Geochimica et Cosmochimica Aca Vol. 57,pp. 2227-2237 Copyright D 1993Pergamon PressLtd. Printedin U.S.A. 0016-7037/93/S6.00 + .00 B-Be systematics in subduction-related metamorphic rocks: Characterization of the subducted component GRAY E. BEBOUT, ' * JEFFREY G. RYAN, 2t and WILLIAM P. LEEMAN 3 'Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA 2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA 3 Keith-Wiess Geological Laboratories, Rice University, Houston, TX 77251, USA (Received April 24, 1992; acceptedin revisedform November 18, 1992) Abstract-The mobility of B and Be in H 2 0-rich fluids and felsic silicate liquids produced during meta- morphism of subducted oceanic slab and sediments has been investigated through analysis of subduction- zone metamorphic rocks of the Catalina Schist, California. In metasedimentary rocks, B/Be and the range in B/Be decrease with increasing metamorphic grade (mean = 72, std. dev. = 41 for lowest-grade lawsonite-albite facies rocks; mean = 21, std. dev. = 11 for higher-grade greenschist and epidote-amphibolite facies equivalents). This decrease to more uniformly low B/Be may be attributed to the preferential removal of B in H 2 0-rich fluids produced by devolatilization reactions over the approximate temperature interval of 350-600'C. Metamafic rocks do not show pronounced decrease in B/Be with increasing metamorphic grade; however, all metarnafic samples have B /Be < 30, lower than values for many altered seafloor basalts. In amphibolite-grade exposures, felsic leucosomes and pegmatites reflecting partial melting have low B/Be similar to their metasedimentary and metamafic hosts, which presumably experienced prior reduction in B/Be during lower temperature devolatilization. This evidence for B and Be mobility during high-P/T metamorphism complements studies of B-Be systematics in arc volcanic rocks in further characterizing mechanisms by which slab-derived elements can be added to the source regions of arc lavas. Before subducted mafic and sedimentary rocks reach Wadati-Benioffzone depths beneath arcs (80- 150 km), the B/ Be of these rocks is likely to have decreased to <30. Thus, highly fractionated, slab-derived hydrous fluids may be necessary to generate the high-B/ Be signatures observed in many arcs (B/Be of up to -200). The B-Be data, together with previously presented stable isotope data for the Catalina Schist, demonstrate that subduction-zone metamorphic processes are capable of homogenizing presubduction variability in the concentrations of particularly "fluid-mobile" elements in rocks and may, through mixing, produce fluids which trend toward uniform trace element and isotopic compositions. These homogeneous fluids could infiltrate parts of the mantle wedge and contribute to the characteristic trace element and isotopic signatures of arc-magma source regions. In hotter subduction zones (e.g., involving subduction of young, hot oceanic lithosphere), silicate melts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low- B/Be signatures to arc source regions. Thus, significant variations, among arcs, in the ranges of B/Be observed in front-rank volcanoes (e.g., for the Bismarck arc, B/Be = 20-190; for the Cascades arc, B/ Be < 5) may be related in part to varying thermal structure, which could govern both the B/Be of hydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subducted slab and sediments. INTRODUCTION ENRICHMENTS IN B AND ' 0 Be in arc volcanic rocks have been interpreted to reflect slab additions to arc source regions via hydrous fluids or silicate liquids (MORRIS et al., 1990a; RYAN and LANGMUIR, 1993). The two-component mixing relations observed for ' 0 Be/ 9 Be and B/ 9 Be (MORRIS et al., 1990a) are believed to result from the addition of a homogenized, slab- derived component (probably a hydrous fluid which strongly fractionates B from Be). The concentrations of B and Be in seafloor sediments and oceanic crustal materials believed to be subducted are reasonably well characterized (Fig. I), and B and Be data sets for arc volcanic rocks (TERA et al., 1986; RYAN and LANGMUIR,1988, 1993; MORRIS et al., 1990a; LEEMAN et al., 1990b; PALMER, 1991) allow estimation of the volcanic output of these elements (for discussion of B, see, RYAN and LANGMUIR, 1993). Less is known, however, about the effects of slab metamorphism on the concentrations of B, Be, and other trace elements in subducted rocks (shaded region along slab-mantle interface in Fig. 1). These meta- morphic processes may dictate the efficiency with which each of these elements is transferred from oceanic reservoirs to arc source regions (see MORAN et al., 1992, for discussion of B recycling). Subduction-zone metamorphic processes may be responsible in part for the trace element fractionations observed among arc volcanic rocks (e.g., differences between the ranges of B/Be in first-rank volcanoes of different arcs, and variations in B/Be from first-rank to behind-the-front volcanoes across individual arcs; see MORRIS et al., 1990a). Subduction-zone metamorphic processes may also pro- foundly affect the trace element compositions of rocks sub- 2227 * Present address: Department of Earth and Environmental Sci- ences, Lehigh University, Bethlehem, PA 18015, USA. t Present address: Department of Geology, University of South Florida, Tampa, FL 33620, USA.
Transcript
Geochimica et Cosmochimica Aca Vol. 57, pp. 2227-2237Copyright D 1993 Pergamon Press Ltd. Printed in U.S.A.
0016-7037/93/S6.00 + .00
B-Be systematics in subduction-related metamorphic rocks:Characterization of the subducted component
GRAY E. BEBOUT, ' * JEFFREY G. RYAN, 2t and WILLIAM P. LEEMAN3
'Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
3 Keith-Wiess Geological Laboratories, Rice University, Houston, TX 77251, USA
(Received April 24, 1992; accepted in revisedform November 18, 1992)
Abstract-The mobility of B and Be in H 20-rich fluids and felsic silicate liquids produced during meta-morphism of subducted oceanic slab and sediments has been investigated through analysis of subduction-zone metamorphic rocks of the Catalina Schist, California. In metasedimentary rocks, B/Be and therange in B/Be decrease with increasing metamorphic grade (mean = 72, std. dev. = 41 for lowest-gradelawsonite-albite facies rocks; mean = 21, std. dev. = 11 for higher-grade greenschist and epidote-amphibolitefacies equivalents). This decrease to more uniformly low B/Be may be attributed to the preferentialremoval of B in H20-rich fluids produced by devolatilization reactions over the approximate temperatureinterval of 350-600'C. Metamafic rocks do not show pronounced decrease in B/Be with increasingmetamorphic grade; however, all metarnafic samples have B /Be < 30, lower than values for many alteredseafloor basalts. In amphibolite-grade exposures, felsic leucosomes and pegmatites reflecting partial meltinghave low B/Be similar to their metasedimentary and metamafic hosts, which presumably experiencedprior reduction in B/Be during lower temperature devolatilization.
