblage of minor
and accessory phases: hornblende, clinozoisite
(Al/[Al1Fe31] 5 0.93), titanite, apatite (100–
200 mm radius), and zircon (#10–20 mm radius) in addition to chlorite. Clinozoisite and
titanite occur as neoblasts within chlorite but
are present in nearly every chlorite grain. All
of these minor phases appear to be neoblastic.
Relict Cr-spinels are also present in Group 4
rocks, but are highly reduced in size
(#100mm).
Whole-Rock Geochemistry
The geochemical effects of fluid flow and
metasomatism on the ultramafic blocks were
investigated via whole-rock compositional
systematics. A plot of CaO versus SiO2 reveals the first-order consequences of serpentinization and metasomatism (Fig. 3A). CaO
for Group 1 is highly variable, with lower
CaO corresponding to greater serpentinization.
This depletion reflects the incompatibility of
Ca in serpentine-group minerals, magnetite,
and chlorite such that Ca is generally removed
by through-flowing fluids; this is a common
feature of serpentinites and the related process
of rodingitization (see Coleman, 1967;
O’Hanley, 1996). Neoblastic diopside is pre-
Figure 3. Bivariate plots of whole-rock (A)
CaO versus SiO2 and (B) TiO2 versus Al2O3
as a function of metasomatic reaction
progress.
sent in Group 1 but absent in Group 2, further
suggesting that Ca was removed from the rock
by fluids as serpentinization proceeded. The
variability of SiO2 contents for Group 2 samples probably reflects different extents of Si
metasomatism. The presence of talc in Group
3 samples is associated with enrichments in
SiO2. Similarly-low CaO contents for both
Groups 2 and 3 suggests that talc formed following serpentinization, which agrees with
petrographic observations. Group 4 rocks are
more enriched in CaO than any other sample;
this is easiest to explain by derivation of Ca
from a fluid-mediated source external to the
blocks. SiO2 contents for the Group 4 samples
are similar to those for Group 3 if dilution of
the blocks by other chemical species—i.e.,
CaO—is considered.
The potential redistribution of other major
and minor elements by fluids during metasomatism was evaluated using the isocon diagram of Grant (1986). As the ratio of TiO2 to
Al2O3 in Group 1 and 2 samples is relatively
constant (Fig. 3B), these elements were assumed to be conservative during serpentinization and used to define the slope of the is-
Geological Society of America Bulletin, September 2003
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KING et al.
ocon for all samples. TiO2 and Al2O3 contents
of Groups 3 and 4 appear to be enriched relative to Groups 1 and 2, probably due to mobility in fluids; this mobility is addressed below. When included in the construction of the
isocon, TiO2 and Al2O3 data for Groups 3 and
4 yield implausible results. For example, mass
losses of ;60% occur in Group 3 samples, in
which all other lines of evidence suggest an
increase in mass (i.e., addition of Si to form
talc; a full discussion can be found in King,
2001). Results for Fe2O3 suggest that iron has
been immobile during alteration, providing an
independent reference for the plotting assumptions. This result is surprising in that substantial changes in the oxidation of Fe must have
occurred during metasomatism, in which Fe
oxidizes from olivine and pyroxene to form
magnetite, but magnetite is absent during talc
formation; however, our results suggest little
if any mobility of Fe in fluids. A Group 1
sample (98RKSD-13a) was used to represent
the protolith composition (CO), as it is the
least visibly altered rock and has the lowest
loss on ignition in Group 1. Representative
samples from Groups 1–4 were used as altered
compositions (CA). Scaling factors were used
for some elements to clarify the diagram, but
it should be noted that this does not affect the
method (cf. Olsen and Grant, 1991).
Isocon results indicate that large relative
changes occurred in the concentrations of
most elements over the course of metasoma-
tism (Fig. 4), and the slope of the isocon
(0.88) indicates a net increase in mass. Several
sets of elements can be treated together due to
similar patterns of enrichment or depletion relative to the assumed protolith composition:
Both SiO2 and P2O5 are enriched in all groups,
and Al2O3 and TiO2 are enriched in Groups 3
and 4. MgO is enriched in Groups 1 and 2,
but depleted in Groups 3 and 4, probably due
to dilution by additions such as SiO2. CaO and
MnO are also depleted in Groups 1–3, but enriched in Group 4. Na2O systematics are similar to CaO and MnO, but could reflect protolith heterogeneities. Fe2O3 appears to be
conservative at all stages of metasomatism.
Ternary diagrams depicting the measured
whole-rock geochemistry further illustrate relative compositional variations in the systems
MgO-SiO2-H2O (Fig. 5, A and B), MgO-AlO3/
2-SiO2-H2O (projected from H2O; Fig. 5, C
and D), and CaO-MgO-AlO3/2-SiO2-H2O (projected from H2O 1 SiO2; Fig. 5, E and F).