This evidence for B and Be mobility during high-P/T metamorphism complements studies of B-Besystematics in arc volcanic rocks in further characterizing mechanisms by which slab-derived elementscan be added to the source regions of arc lavas. Before subducted mafic and sedimentary rocks reachWadati-Benioffzone depths beneath arcs (80- 150 km), the B/ Be of these rocks is likely to have decreasedto <30. Thus, highly fractionated, slab-derived hydrous fluids may be necessary to generate the high-B/Be signatures observed in many arcs (B/Be of up to -200). The B-Be data, together with previouslypresented stable isotope data for the Catalina Schist, demonstrate that subduction-zone metamorphicprocesses are capable of homogenizing presubduction variability in the concentrations of particularly"fluid-mobile" elements in rocks and may, through mixing, produce fluids which trend toward uniformtrace element and isotopic compositions. These homogeneous fluids could infiltrate parts of the mantlewedge and contribute to the characteristic trace element and isotopic signatures of arc-magma sourceregions. In hotter subduction zones (e.g., involving subduction of young, hot oceanic lithosphere), silicatemelts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low-B/Be signatures to arc source regions. Thus, significant variations, among arcs, in the ranges of B/Beobserved in front-rank volcanoes (e.g., for the Bismarck arc, B/Be = 20-190; for the Cascades arc, B/Be < 5) may be related in part to varying thermal structure, which could govern both the B/Be ofhydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subductedslab and sediments.
INTRODUCTION
ENRICHMENTS IN B AND '0Be in arc volcanic rocks have beeninterpreted to reflect slab additions to arc source regions viahydrous fluids or silicate liquids (MORRIS et al., 1990a; RYANand LANGMUIR, 1993). The two-component mixing relationsobserved for '0Be/9Be and B/9 Be (MORRIS et al., 1990a) arebelieved to result from the addition of a homogenized, slab-derived component (probably a hydrous fluid which stronglyfractionates B from Be). The concentrations of B and Be inseafloor sediments and oceanic crustal materials believed tobe subducted are reasonably well characterized (Fig. I), andB and Be data sets for arc volcanic rocks (TERA et al., 1986;
RYAN and LANGMUIR, 1988, 1993; MORRIS et al., 1990a;LEEMAN et al., 1990b; PALMER, 1991) allow estimation ofthe volcanic output of these elements (for discussion of B,see, RYAN and LANGMUIR, 1993). Less is known, however,about the effects of slab metamorphism on the concentrationsof B, Be, and other trace elements in subducted rocks (shadedregion along slab-mantle interface in Fig. 1). These meta-morphic processes may dictate the efficiency with which eachof these elements is transferred from oceanic reservoirs toarc source regions (see MORAN et al., 1992, for discussionof B recycling). Subduction-zone metamorphic processes maybe responsible in part for the trace element fractionationsobserved among arc volcanic rocks (e.g., differences betweenthe ranges of B/Be in first-rank volcanoes of different arcs,and variations in B/Be from first-rank to behind-the-frontvolcanoes across individual arcs; see MORRIS et al., 1990a).Subduction-zone metamorphic processes may also pro-foundly affect the trace element compositions of rocks sub-
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* Present address: Department of Earth and Environmental Sci-ences, Lehigh University, Bethlehem, PA 18015, USA.
t Present address: Department of Geology, University of SouthFlorida, Tampa, FL 33620, USA.
Fib. 1. Schematic illustration of a continental margin-type subduction zone and the various B and Be reservoirs atsuch convergent margins. Indicated are ranges in B and Be concentration and B/ Be of fresh and altered MORB (RYANand LANGMUIR, 1988, 1993; MORAN et al., 1992; SPIVACK and EDMOND, 1987), seafloor sediments (RYAN andLANGMUIR, 1988, 1993; MORAN et al., 1992), seawater (SCHWARCZ et al., 1969), and arc volcanic rocks (MORRIS etal., 1990a; RYAN, 1989; RYAN and LANGMUIR, 1993; LEEMAN et al., 1990b). Concentrations in continental crust areextremely variable but are believed to fall in the ranges of -5 to 100 ppm (or more) B and I to 3 ppm Be (B/Be maybe extremely high; MORRIS et al., 1 990a; LEEMAN et al., 1992). Values for the mantle wedge are also less certain, butare inferred from MORB data to range from 0.05 to 0.10 ppm B and 0.04 to 0.06 ppm Be, with B/Be of - Ito 3(RYAN and LANGMUIR, 1988, 1993). Subduction-zone metamorphism at depths shallower than those beneath arcs(80 to 150 km; CRAWFORD et al., 1989; shaded area along slab-mantle interface) results in selective removal of B andreduced B/Be ratios (probably to <30). Parentheses indicate most reasonable ranges, omitting extremes.
ducted to greater depths in the mantle beyond arcs (HARTand STAUDIGEL, 1989; MORRIS et al., 1 990a; HART and REID,1991).
To directly assess the mobilities of B and Be and othertrace elements during progressive metamorphic devolatiliza-tion and partial melting, we have investigated trace elementsystematics in the Catalina Schist, a subduction-relatedmetamorphic complex exposed on Santa Catalina Island insouthern California. The Catalina Schist is well-suited forthis study as it contains metamorphosed sedimentary rocks(largely sandstones and shales) and mafic rocks (basalts andgabbros) ranging in grade from lawsonite-albite to amphib-olite facies (inferred metamorphic conditions of 350° to750'C, 5-11 kbar; PLATT, 1975; SORENSEN and BARTON,1987; BEBOUT, 1989, 199 la). Thus, the effects of progressivemetamorphism on trace element and stable isotope com-positions may be examined (BEBOUT and BARTON, 1989;BEBOUT, 1991 a; BEBOUT and FOGEL, 1992). The rock types(e.g., metamorphosed trench and pelagic sedimentary rocksand seafloor basalts) are characteristic of those in many sub-duction-zone metamorphic terranes (see BEBOUT, 199 Ia) andthose observed unmetamorphosed in deep-sea cores fromtrench and off-trench environments (e.g., Shipboard ScientificParty, 1991 ). Evidence for fluid transport and associated masstransfer during metamorphism includes the occurrence ofveins, reaction zones between disparate lithologies, changesin bulk chemical composition, and changes in isotopic com-position (BEBOUT and BARTON, 1989). Stable isotope andpetrologic studies of the Catalina Schist have yielded evidencefor progressive devolatilization and km-scale transport ofH20-rich C-O-H-S-N fluid during metamorphism (molefraction H20 perhaps > 0.98; BEBOUT and BARTON, 1989;
BEBOUT, 1991 a; BEBOUT and FOGEL, 1992). Pegmatites inthe amphibolite unit are interpreted to represent the productsof vapor-saturated partial melting of sedimentary and maficrocks (SORENSEN and BARTON, 1987; BEBOUT and BARTON,1989).
We present B and Be concentration data for metamor-phosed sedimentary and mafic rocks, veins, and pegmatitesfrom the Catalina Schist. We discuss the principal minera-logical reservoirs for these elements and the extent to whichB and Be were mobilized during progressive devolatilization(i.e., dehydration) reactions and partial melting. Using themean concentration data for the Catalina Schist metasedi-mentary rocks, we calculate rock and fluid B-Be systematicsand discuss potential consequences of B and Be fluid-rockpartitioning for the B/Be evolution of subducted rocks andof hydrous fluids released by devolatilization reactions duringthe prograde metamorphism of these rocks. Finally, we dis-cuss possible implications of the inferred relative mobilitiesof B and Be during metamorphic processes in the CatalinaSchist for interpretation of the B-Be systematics in arc vol-canic rocks.
BORON AND BERYLLIUM DATA
Boron and Be concentration data for ninety-six metasedimentary,metamafic, and metaultramafic rocks, mineral separates, veins, andpegmatites (Table I) were obtained by inductively coupled plasmaemission spectrometry (ICP) techniques following Na2CO3-fluxedfusions (see RYAN, 1989; RYAN and LANGMUIR, 1993). Analyticalprecision for the techniques is +5% for Be; for B, precision is ±5%at concentrations greater than 10 ppm and ± 10% for concentrationsof less than 1o ppm. Comparative analyses for B in selected sampleswere obtained by prompt-gamma neutron activation (see LEEMAN,
1988): all analyses by the two methods agree to within 10%, andmost to within 5%.
Seafloor-Altered MORBB = 0.3 to 300/pm (0.3 to 40)
*fine-grained interlayer in layered metasandstone/metashale sequencefcoarse-grained interlayer in layered metasandstone/metashale sequence**retrograde vein in epidote-amphibolite grade metamafic exposurettprograde albite-graphite vein from blueschist grade metasedimentary exposure***other major/trace element data in BEBOuT (1989) and SORENSEN and GROSSMAN (1989)
Boron concentrations and B/Be of metasedimentary rocks decreaseprogressively with increasing metamorphic grade (Figs. 2a, 3) andcorrelate with a decrease in H 20 content which reflects progressivedevolatilization over an approximate temperature interval of 350°to 600°C (BEBOUT, 199 la). The decrease in B content is consistentwith results for other metamorphic suites which show a wide rangein grade (e.g., SHAW et al., 1988; NABELEK et al., 1990; MORAN etal., 1992). Lowest-grade lawsonite-albite rocks (inferred metamorphicconditions of -350°C, 5 to 8 kbar) range in B content from 12-181 ppm B with a mean of 73 ppm (std. dev. = 55 ppm). Highergrade greenschist and epidote-amphibolite metasedimentary rocks(excluding samples 6-341 A, B; see following text) range from 1.3-30 ppm B with a mean of 15 ppm (std. dev. = 8 ppm). Highest-grade amphibolite equivalents (inferred conditions of 6500 to 750°C,8-11 kbar) range from 4.0-19 ppm B with a mean of 11 ppm (std.dev. = 6 ppm). Thus, both the absolute concentrations of B and thevariability within an individual grade decrease with increasing meta-morphic grade. Mean Be concentration in the rocks decreases from0.95 ppm (std. dev. = 0.31 ppm) in the lowest-grade lawsonite-albite
grade rocks to 0.70 (std. dev. = 0.26 ppm) in the greenschist andepidote-amphibolite facies rocks (excluding samples 6-3-41A and B,which have unusually high Be contents of >4.0 ppm) and 0.63 (std.dev. = 0.24 ppm) in the highest grade amphibolite facies rocks. How-ever, the metasedimentary rocks at each grade show similar rangesin Be concentration (-0.25 to 1.25 ppm Be; see Fig. 2b). The decreasein B/Be from a mean of -72 for the lawsonite-albite grade meta-sedimentary rocks to a mean of -21 for greenschist and epidote-amphibolite grade equivalents can be explained by selective loss ofB (Fig. 3). Twelve samples of metamafic rocks contain from 3-60ppm B and from 0.2-2.0 ppm Be (overlapping the range for Be inmid-ocean ridge basalt; see RYAN and LANGMUIR, 1988); the lowervalues for both elements are, in general, for the highest grade (am-phibolite facies) rocks. Metamafic B/Be shows no significant variationwith increasing metamorphic grade and, with only one exception (aglaucophanic greenschist sample with B/Be of 30), ranges from 3-18 (Fig. 3).
In order to decipher the behavior of H and Be during devolatilizationand partial melting, the mineral reservoirs in which these elements
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G. E. Bebout, J. G. Ryan, and W. P. Leeman
I I I * l in the rock relative to B(cf. BusHLYAKOv and GRIGOR'YEV, 1988).so , a Berylliun. concentrations of muscovite and feldspar separates from
b 3 wthe metasedimentary rocks and pegmatites are similar to those of theLawsonte-Albie (31to5.5 wt. % H 20) whole-rock samples (see Table 2), indicating relatively uniform par-
Blueschist (3.2 to 5.4 wt. % H2 0) titioning of Be among the various minerals in the rocks (compared
GreenSChist*Epidole with the distribution of B) .Other studies of Be distributions in igneousAmphlbolite (1.0 to 3 w % H 2°) and metamorphic rocks have documented that significant, systematic
variations in Be concentration exist among coexisting silicate minerals(~I Amphibolite (1.0 to 3.0 wt. % H 2°) ((see WUENCH and HORMAN, 1978; DOMANIK, 1992). In a study of
' ' 'B and Be distributions in subduction-related metamorphic rocks (in-0 40 80 120 160 200 cluding some Catalina Schist samples), DOMANIK (1992) docu-
Boron (ppm) mented that Be is enriched in white micas and biotite, amphibole,lawsonite, and omphacite, and that Be occurs at only low concen-trations in plagioclase, quartz, calcite, clinozoisite/zoisite, garnet, il-
b mentite, apatite, sphene, and rutile.Lawsonite-Aibite (a) 0 @To further evaluate B-Be hydrous fluid-mineral and melt-mineral
partitioning, we measured the B and Be concentrations of veins whichBlueschist @precipitated from H2 0-rich fluids at blueschist conditions (BEBOUT
and BARTON, 1989), and of pegmatites produced by migmatization
GrenAmphibslite processes during amphibolite-grade metamorphism (SORENSEN andBARTON, 1987; BEBOUT, 1989; BEBOUT and BARTON, 1989). In