Because the activities of SiO2 and H2O (aSiO2
and aH2O, respectively) undoubtedly varied
over the course of metasomatism, these diagrams are not phase diagrams and have no
thermodynamic significance. Nonetheless, the
diagrams illustrate some of the major relative
chemical variations resulting from metasomatism. Within the system MgO-SiO2-H2O,
Groups 1 and 2 plot near the serpentine composition and within the peridotite 1 H2O field
(i.e., the field forsterite-enstatite-H2O). No
sample plots within the brucite-serpentineH2O field, which agrees with the absence of
brucite in any sample. Group 3 and 4 samples
plot near the talc end-member outside of peridotite 1 H2O, which requires the addition
(i.e., an external source) of SiO2.
Systematics in the system MgO-AlO3/2-SiO2H2O (Fig. 5, C and D) indicate that following
the addition of SiO2 to form talc, AlO3/2 was
added to the overall bulk composition. Groups
1 and 2 are indistinguishable from one another
in the system and plot near the serpentine endmember. Group 3 and 4 samples generally plot
within the talc-chlorite (-quartz) field and have
enrichments in AlO3/2 that are considerably
higher than any Group 1 or 2 sample.
CaO-MgO-AlO3/2-SiO2-H2O is perhaps the
most informative system for the ultramafic
blocks, in that the dominant mineralogy for
each group is present (Fig. 5, E and F). Although differences in the activity of aSiO2 undoubtedly existed during metasomatism, this
discrepancy was tolerated to evaluate the relative behavior of Ca, Mg, and Al during metasomatism. Group 1 samples define an array
extending from the initial Ca contents of the
peridotite toward the serpentine-chlorite join.
All Group 2 and 3 samples plot along this join
as serpentine and talc plot at the same position
in CaO-MgO-AlO3/2-SiO2-H2O. The plotting
position of the Group 4 samples can best be
explained by a vector toward clinozoisite from
the serpentine/talc plotting position; this in-
Figure 4. Summary isocons for (A) major elements and (B) minor elements in Franciscan Complex ultramafic blocks as a function of
reaction progress. The isocon diagram is constructed using an assumed original protolith composition plotted against representative
samples for each metasomatic group. The isocon is a best-fit line drawn through elements considered to be immobile during alteration;
in this diagram, Al2O3 and TiO2 for Group 1 and 2 samples are used to define the slope of the isocon. Elements plotting above the
isocon are interpreted as enriched relative to protolith composition, while elements below the isocon are interpreted as relatively depleted.
See text for full assumptions used in isocon analysis.
1102
Geological Society of America Bulletin, September 2003
SUBDUCTION ZONE METASOMATISM OF FRANCISCAN COMPLEX ULTRAMAFIC BLOCKS
Figure 5. Ternary diagrams of measured whole-rock geochemistry for Franciscan Complex ultramafic blocks. (A) The system MgO-SiO2-H2O (MSH); shaded region is area of
(B) plot of measured data in MSH as a function of reaction progress. (C) The system
MgO-AlO3/2-SiO2-H2O (MASH); shaded region is area of (D) plot of measured data in
MASH as a function of reaction progress. (E) The system CaO-MgO-AlO3/2-SiO2-H2O
(CMASH); shaded region is area of (F) plot of measured data in CMASH as a function
of reaction progress. Abbreviations used are: Brc—brucite; Chl—chlorite; Czo—
clinozoisite; Di—diopside; En—enstatite; Fo—forsterite; Lws—lawsonite; Pmp—
pumpellyite; Qtz—quartz; Srp—serpentine; Tlc—talc; Tr—tremolite.
dicates co-enrichment of Ca and Al to the bulk
rock composition.
Oxygen Isotope Geochemistry
Synmetamorphic calcite 1 serpentine veins
were sampled to evaluate the oxygen isotope
composition of fluids affecting the ultramafic
blocks. All vein samples (n 5 9) were collected from the Jade Cove locality, where they
are found within Group 2 and Group 3 rocks.
Mineral separates recovered from veins are
limited to calcite and serpentine; no other
mineral was observed in any vein sample. We
interpret all calcite as precipitated from veinforming fluids; in contrast, the serpentine
might have been mechanically incorporated
into the veins; i.e., potentially, pieces of wall
rock may have been disaggregated and deposited into veins as inclusions. However, we
think it likely that most or all serpentine was
precipitated from fluids because the population of vein serpentine has relatively well
formed crystals; its color is distinct from wall
rocks, as it is lime-green and translucent as
opposed to dark green and opaque; and it is
free of magnetite. Most veins analyzed were
bimineralic, although some were composed
solely of calcite or serpentine.