Amphibolite (Z (ID Q blueschist metasedimentary exposures, nearly monomineralic sodic-I I I I I amphibole veins have B and Be contents (7.5-66 ppm and 0.17-1.60 0.50 1.00 1.50 ppm, respectively) and mean B/Be of 33 ppm (std. dev.= 12 ppm;
n = 9) similar to those of host rocks (Fig. 4; host rock mean B/BeBeryllium (ppm) = 33 ppm, std. dev. = 11 ppm; M = 8). One albite-graphite vein
contains 4.6 ppm B and 0.13 ppm Be and has B/Be of '-35, whichFIG. 2. Boron and beryllium concentrations in whole-rock meta- is also similar to that of the surrounding blueschist grade metasedi-
sedimentary samples from the Catalina Schist (see Table I ). (a) B mentary rocks. The low B concentration in this vein is consistentconcentration data. Concentrations are as follows: for fourteen law- with the low B concentrations in feldspar mineral separates (Tablesonite-albite facies rocks, mean = 73, std. dev. = 55 ppm; for eight 2). Pegmatites in the amphibolite unit show wide ranges in B contentblueschist facies rocks, mean = 25, std. dev. = 16 ppm; for thirteen (excludingsample 7-3-23, 6.6-37 ppm; n = 7)and Be content(0.66-greenschist and epidote amphibolite facies rocks, mean = 15 ppm, 4.1 ppm; M = 7), but have relatively uniform B/Be similar to orstd. dev. = 8 ppm; for seven amphibolite facies rocks, mean = II slightly lower than that of their mafic and sedimentary hosts (Fig.ppm, std. dev. = 6 ppm. Despite metamorphism to temperatures 4). A tourmaline-bearing pegmatite (sample 7-3-23) contains 970near 350'C, the lowest grade lawsonite-albite facies rocks retained a ppm B and 2.1 ppm Be (with a B/Be of -460).large fraction of their protolithic B, as judged by the similarity in Bconcentration range with that of seafloor sediments (range of -50
to 200 ppm); the large range in B concentration of the low-graderocks probably reflects variability in sedimentary protoliths. Note thedecrease in the mean and standard deviation of the B concentrationswith increasing metamorphic grade. Also shown, for each metamor-phic grade, are ranges in H 2 0 content obtained using standard hy-drogen isotope extraction techniques; (b) Be concentration data.Concentrations are as follows: for fourteen lawsonite-lbite faciesrocks, mean 0.95, std. dev. = 0.31 ppm; for eight blueschist faciesrocks, mean = 0.77, std. dev. = 0.36 ppm; for thirteen greenschistand epidote amphibolite facies rocks, mean = 0.70 ppm, std. dev.= 0.26 ppm; for seven amphibolite facies rocks, mean = 0.63 ppm,std. dev. = 0.24 ppm.
reside must be known. Table 2 contains data for whole-rock andmineral separate samples from several high-grade metasedimentaryrocks and pegmatites. These data are compatible with the preferentialincorporation of B in white mica; for all but sample 7-3-23, whichcontains tourmaline, muscovite separates have higher B than therespective whole-rock samples. Enrichment of B in mica (white micaand, where present, biotite) in metasedimentary (and metamafic)rocks of the Catalina Schist is further indicated by the ion microprobedata of DOMANIK et al. ( 1991 ) and A. E. Bebout (unpubl. data);A. E. BEBOUT et al. (1992) demonstrated enrichment of B in micasin similar rocks of the Pelona Schist metamorphic complex, Cali-fornia. Feldspar does not appear to strongly incorporate B, as B con-centrations are low for two feldspar separates relative to whole-rockconcentrations (see Table 2); this finding is also compatible with theion microprobe results of DOMANIK et al. ( 1991 ), A. E. Bebout (un-publ. data), and A. E. BEBOUT et al. (1992). With the exception ofseveral occurrences in felsic pegmatites in the amphibolite unit andthe greenschist/epidote-amphibolite-grade metasedimentary rocks,tourmaline is not present to host B in the Catalina Schist. Berylliumappears to be somewhat more evenly distributed among the minerals
DISCUSSION
Boron and Be concentrations in the Catalina Schist reflectthe differential removal of B and Be from subducted sedi-mentary and mafic rocks by H20-rich fluids and felsic silicateliquids. The B-Be signatures of these two fluids were appar-ently dramatically different. The H20-rich fluids producedduring progressive devolatilization are inferred to have hadcomparatively high B/Be, because their removal resulted ina decrease in the B/Be of the rocks (Figs. 2a, 3). If the peg-matites are directly representative of silicate liquid compo-sitions produced during melting (see discussions in SORENSENand BARTON, 1987; BEBOUT and BARTON, 1989; SORENSENand GROSSMAN, 1989), the silicate liquids had B/ Be similarto those of the host rocks. The relatively low B/Be of thepegmatites may thus reflect melting of rocks which had al-ready experienced selective B loss during devolatilization(Figs. 2a, 3).
Fluid-Rock Mass Balance of Boron and Beryllium
Observations regarding B and Be behavior during pro-gressive devolatilization of the metasedimentary rocks of theCatalina Schist may allow some constraints to be placed onthe relative partitioning of B and Be between the hydrousfluids and the rocks and, thus, the B/Be of the removed hy-drous fluid component. A variety of models for loss of B andBe can be envisaged. Important factors in these models in-clude the bulk rock-fluid partition coefficients (which arepresumably dependent on pressure, temperature, and vari-
B-Be systematics in subduction-related metamorphic rocks
a)mm
11 10 100 1000
B (ppm)
FIG. 3. B/Be vs. B content of metasedimentary rocks (labeled unshaded fields) and metamafic rocks (shaded field).Boron and B/Be of metasedimentary rocks decrease with increasing metamorphic grade; the B loss accompanies H2 0loss attributed to devolatilization reactions over the approximate temperature interval of 3500 to 600 0C (BEBOUT,1991a; see Fig. 2a). Beryllium concentration in these rocks shows no systematic trend with increasing metamorphicgrade. With the exception of one glaucophanic greenschist sample with 60 ppm B and 2.0 ppm Be (sample 8-3-4; B/Be of 30; box with same shaded pattern), all of the measured metamafic rocks have B/Be • 18. Because seaflooraltered basaltic rocks can contain up to 300 ppm, it is likely that some metamafic rocks lost significant B duringdevolatilization at temperatures lower than those recorded by the Catalina Schist ( <350°C; MORAN et al., 1992, reportevidence for low-temperature loss of B in other metamafic suites).