Results from the veins suggest a remarkably
homogeneous fluid composition (Fig. 6). Of
the 32 calcite analyses, three are strongly enriched in 18O and 13C (13C is not shown, but it
is included in the Data Repository) compared
to the other analyses. This suggests that these
portions of the veins were either formed by
fluids at a different temperature, by fluids of
a different composition, or were reset by diffusion following their formation. However, the
remaining 29 samples from the veins define a
relatively restricted range of 12.86‰ 6
0.46‰ (1s). Separates of vein serpentine d18O
are even more notably homogeneous, with a
mean value of 8.24‰ (6 0.16‰, 1s). Such a
relatively constant value for d18O in these samples suggests that this serpentine almost certainly
originated from fluids, in that reworked grains
of serpentine mechanically incorporated into the
veins would be very unlikely to have such a
homogeneous composition unless they were
completely re-equilibrated with the fluids (see
disseminated serpentine d18O below).
Mineral separates from the ultramafic blocks
are limited to the dominant rock-forming mineral at each stage of metasomatism, and relict
peridotite phases, where possible. Peridotite
phases from two Group 1 samples were analyzed. Sample 98RKSD-9a has d18O values for
olivine (;5.32‰) and clinopyroxene (;5.72‰)
that are within the range of typical mantle values and display a mantle D18Ool-cpx for these
minerals (i.e., 10.4‰; Mattey et al., 1994). A
second sample (98RKSD-8a) has values for
olivine (;6.29‰) and clinopyroxene (;5.96‰)
that are significantly higher and display a reversed D18Ool-cpx. This is probably due to contamination of the separates by metamorphic
serpentine, despite our efforts to obtain clean
mineral separates.
Serpentine separates from Group 2 samples
display a wide range in d18O, from ;6.3‰ to
;8.1‰. These values probably reflect contributions of oxygen from the peridotite protolith
and the metasomatic fluid and are considered
in more detail below. Mineral separates of talc
from Group 3 samples are rather homogenous
with respect to d18O within each individual
sample and display an intersample range of
;0.5‰ (;9.65 to ;10.10‰). Group 4 tremolite separates also show a limited range of
Geological Society of America Bulletin, September 2003
1103
KING et al.
d18O compositions (;8.5‰ to ;9.0‰); however, the full range is expressed in one sample,
while the other sample is homogenous (8.63
6 0.01‰; n 5 2).
DISCUSSION
Quantitative Measures of Fluid-Rock
Interaction
The oxygen isotope ratios of the ultramafic
blocks considered in this study offer a useful
constraint on the overall history and context
of metamorphism and fluid-rock interaction.
Foremost, oxygen isotope fractionation between calcite and serpentine in veins indicates
that metamorphism of the blocks occurred prior to their emplacement and association with
the lower grade mélange matrix of the field
area. Review of published fractionation factors (Kohn and Valley, 1998a, b; Saccoccia et
al., 1998) suggests that the fractionation constant (A) for this pair lies between 2.2 and 2.6;
this range of values results in a temperature
of 450 6 30 8C. When combined with analytical uncertainties (6 408C), the total error
for the vein analyses is 450 6 50 8C (Fig. 6).
The potential diffusional reequilibration of
vein calcite was evaluated using the Dodson
(1973) equation with the diffusion data of
Farver (1994), a spherical, 1-mm grain, and a
cooling rate of 10 8C/Ma. This grain size and
cooling rate are considered minimum values
for these two variables and were chosen to
maximize the potential diffusion of oxygen.
These values imply a closure temperature of
;400 8C for calcite. Higher isotopic temperatures for the veins suggest faster cooling or
larger effective grain sizes for calcite and limited oxygen diffusion.
Furthermore, since the range in serpentine
d18O for Group 2 falls within the range bounded by the initial bulk peridotite (modeled as
5.42‰; see Table 1) and vein serpentine
(8.24‰), we constructed a relatively simple
linear mixing model to evaluate reaction progress in Group 2 samples via the extent of
isotopic equilibrium between serpentine and
fluids (Table 1). This model is not meant to
represent physical or mechanical mixing of
the two oxygen reservoirs, nor does it make
assumptions regarding the physical mechanism(s) of oxygen exchange or fluid fluxes; it
merely evaluates the required contributions
from the two oxygen reservoirs to produce the
measured values for serpentine d18O from
Group 2. The only fundamental assumption in
the model is that serpentine in isotopic equilibrium with metasomatic fluids will have a
d18O composition identical to that measured
1104
Figure 6. Variations in d18O of calcite and serpentine from metamorphic veins within
Franciscan Complex ultramafic blocks. The thermometry equation contains two sources
of error, from uncertainties in value of A for calcite-serpentine pair (6 30 8C) and analytical error (6 40 8C). The equation’s total error is 6 50 8C. All samples are 98RKJC(sample number).