ations in the modal abundances of minerals) and the mech-anisms of fluid-rock exchange. Loss of hydrous fluid duringcontinuous metamorphic reactions with efficient removal offluid may approximate Rayleigh distillation (sequential re-moval of infinitesimal aliquots of fluid equilibrated with therock), whereas B and Be loss during discontinuous meta-morphic reactions, particularly with inefficient fluid removal,may approximate a multi-batch volatilization process (se-quential removal of relatively larger fluid aliquots equilibratedwith the rock). A process similar in behavior to Rayleighdistillation (or multi-batch volatilization) might, in general,be expected for cases where continuous and discontinuous
reactions occur and fluid release is controlled by the rheologyof the rocks and pore fluid pressures (see discussions byWALTHER and ORVILLE, 1982; BEBOUT and FOGEL, 1992).Modelling of B and Be losses purely as closed-system volatiledistillation (loss of fluid released by devolatilization reactionswithout influx of externally derived fluid) is very likely anoversimplification, as the rocks may have exchanged withfluids introduced from external sources. A large number ofexchange and enrichment/depletion scenarios can be imag-ined for open system cases (see NABELEK, 1987; BANNERand HANSON, 1990). The impact of externally derived fluidswould depend largely on the degree to which the fluids had
Table 2. Boron and beryllium concentrations of mineral separates (in ppm).
not determinedttourmnaline-bearing pegmatite from metasedimentary exposure
223 1
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I I I ' ' 'alina Schist for DB/DB, of less than approximately 5 and forem * * Blueschist Metasedimentary the appropriate amount of B loss (80%). Figure 5b shows
c Blueschist Veins that, for the same calculations, the rock B/Be is decreasedfrom the average value for low-grade metasedimentary rocksto the average B/Be for the high-grade rocks by B and Be
Coo Host-Rocks k Metasedimena removal at DB/DBe of approximately 5. Together, Fig. 5a and
( 0 egmatites 0 Metamafic b demonstrates that the use of a DB/DBe of -5 or greater_ I I I I ,_,_,_,_,_,_(because the value of 25% Be loss is considered to be a max-0 20 40 60 80 100 imum) will satisfy the averaged B-Be data for the Catalina
B/Be Schist metasedimentary rocks. Recognizing the limitationsimposed by protolith variability (i.e., variability in B and Be
FiG.4. ompnsos o B/e o veis ad pgmaite wih tose concentrations and B/ Be of the low- and high-grade samples ),'host-rocks from the blueschist and amphibolite units (see data inible I and discussion in text). it appears that the use of DB/DBe of less than -5 would be
incapable of producing the observed decrease in B concen-tration and B/Be of the rocks. For the same calculations,
reviously equilibrated with the rocks (i.e., inheritance of Fig. 5c shows the evolution in fluid B/Be resulting from pro-aid B-Be signatures through exchange with similar rocks gressive fluid-rock exchange over the same range in DB/DBe.
pstream). Note that the B/Be of the fluid decreases to varying extentsTo demonstrate simplified mass-balance relations for B (depending on DB/DBe) as the removal of B and Be from the
id Be during fluid-rock exchange, Fig. 5 presents calculations rock proceeds.F evolving fluid and rock composition as B and Be are re- The mass-balance calculations in Fig. 5a, b, and c applyioved by fluids, using as an example the mean concentration in general to fluid-related B and Be losses; they are not specificita for lawsonite-albite facies and greenschist/epidote-am- to particular fluid-rock exchange mechanisms for loss, suchhibolite facies metasedimentary rocks of the Catalina Schist as closed-system devolatilization (i.e., partitioning only intoee Fig. 2a and b) as constraints on rock B-Be evolution. In fluid derived locally by devolatilization of the rock), or theig. 5a and b, data for the lowest-grade, lawsonite-albite facies case where B and Be partition into fluids solely introduced)cks are shown by the black-filled circles and data for higher from external sources. The calculations are simplistic in that-ade equivalents (greenschist and epidote-amphibolite fa- they do not allow for the effects of varying mineral modes,es) are shown by the circles filled with the pattern; bars on pressure, temperature, or fluid chemistry on bulk fluid-rockhe average compositions of the higher grade metamorphic partition coefficients and do not attempt to take into accountocks (circle filled with pattern) indicate one standard de- protolith variability quantitatively (see ranges in B and Beiation. We infer an approximate 80% loss of B based on the concentration in Table l; see BEBOUT and FOGEL, 1992, forecrease in mean concentration from -73 ppm B in law- discussion of protolith variability in N concentrations in thernite-albite rocks to - 15 ppm B in greenschist and epidote- same rocks). However, the calculations do demonstrate (1)mphibolite equivalents (see Fig. 2a). The mean Be concen- the magnitude of the dependencies of B-Be signatures in de-ation of the greenschist and epidote-amphibolite samples volatilized rocks on fluid-rock partition coefficients, and (2)).70 ppm) is approximately 25% lower than that of the low- the potentially dramatic nature of the evolution in fluid B/t-grade samples (0.95 ppm); however, we consider 25% Be Be during progressive fluid-rock exchange. The models inss to be a maximum value because the low- and high-grade Fig. 5a and b demonstrate that the B/Be of the removed fluidamples show similar ranges in Be content (see Fig. 2b). Data was likely significantly higher than the B/Be of the rocks.ir the amphibolite-facies rocks were not used because these Based on these calculations, the B/Be of the hydrous fluidsocks have experienced partial melting reactions, subsequent which equilibrated with these rocks could have been ex-o devolatilization, which could have further affected the B- tremely high, as expected from data for hydrothermal fluidse systematics. The fluid was assumed to have been externally (see MEASURES and EDMOND, 1983; SPIVACK and EDMOND,
erived and to have contained no B or Be prior to exchange 1987; CAMPBELL et al., 1988) and data from experiments atith the rock. In the calculations, the amounts of B and Be high pressures and temperatures (TATSUMI and ISOYAMA,