TABLE 1. REACTION PROGRESS MODEL FOR SERPENTINE d18O COMPOSITIONS
d18O
Simultaneous equations
Initial rock
Vein serpentine
5.42‰
8.24‰
d O 5 (Xr * d18Or) 1 (Xf * d18Of)
1 5 (Xr 1 Xf)
Group 2 sample
Serpentine d18O
X rock (Xr)
X fluid (Xf)
6.37‰
7.19‰
7.84‰
7.84‰
8.18‰
0.664
0.371
0.141
0.141
0.022
0.336
0.629
0.859
0.859
0.978
Model end-members
98RKSD-12A
98RKSD-17A
98RKJC-4A
98RKJC-12A
00RKJC-109
18
Note: ‘‘Initial Rock’’ is the calculated d18O of the initial peridotite using measured values of olivine (5.32‰) and
clinopyroxene (5.72‰) from sample 98RKSD-9a with an assumed value for orthopyroxene (5.9‰; Mattey et al.,
1994; Hoefs, 1997) combined in modal proportions of 80:10:10 (Ol:Cpx:Opx); ‘‘Vein serpentine’’ is the average
serpentine d18O of vein samples (see Figure 6). Results from the model are presented in Figure 7a.
for vein serpentine. This assumption is convenient in that isotopic equilibrium between
Group 2 serpentine and fluids can be expressed relative to vein serpentine d18O without introducing additional uncertainties from
calculating the fluid d18O. Therefore, the model is essentially a measure of reaction progress
in Group 2 samples as peridotite-derived oxygen is replaced by fluid-derived oxygen during metasomatism. Results of the reaction progress model (Table 1, Fig. 7A) indicate that
the increasing d18O of serpentine from Group
2 samples can be explained by increasing
amounts of fluid-equilibrated oxygen in the
ultramafic blocks and suggest that alteration
serpentine approached equilibrium with fluids
as the serpentine formed. In addition, the good
positive correlation between enrichments in
Group 2 serpentine d18O and the increasing
SiO2 in the whole-rock geochemistry for the
same samples (Fig. 7B) implies that SiO2 added to the ultramafic blocks during this stage
of metasomatism was derived from fluids.
If the ultramafic blocks did approach equilibrium with the fluids during serpentinization,
then it should be expected that Group 3 and
4 samples should be in isotopic equilibrium
with fluid-derived oxygen at a constant temperature if they were, in fact, derived from
pre-existing serpentinites. Comparison of d18O
values of vein serpentine (ideally in equilibrium with the fluid d18O) with talc (from
Geological Society of America Bulletin, September 2003
SUBDUCTION ZONE METASOMATISM OF FRANCISCAN COMPLEX ULTRAMAFIC BLOCKS
TABLE 2. COMPARISON OF OBSERVED AND CALCULATED d18O FRACTIONATIONS
Mineral
Mean d18O
Talc
9.68‰
Tremolite
8.56‰
Serpentine
8.24‰
Mineral-mineral fractionations
Calculated
Tlc d18O-Tr d18O
Temperature
2008C
3.13‰
3008C
2.13‰
4008C
1.54‰
5008C
1.17‰
6008C
0.92‰
Observed
1.12‰
1s
Mineral pair
A
60.42‰
60.34‰
60.16‰
18
18
d Oa–d Ob 5 (A x 106)/(T2)
Talc-Tremolite
Talc-Serpentine
Tremolite-Serpentine
0.7
0.89
0.19
Tlc d18O-Srp d18O
3.98‰
2.71‰
1.96‰
1.49‰
1.17‰
1.44‰
Tr d18O-Srp d18O
0.85‰
0.58‰
0.42‰
0.32‰
0.25‰
0.32‰
Note: Values for A are from: talc-tremolite, Kohn and Valley (1998a, 1998b); talc-serpentine, calculated from
data of Saccocia et al. (1998); there is excellent agreement between the observed and calculated fractionations
for all three mineral pairs for a temperature of 5008C.
Figure 7. Results of linear mixing model (A,
see Table 1) for serpentine d18O from
Group 2 samples (open diamonds); filled
circles are end-members in the mixing
model. (B) Best-fit correlation (handdrawn) between serpentine d18O and wholerock SiO2 is shown. These relations suggests
increasing serpentine d18O correlates with
greater equilibration with metasomatic fluids and greater extents of Si metasomatism
during serpentinization.
Group 3 rocks) and tremolite (from Group 4
rocks) in the ultramafic blocks consistently
yield temperatures of ;500 8C (Table 2). This
temperature is interpreted as a preserved equilibrium temperature, even though likely closure temperatures for serpentine and talc are
probably only ;300 8C, while tremolite is
probably near 500 8C (Kohn and Valley,
1998a). Diffusional exchange in Group 2–4
samples was probably limited in that the diagnostic mineral for each group dominates the
mineral mode. The preserved oxygen fractionations among different rocks suggest that the
Group 3 and 4 samples were most likely derived from pre-existing serpentinites due to
further extents of reaction progress with a single fluid at a rather constant temperature.
Collectively, all stable isotope thermometers considered in this study are consistent
with a single temperature of ;450–500 8C.
The simplest interpretation of this temperature
is that the peridotites were incorporated into
mélange at depth in the subduction channel,
then infiltrated and metasomatized at ;450–
500 8C. Brittle deformation of the rigid peridotite blocks in the viscous mélange matrix
created pathways for fluids, leading to the observed metasomatic zonations. No evidence
for metamorphism after the metasomatic event
is apparent, indicating that additional metamorphism of the blocks as they traveled upward (down P-T) within upwelling flow mélange (i.e., Cloos and Shreve, 1988) was
limited or absent. As the ultramafic blocks
traveled upward in the subduction channel due
to upwelling mélange, they were passed to different packets of mélange, each experiencing
lower peak P-T conditions in the subduction
channel in addition to upwelling to progressively higher structural levels in the subduction channel (cf. Cloos and Shreve, 1988). Ultimately, the blocks were transferred to a zone
of mélange that had only experienced peak
metamorphism in the subduction channel of
150–200 8C and 4–6 kbar (Ernst, 1988; Underwood and Laughland, 2001), with which it
was underplated to the hanging wall of the
Franciscan subduction zone. This scenario is
analogous to explanations for tectonic associations of higher-grade blueschist blocks in
lower-grade sedimentary mélange in the Franciscan Complex (Cloos and Shreve, 1988).
Subduction-Zone Metasomatism
Given the isotopic evidence for a single, homogeneous fluid affecting the ultramafic
blocks, it seems likely that the metasomatic
mineralogy of the blocks was produced by interaction with a fluid that was also homogeneous with respect to major solutes. If the sol-
ute load of the fluids was indeed homogeneous
during metasomatism, then apparent differences in the metasomatic mineralogy and geochemistry of the ultramafic blocks must have
been controlled by the ultramafic blocks themselves. That is, the whole-rock geochemistry of
each group was determined by the group’s bulk
crystal chemistry and by the activities of major
components buffered by the metasomatic mineralogy. Therefore, during reactive fluid flow,
as the ultramafic blocks attempted to reach
equilibrium with the infiltrating fluid, each
Group interacted with the fluid in a different
manner. Here we describe our interpretation of
the fluid-rock history and the corresponding
implications for subduction zone geochemistry
in the framework of the idealized reaction history preserved by the blocks.
The most obvious changes in major-element
geochemistry during the serpentinization of
the ultramafic blocks are the addition of water
and Si and removal of Ca. First, it is useful to
assess whether sufficient Si was present within
the peridotite for the serpentinization reaction
to go to completion without requiring an external source. Despite the aforementioned evidence for the addition of Si to the blocks, this
exercise is warranted in that the models of
Manning (1995, 1997) may underestimate the
Si budget for a peridotite since these models
are restricted to the system MgO-SiO2-H2O; of
most importance is the role of clinopyroxene
during serpentinization. Since Ca was efficiently removed from the blocks via fluids, additional Si for the serpentinization reaction
could have been derived from clinopyroxene.
This hypothesis was addressed by balancing
six end-member serpentinization reactions in
the system CaO-FeO-MgO-SiO2-H2O using
the modal abundances of olivine, orthopyroxene, and clinopyroxene combined with the
relative proportions of the Mg and Fe endmembers for each phase as measured by the
electron microprobe (Table 3). Spinel was as-
Geological Society of America Bulletin, September 2003
1105
KING et al.