*ss in fluid equilibrated with the rock reflect varying fluid/ 1988). In addition, because of the preferential removal of Bick distribution coefficients for the two elements (expressed (relative to Be) during progressive devolatilization, the B /s DB(f/rk) and DBe(fl/rk); hereafter referred to as DB and DBe). Be of the fluid may decrease dramatically during the pro-n Fig. 5a and b, B and Be are removed from the rock (be- gressive devolatilization of the rocks (Fig. 5c). In the cal-nning composition shown by black-filled circle) to degrees culations of Fig. Sc, this effect is particularly dramatic for)rresponding to a range in D/DR. (see Fig. S caption for cases where DB/DBe is -5.!scription of calculations). The curves in Fig. 5a demon- Approximation of devolatilization in Catalina Schistrate the effect of B and Be removal in multiple, very small metasedimentary rocks by a Rayleigh distillation model (se-itches at varying DB/DB,; these calculations approximate quential removal of very small batches equilibrated with theRayleigh distillation process (see Fig. 5a caption; cf. NA- rock) is supported by nitrogen isotope systematics in the sameMEK, 1987; BANNER and HANSON, 1990). The calculations rocks (BEBOUT and FOGEL, 1992). Boron and N show cor-i Fig. 5a show that, for a rock with initial B and Be con- related decreases in concentration with progressive volatileentrations of 73 and 0.95 ppm, respectively, Be depletions loss in the Catalina metasedimentary rocks, and both ele--e significantly greater than the average observed in the Cat- ments appear to be concentrated in mica (see Fig. 7 in BE-
2232
B-Be systematics in subduction-related metamorphic rocks
E0L0L
01)
mco
0
C)
0
Rock Boron (ppm)
1500
1000
500-
0 20 40 60
Rock Boron (ppm)
FIG. 5. Calculations of B and Be removal from metasedimenrocks, scaled to the Catalina metasedimentary rocks. In theseculations, the following general equation was used for each increnof fluid-rock exchange: Cf(rock) = C1(rock)/(FD + (1-F)), wlCf(rock) and Ci(rock) indicate the concentrations of B and Bthe rock after and prior to the increment of exchange, respectiiF is the weight fraction of fluid exchanging with the rock in icalculation, and D is the bulk fluid-rock partition coefficient (DB
DB,). Calculations for B and Be fluid-rock exchange were perfonsimultaneously in each increment of exchange (see Eqn. 3 ofBELEK, 1987, and Eqn. 7 of BANNER and HANSON, 1990, for sibatch exchange). Because small fluid increment sizes were used, tcalculations produce results indistinguishable from those using I10 of NABELEK ( 1987) for "open system" or Rayleigh fractionabehavior. For (a) and (b), filled circles represent constraints onof B and Be and B/ Be decrease from comparison of mean datathe lowest grade, lawsonite-albite facies rocks (filled with black),higher grade, greenschist to epidote-amphibolite facies rocks whave undergone devolatilization (filled with pattern; one std. devB, Be, and B/Be shown with bars). Numbers on lines and curve(a)-(c) indicate the ratio of the bulk fluid-rock B and Be particoefficients (D 5 /D&.) used in each of the calculations. (a) BorolBe concentration of a rock with initial B and Be concentratior73 and 0.95 ppm, respectively, during progressive B and Be losRayleigh distillation processes (multiple infinitesimal batches;labelled curves). (b) B/Be vs. B concentration for the same c2
I BOUT and FOGEL, 1992). Tourmaline does not occur in suf-7 ficient abundance to be a significant host for B. Thus, de-
creases in B content may also reflect progressive partitioningout of B-rich minerals (primarily mica; see Table 2) intoH20-rich fluids derived by reactions involving the breakdownof chlorite and phengitic white mica, and the stabilization ofless phengitic white mica, biotite, garnet, and kyanite (BE-BOUT and FOGEL, 1992). Boron loss, like the loss of N, maynot simply reflect the breakdown of mica (muscovite andbiotite) to anhydrous, less B- and N-rich minerals such as K
80 feldspar; muscovite and biotite show no obvious variationin their combined modal abundance with grade. Continuousexchange reactions like those which stabilized micas during
- progressive metamorphism of the Catalina Schist may allowthe continual reequilibration of fluid and rock isotopic andtrace element compositions during progressive devolatiliza-tion and/or reaction with externally derived fluids. In thecalculations of Fig. 5, fluid-to-rock ratios are inversely relatedto the absolute values of DB and Dik that are used; if smallerabsolute values for DB and D& are used at constant DB/DBe,
larger fluid-to-rock ratios are required to produce the sameobserved shifts in B and Be concentrations. As an example
- of the dependency of fluid-to-rock ratios on the DB that is80 used, for a DB of 10, a fluid-to-rock ratio (fluid/rock) of
-0.18 (by weight) is necessary to remove -80% of the B.For a DB of 2, a fluid-to-rock ratio of - 1.6 is necessary toresult in -80% B loss. Calculated fluid-to-rock ratios arehighly dependent on the initial B and Be concentrations as-sumed for the fluid with which the rock equilibrates in eachstep; in the calculations in Fig. 5, and in the preceding cal-culations of fluid-to-rock ratios, the fluid is assumed to haveinitially contained no B or Be (see previous discussion).
Identification of a minimum B/ Be for the removed B-Becomponent based on the data for the Catalina Schist is model-dependent and complicated by the ranges in the B and Be
-0 concentrations of the low- and high-grade metasedimentary80 rocks (see Fig. 2a,b). In Fig. 5a and b, only the calculations
which predict initial fluid B/Be of approximately 400 (cor-tary responding to DB/DB, of -5) or greater successfully explain
Ia the observed mean B and Be losses. Limits on minimum DB
ncent and DBe may be estimated for the endmember (and ratherthere unlikely) closed-system case where loss is attributed solelye in to partitioning of the elements into fluids evolved internally
ielyh by devolatilization reactions. These calculations are similar,and to the calculations of Fig. 5, except that they incorporate amed small correction for rock weight loss during progressive lossNA- of H2 0. We estimate the range in H20 losses by comparingngle the H20 content of the low- and high-grade metasedimentaryheseiqn. rocks (3-5.5 wt% in lowest grade, lawsonite-albite and blue-ltion schist facies rocks to 1-3 wt% in highest grade, amphiboliteloss facies rocks; see BEBOUT, 199 la; Fig. 2a). In these calcula-i for tions, the use of a DB value less than approximately 35 resultshwih in insufficient B loss during loss of 4.5 wt% H20; a D& of
,. fores initiona vs.is ofs by; seeLlcu-
lations as those in (a), showing coupled decreases due to progressiveB and Be loss at varying DB/DB,. Initial B/Be used (77) is the ratioof the mean B and Be values. (c) B/Be of "fluid" component vs.progressive devolatilization (progressive B and Be loss; e.g., see arrowon uppermost curve) of a rock with initial B and Be of 73 and 0.95ppm, respectively.