TABLE 3. CALCULATIONS OF NET SiO2 DURING SERPENTINIZATION
Mineral end-member
Molar volume (cm3/mole)
Forsterite
Fayalite
Enstatite
Ferrosilite
Diopside
Hedenbergite
Modal proportion
43.79
46.39
31.44
33.00
66.09
68.30
0.72
0.08
0.09
0.01
0.093
0.007
Net moles SiO2†
21
3
1
3
4
6
Serpentinization reactions
3 Forsterite 1 1 SiO2(aq) 1 4 H2O 5 2 Serpentine
3 Fayalite 1 2 H2O 5 2 Magnetite 1 3 SiO2(aq) 1 2 H2
3 Enstatite 1 2 H2O 5 1 Serpentine 1 1 SiO2(aq)
3 Ferrosilite 1 1 H2O 5 1 Magnetite 1 3 SiO2(aq) 1 H2
3 Diopside 1 2 H2O 5 1 Serpentine 1 3 CaO(aq) 1 4 SiO2(aq)
3 Hedenbergite 1 1 H2O 5 1 Magnetite 1 3 CaO(aq) 1 6 SiO2(aq) 1 H2
Mineral end-member
Forsterite
Fayalite (to Srp)
Fayalite (to Mgt)
Enstatite
Ferrosilite (to Srp)
Ferrosilite (to Mgt)
Diopside
Hedenbergite (to Srp)
Hedenbergite (to Mgt)
TOTAL
Model 1: All Fe goes to magnetite
Moles/cm3
Net moles SiO2/cm3‡
16.4 3 1023
n/a
1.7 3 1023
2.9 3 1023
n/a
0.3 3 1023
1.4 3 1023
n/a
0.1 3 1023
25.47 3
n/a
1.73 3
0.97 3
n/a
0.3 3
1.87 3
n/a
0.2 3
–0.4 3
1023
1023
1023
1023
1023
1023
1023
Model 2: Serpentine Mg/[Mg1Fe] 5 0.96
Moles/cm3
Net moles SiO2/cm3†
16.4
0.9
0.9
2.9
0.2
0.2
1.4
0.04
0.06
3
3
3
3
3
3
3
3
3
1023
1023
1023
1023
1023
1023
1023
1023
1023
25.47
20.3
0.87
0.97
0.07
0.17
1.87
0.17
0.07
21.58
3
3
3
3
3
3
3
3
3
3
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
Note: Modal proportions of end-members calculated from microprobe compositions of olivine, orthopyroxene,
and clinopyroxene in Group 1 samples.
†
Negative values for net moles of SiO2 indicate net consumption of SiO2 by the reaction; positive values indicate
net production of SiO2 by the reaction.
‡
Totals for the net moles of SiO2/cm3 are calculated by multiplying the moles/cm3 of each mineral by the net
moles of SiO2 for each reaction. The net moles SiO2/cm3 are divided by 3 to account for the fact that 3 moles of
each mineral end-member are on the left side of each balanced equation.
sumed to be isolated from the system by chloritic coronas to simplify the calculations. Two
situations were considered: one in which all
Fe present is partitioned into magnetite, and a
second where sufficient Fe is partitioned into
serpentine to produce the observed serpentine
Mg/[Mg1Fe] composition of 0.96. The calculations demonstrate that, for these peridotites, additional Si is required for the reaction
to go to completion, but far less is needed than
is implied in a model MgO-SiO2-H2O system.
The fact that brucite was never observed in
Groups 1 or 2 suggests that during serpentinization, sufficient amounts of Si were probably available locally within the serpentinizing
peridotite from clinopyroxene so that the bruciteserpentine buffer of Manning (1995, 1997; Reaction 1) was never stable. These results from
the modeled CaO-FeO-MgO-SiO2-H2O reactions strengthen the evidence provided by the
isocon results and the coupled increases in
d18O and SiO2 (for Group 2) to indicate that
Si-metasomatism must have occurred during
serpentinization for the reaction to go to
completion.
It is difficult to reconcile the depletion of
Ca from the blocks without a significant fluid
flux out of the blocks. Other elements were
probably affected similarly during serpentini-
1106
zation if they were sufficiently soluble in the
fluid (e.g., Na), and fluid-mobile trace elements were also likely removed by the fluid.
This is the first of several significant implications of the metamorphic history for metasomatic processes in the mantle wedge: fluids
advancing ahead of the serpentinization front
may be substantially enriched in elements derived from peridotite that are incompatible in
serpentinite. The only species that will be partitioned out of the infiltrating fluid during serpentinization will be water, Si, and perhaps trace
elements that are compatible in serpentine-group
minerals, such as lithium, boron, and chlorine
(Grew, 1996; O’Hanley, 1996).
The formation of talc from serpentine in the
ultramafic blocks undoubtedly removed significant amounts of Si from solution. In addition, Al and Ti appear to have been incorporated into the rock as well. These two
elements must be contained in chlorite. Observations show that no other elements have
been partitioned into the rock at this stage of
metasomatism, and this is most likely due to
the absence of suitable mineral hosts. While
these chemical effects are obvious, they probably had little impact on the dissolved chemical budget of other elements in fluids. The
most important aspect of the talc-in reaction
front is that approximately half of the structurally bound water in the system is remobilized. Even though this reaction progresses
due to an influx of fluid, the reaction is controlled by the concentration of aqueous Si and
is a dehydration reaction (Reaction 2). This is
an important aspect of the metamorphic history for the fluid budget and is the next significant implication for fluid processes in subduction zones. The additional water liberated
by the talc front will effectively dilute the concentration of dissolved species in the fluid, enhancing the ability of the fluid to leach out
additional amounts of components from the
rock that are not compatible with serpentine
or talc. This process may take on even more
importance if slab-derived boron or lithium
are present in the serpentinite; the combined
effects of liberated water from the talc-in reaction, the generally lower concentrations of
these elements in talc as compared to serpentine (Grew, 1996), and recognized isotopic
fractionation of these elements by fluid-rock
interactions in nature (e.g., Palmer and Swihart, 1996; Chan and Kastner, 2000) imply
that these tracers of slab inputs to the volcanic
arc may behave in a much more complicated
manner in the mantle wedge than previously
considered.
The Group 4 samples underwent reactions
not previously considered in metasomatic
models due to limits in solubility and thermodynamic data. The aqueous SiO2 buffering
reactions of Manning (1995, 1997; Reactions
1, 2) provide a framework in which it is possible to write a similar reaction for the production of tremolite from talc. Given the presence of CaCO3 veins in the ultramafic blocks,
we propose that the form of the talc to tremolite reaction in the system CaO-MgO-AlO3/2SiO2-H2O (-CO2) is:
5Mg3Si4O10 (OH)2 1 4SiO2(aq) 1 6CaCO3(aq)
5 3Ca2Mg5Si8O22 (OH)2 1 2H2O
1 6CO2 .
(4)
This reaction has the same form as the
aqueous SiO2 buffers of Manning (1995,
1997), where SiO2 is added to the rock and a
new mineral is formed by a dehydration reaction. Note that the Ca required for the reaction need not be derived from carbonate.
Several features of rocks that underwent the
tremolite-in reaction suggest that they have
approached chemical equilibrium with the
metasomatic fluids. Most obvious is the increase in mineralogic diversity, which allows
the rocks to buffer the chemical potentials of
Geological Society of America Bulletin, September 2003
SUBDUCTION ZONE METASOMATISM OF FRANCISCAN COMPLEX ULTRAMAFIC BLOCKS
many more elements. The mineralogic diversity may indicate that the chemical potentials
of components in the fluid and rock are less
different at this stage than in earlier stages of
metasomatism, leading to at least a quasi-rock
buffered system. In addition, the large number
of elements added to the overall system by
fluids suggests that the aSiO2 was high enough
for minerals such as tremolite, clinozoisite,
and zircon to become stable. We suggest that
aSiO2 was the dominant control on the formation of the accessory minerals, but other elements, such as Ca, Al, Ti, Zr, etc., must have
been in sufficient concentrations to stabilize
them.
We reach several conclusions about the geochemistry of subduction zones based on properties of the Group 4 mineral assemblage. The
first is that many elements traditionally held
to be ‘‘immobile’’ in slab-derived fluids must
be at least modestly soluble. Specifically, high
field-strength elements such as Zr and Ti appear to have been added to the blocks by fluids. Such a conclusion supports experimental
evidence for rutile solubility in subduction
zone fluids (Ayers and Watson, 1993) as well
as several studies of subduction zone geochemistry, which have found evidence for mobility of high field-strength elements in eclogitederived fluids (Philippot and Selverstone,
1991) and Hf isotopic evidence from arc volcanic rocks for slab additions of high fieldstrength elements to the sub-arc mantle
(Woodhead et al., 2001). Given our evidence
for at least modest high field-strength element
mobility in subduction zone fluids, we suggest
that additions of these elements to the sub-arc
mantle are generally not reflected in arc volcanics due to incorporation in minerals within
Group 4-like rocks along the slab-mantle interface. This conclusion is similar to models
suggested by Hofmann (1988) and Ionov and
Hofmann (1995), in which slab-derived high
field-strength elements are retained in metasomatic phases stabilized in the mantle wedge.
Similarly, our second important conclusion
from the accessory mineral assemblage in
Group 4 is their potential ability to fractionate
many trace elements in addition to high fieldstrength elements from fluids. For example, titanite is capable of partitioning both high
field-strength elements and rare earth elements
from solution (Tiepolo et al., 2002), zircon
may sequester U and high field-strength elements from fluids, while the presence of clinozoisite and apatite suggests that the stability
of allanite and monazite is possible, given sufficient concentrations of rare earth elements in
fluids. Thus, the accessory minerals in Group
4 suggest the possibility that the trace element
Figure 8. Proposed geochemical behavior of the ‘‘metamorphic buffer.’’ Metasomatic reactions between slab-derived fluids and the ultramafic mantle wedge will follow a specific
series of reactions, creating mineral zonations similar to those observed for ultramafic
blocks in this study. Each mineral zone will interact differently with presumably homogeneous fluids derived from metamorphic reactions occurring within subducting
lithosphere.
budget of slab-derived fluids may be susceptible to fractionation in this zone once it has
become stable along the slab-mantle interface.
Metasomatic zones of Group 4-like rocks
may, therefore, potentially act to change the
character of slab-derived fluids, so that volcanic rocks do not truly sample slab additions
to the mantle wedge. This conclusion is similar to the ‘‘zone refining’’ of slab-derived fluids within the mantle wedge in the trace element modeling of Ayers (1998) and only
differs in that the alteration of fluid chemistry
is here considered to be an effect of metasomatic minerals in the mantle wedge rather
than movement of fluids that continuously reequilibrate with peridotite mineral assemblages during ascent to the arc magma source region (Ayers, 1998).