Rock Boron (ppm)
01)M
co
E2
00" / 20
10
2-2
2233
G. E. Bebout, J. G. Ryan, and W. P. Leeman
approximately 7.5 is necessary to result in a loss of 25% ofthe Be during loss of 4.5 wt% H20. For smaller amounts ofH20 loss, larger values of D5 and DB, would be required.Note that, for this case of endmember closed-system volatileloss (B and Be partitioning only into fluids derived internallyby devolatilization reactions), DB and DB& must be >I toresult in decrease in the concentrations of the elements inthe rocks. Although smaller amounts of H 20 loss (<4.5 wt%)would require yet higher D5& in these calculations, the valueof -7.5 for D& should be regarded as an extreme maximumvalue because of (I) the large degree of Be loss assumed inthe calculations (25%), and (2) the unlikelihood that theendmember closed-system model is applicable (see discussionin BEBOUT and FOGEL, 1992). The Da, value of - 7.5 inferredfrom these calculations is dramatically higher than that ex-pected based on high-temperature, high-pressure experiments(inferred DB far less than 1.0; TATSUMI and ISOYAMA, 1988;see discussion in MORRIS et al., 1990a). However, if verylittle Be was lost (based on the similar ranges of Be concen-tration at all metamorphic grades), and the rock systemswere not entirely closed, the DB, values were probably con-siderably lower than 7.5 and, perhaps, more compatible withthe experiments of TATSUMI and ISOYAMA (1988).
To explain the similarity in B and Be concentrations andB/Be of the Na-amphibole veins and blueschist metasedi-mentary rocks, we envisage a multi-stage fractionation processwhereby veins precipitated from fluids which were previouslyequilibrated with respect to B and Be with host rocks (orwith similar rocks upstream of current hosts). In this model,DB and DB, for the crystallizing vein assemblages would havebeen similar to those of the host rocks. Similarities in the 0,H, C, and N isotope compositions of vein minerals and thesame minerals in host rocks (BEBOUT and BARTON, 1989;BEBOUT, 199 1a,b, 1992) are consistent with the equilibrationof veins with fluids which previously equilibrated with host-rocks (the fluid sources). Further data are required to morerigorously test this hypothesis.
Implications for B-Be Systematics in Arc Volcanic Rocks
Our results argue strongly against the bulk addition to themantle reservoir of a sediment/slab mixture or of a meltderived from metamorphosed sediment/slab sources as ex-planations of the B-Be systematics in high-B/Be volcanic arcs.Before subducted mafic and sedimentary rocks reach Wadati-Benioff zone depths below arcs (80-150 km), the B/Be ofthese rocks is likely to have decreased to <30. Sediment/slab mixtures will not provide a mixing component with suf-ficiently high B/Be to explain the linear trends in the arcdata for many arcs, nor will melts of sediment/slab, assumingsimilar melt-rock partitioning of B and Be (and barring themetasomatic stabilization/enrichment of tourmaline, otherB-rich phases, or Be-rich phases, at greater depths, in sufficientquantities to significantly impact melt compositions; see dis-cussion of B-metasomatism by MORGAN and LONDON,1989). These results strengthen the arguments for the additionto arc magma sources of a fractionated (high-B/Be), slab-derived, hydrous fluid component (see MORRIS et al., 1 990a),particularly for the higher-B/Be arcs.
The B and Be data, together with oxygen and hydrogenisotope data for the Catalina Schist (BEBOUT and BARTON,
1989; BEBOUT, 1991 a,b), demonstrate the potential of sub-duction-zone metamorphic processes for producing fluids(H2 0-rich solutions or felsic silicate liquids) and/or rocks(subducted sediments and oceanic crustal igneous rocks) thatare homogenized with respect to certain trace element andisotopic ratios. In the Catalina Schist, sedimentary and maficrocks which, on the seafloor, had a wide range of B contentsand B/Be, became more depleted and uniform in B concen-tration and more uniform in B/Be due to devolatilization(Figs. I, 2a, 3; see discussion for other fluid-mobile elementsby HEIER, 1973; LEEMAN et al., 1990a). Although the am-phibolite-grade mafic and sedimentary rocks of the CatalinaSchist have dramatically different major and trace elementcompositions, they produced silicate melts (represented bypegmatites) with similar and relatively uniform B/Be (Fig.4) because of this prior reduction and homogenization ofsource-rock B/Be by devolatilization. In the Catalina Schist,devolatilization of a lithologic package that was initially het-erogeneous with respect to oxygen and hydrogen isotopecompositions resulted in the production of H20-rich fluidswith relatively uniform oxygen and hydrogen isotope com-positions, based on data for veins and other metasomaticfeatures in each of the major units (fluid 6 8 OsMow = + 13± 1 Too; 1DsMow = -15 ± 1 5%o; see discussions of fluid ho-
mogeneity in BEBOUT and BARTON, 1989; BEBOUT, 1991 a,b).
The uniformity in vein B/Be (Fig. 4) could conceivably reflecthomogenization of B/Be in the H20-rich fluids; the paucityof mineral-hydrous fluid partitioning data precludes a morequantitative assessment of this possibility. Because of thedramatic range in possible B/Be for fluids evolved at variousstages of rock devolatilization (e.g., see Fig. 5c), efficientmixing of fluids with B/Be inherited from heterogeneoussources may be required to deliver a homogeneous B/Besignature to the sub-arc mantle wedge (cf. MORRIS et al.,1990a). Such a mixing process has been called upon to ex-plain the regularity in stable isotope compositions of the Cat-alina metamorphic fluids (BEBOUT, 1991a). That thesemetamorphic processes resulted in the homogenization offluid and rock composition is compatible with arguments,based on study of arc volcanic lavas, for the regulation ofslab B and Be addition by production of a homogenized slabcomponent (MORRIS et al., 1990a). Metamorphic processessuch as these could conceivably also deliver slab componentshomogenized with respect to Rb, Cs, and Ba, elements whichare similarly believed to be relatively mobile in hydrous fluids(see LEEMAN et al., 1990a). Delivery, to the mantle wedge,of a slab component homogenized with respect to these latterelements has been inferred from data for some arc volcanicrocks (see MORRIS et al., 1990b). Such homogenization mightalso result in the contribution of slab components with rel-atively uniform strontium, neodymium, and lead isotopiccompositions (see EDWARDS et al., 1991) .