The cumulative effect of these mineral
zones changes the trace element chemistry of
fluids progressing to regions of the mantle farther from the slab-mantle interface (Figs. 8
and 9). We propose that this system, the
‘‘metamorphic buffer,’’ is a fundamental component in the geochemistry of subduction
zones. Two main factors will determine the
geometry and mineralogy of the metamorphic
buffer: 1) at shallow, forearc levels of a subduction zone, the metamorphic buffer will be
less extensive due to lower Si solubility at
these P-T conditions (Manning, 1995, 1996,
1997) but will grow in extent as Si solubility
increases as a function of both P and T (i.e.,
the metasomatic reaction fronts will progress
farther into the mantle for an identical fluid
flux); and 2) at deeper levels, the antigoriteout reaction at temperatures near 600 8C will
limit the down-dip extent of the metamorphic
buffer as proposed here. The buffering reac-
tions are likely to have a significant impact on
the geochemistry of arc magmas in more mature subduction zones where older oceanic
lithosphere or faster subduction rates lead to
greater depression of isotherms (e.g., Peacock,
1996). However, regardless of fluid flux, Si
concentration in fluids, or buffer geometry, a
metamorphic buffer should always exist along
the slab-mantle interface and exert a major influence on fluid and, therefore, arc geochemistry. Furthermore, if viscous coupling of hydrated mantle to the subducting slab occurs
(i.e., Tatsumi and Eggins, 1995), then entrainment of slab-derived components in the metamorphic buffer (i.e., in Group 4-like rocks)
and subsequent sub-solidus breakdown or prograde devolatilization of this layer at sub-arc
depths may promote arc magma formation. A
test of this model may be provided by detailed
geochemical studies of ultrahigh-pressure
metamorphic suites. If, for example, the fluids
in equilibrium with ultrahigh-pressure metamorphic rocks at sub-arc depths are inconsistent with those required to produce arc magmas, then an alternative source is needed (i.e.,
potentially, a down-dragged metasomatic peridotite; Tatsumi and Eggins, 1995).
The metasomatic processes hypothesized by
this study to occur along the slab-mantle interface are likely to be merely one component
in a much more complex system. Metasomatic
products such as serpentine and talc will
strongly influence the mechanical behavior of
the slab-mantle interface (e.g., Reinen et al.,
1991; Peacock and Hyndman, 1999), and ultimately promote distributed deformation and
mechanical mixing of slab and mantle materials, which will also lead to modification of
fluid chemistries (Bebout and Barton, 2002).
Geological Society of America Bulletin, September 2003
1107
KING et al.
Figure 9. Potential example of a metamorphic buffer in the Cascadia subduction zone,
imaged by Zhao et al. (2001) using seismic P-wave tomography (modified after Fig. 8 in
Zhao et al., 2001). (A) Surface heat flow for a portion of the Cascadia subduction zone.
(B) Interpretations of Zhao et al. (2001) are shown for low P-wave velocity regions above
the slab; note that low P-wave velocity zone extends along strike of subduction zone. (C)
Possible interpretation of low P-wave velocity zones based upon this study. Fields marked
23% and 26% are percent deviations from mantle P-wave velocity.
We have highlighted the significance of metasomatic processes in this contribution in that
the synergetic combination of a steep chemical gradient between subducted material and
the mantle, high solubility of many major elements in fluids (e.g., Manning, 1998; Manning et al., 2001), and the migration paths of
fluids from the slab to the mantle (e.g., Peacock, 1993b; Manning, 1995, 1997) will lead
to the development of mineral zonations similar to those we have observed even in areas
where mechanical mixing may be limited.
Further studies are required for a more complete understanding of the varied processes
that occur along the slab-mantle interface,
from which it will be possible to more accurately understand magmatism in convergent
margins.
ACKNOWLEDGMENTS
This study presents the results of Robert L.
King’s B.A. and M.S. theses. L. Reinen, C. Davidson, L. Bettison-Varga, R. Varga, J. Shervais, J.
Knapp, C. Parkinson, A. Wilson, J. Ryan, G. Bebout, J. Wakabayashi, and C. Manning provided
constructive discussions or reviews of earlier versions of this study. Formal reviews by J. Ayers, M.
Cloos, and associate editor C. Miller substantially
improved the clarity and presentation of the manu-
1108
script. Support was provided by the Keck Geology
Consortium, the University of South Carolina, and
National Science Foundation grant EAR0073803 (to
Matthew J. Kohn).
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REVISED MANUSCRIPT RECEIVED 9 FEBRUARY 2003
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1109