Significant fractionation of B and Be in the mantle wedgeduring melting processes is unlikely (MORRIS et al., 1990a;RYAN and LANGMUIR, 1993); however, the extent of frac-tionation which may accompany fluid-rock interactions, in-cluding the metasomatic stabilization of hydrous phases (e.g.,micas, amphiboles), in the mantle wedge (see WYLLIE, 1991 )remains uncertain. The varying impacts of hydrous fluid ormelt additions to arc source regions may explain some of the
B-Be systematics in subduction-related metamorphic rocks
B/Be variability observed in arc volcanic rocks. Subductionzones with cooler inferred thermal structures on average showhigher B/Be, consistent with addition of high-B/ Be hydrousfluids (e.g., for the Bismarck arc, B/Be = 20 to 190; RYANand LANGMUIR, 1993). Subduction zones with hotter inferredthermal structures produce volcanic rocks with relatively lowB/Be, perhaps consistent with the addition of a melted com-ponent from a slab previously stripped of B by devolatiliza-tion. As an example, front-rank basalts from the Cascadeshave B/Be < 5 (LEEMAN et al., 1990b; MORRIS et al., 1990a;RYAN and LANGMUIR, 1993), similar to that of the Catalinapegmatites (see Fig. 4). Decreases in B/Be have been doc-umented across individual arcs (MORRIS et al., 1990a; RYANet al., 1990; RYAN and LANGMUIR, 1993), in all cases varyingfrom high front-arc values to lower back-arc values similarto those of mid-ocean ridge basalt (e.g., B/Be of >100 to 9across the Bismarck arc; MORRIS et al., 1990a). These de-creases could presumably represent the diminishing effectsof hydrous fluid addition (i.e., contribution of progressivelysmaller amounts of a high-B/Be hydrous fluid) and/or theonset of melt-dominated B-Be transfer. Alternatively, de-creasing B/Be across individual arcs may reflect progressivedecrease in the B/Be of the released hydrous fluid duringprogressive devolatilization as the rocks are subducted togreater depths (see Fig. 5c).
The results of this study, together with nitrogen isotopestudies (BEBOUT and FOGEL, 1992) and ion microprobe traceelement studies on the Catalina rocks (DOMANIK et al., 1991;A. E. Bebout, unpubl. data) and other similar metamorphicsuites (A. E. BEBOUT et al., 1992), emphasize the importanceof identifying mineral residency of trace elements in attemptsto characterize their behavior during metamorphic processes.Particularly in sediment-rich subduction zones, muscoviteand other micas may serve as important mineral reservoirsfor B and other trace elements characteristically enriched inarc basaltic rocks relative to mid-ocean ridge basalts (e.g.,Rb, Cs, Ba, possibly N; see ZHANG and CLAYTON, 1988).The abundances and stability relations of the micas may sig-nificantly impact histories of trace element release from sub-ducted slab and sediments (cf. DOMANIK, 1992). In the Cat-alina Schist, micas are important reservoirs for these elementsto temperatures exceeding those of H20-saturated partialmelting (6500 to 750°C; see data in Tables I and 2; DOMANIKet al., 1991; A. E. BEBOUT et al., 1992; DOMANIK, 1992;A. E. Bebout, unpubl. data).
Finally, the Catalina Schist records relatively high-tem-perature metamorphism for its inferred depths (15-45 km)compared with many other subduction-related metamorphicrock suites (see BEBOUT, 1991a), subduction zone thermalmodels (e.g., SACKS and KINCAID, 1990; PEACOCK, 1990),or geophysical observations at convergent margins (e.g., SATOand SACKS, 1990). Thus, these rocks are not direct repre-sentatives of subduction-zone metamorphism at sub-arcdepths, but rather shallower analogues produced in a relativelyhot subduction environment, probably during early stages ofsubduction (PLATT, 1975; PEACOCK, 1990). Hydrous fluidsand silicate liquids similar to those produced in the CatalinaSchist could be responsible for slab additions to magmas pro-duced when young, hot oceanic crust is subducted, or duringearly stages of subduction when hot conditions prevail (e.g.,
producing adakites, boninites; see DEFANT and DRUMMOND,
1990; CRAWFORD et al., 1989). Boninites, in particular, mayrecord additions of hydrous fluids variably enriched in sed-iment and slab-derived components (CRAWFORD et al., 1989)from the slab at depths less than 50 km; these depths aresimilar to those inferred for the Catalina metamorphism(overall range of 15-45 km; see BEBOUT, 199 1a,b). The rel-atively high-temperature metamorphism recorded by theCatalina Schist (inferred temperatures of 350 to 750°C) maybe analogous to that which occurs at greater depths in theCascades subduction zone, where young, hot oceanic litho-sphere is currently being subducted (see LEEMAN et al.,1990b). Pegmatites in the Catalina Schist show other char-acteristics similar to those inferred for slab-derived fluids (cf.GILL, 1981; ELLAM and HAWKESWORTH, 1988; DEFANT andDRUMMOND, 1990); they are siliceous, have high LILE/HFSE, are LREE-enriched, and have elevated initial 87Sr/86Sr (see BEBOUT and BARTON, 1989; additional data in So-RENSEN and BARTONj 1987; BEBOUT, 1989; SORENSEN andGROSSMAN, 1989). In some subduction zones, slab devola-tilization at depths of •50 km might result in hydration ofmantle wedge and diapiric uprise of serpentinite bodies, asis observed in some forearc regions (e.g., FRYER and FRYER,1987). Direct extrapolation of the results of this study tocases of devolatilization deeper and in cooler subductionzones is complicated by uncertainties in subsolidus phaseequilibria at greater depths (PEACOCK, 1990; BEBOUT,1991 a). With this necessary qualification, we suggest that theCatalina B-Be systematics demonstrate distinct partitioningbehavior, in hydrous fluid-rock and melt-rock situations, thatmay allow discrimination between hydrous fluid and melttransfer models for arc magmatism. These data constitutedirect evidence for the differential mobilities of B and Beduring progressive devolatilization and migmatization in theforearc region of an Early Cretaceous subduction zone. Ourresults document chemical alteration of subducted rocks inforearc regions which may profoundly affect the chemicalmixing systematics involving these rocks at depths beneaththe volcanic fronts of arcs and at still greater depths in themantle.
CONCLUSIONS
Boron concentrations in subducted mafic and sedimentaryrocks may be profoundly affected by metamorphism and at-tendant fluid-rock exchange in the forearc regions of sub-duction zones (see estimated presubduction B concentrationsfor mafic and sedimentary rocks in Fig. 1; data for meta-morphosed equivalents in Fig. 2a). Whereas the B in meta-sedimentary rocks is noticeably lost over the interval inmetamorphic grade represented by the Catalina Schist (seeFig. 2a), B in metamafic rocks shows less significant loss overthis same interval in metamorphic grade. It is likely, basedon the data for the Catalina Schist and the results of MORANet al. (1992), that the loss of B in metabasaltic rocks beginsat somewhat lower temperatures and shallower levels thanthose recorded by the Catalina Schist. The results for theCatalina Schist demonstrate that B and Be are significantlyfractionated during metamorphic devolatilization, particu-larly of pelitic and psammitic sedimentary rocks, over an
approximate temperature range of 350 to 6000C. This processproduces high-B/Be hydrous fluids and results in a dramaticreduction of the B/Be in the subducted rocks. The assignmentof absolute values for DB and DB, and the quantitative der-ivation of DB/D&e during the devolatilization of the meta-sedimentary rocks of the Catalina Schist are complicated byprotolith variability in B and Be concentrations and the de-pendence of the calculations on models for the fluid-rockinteraction (e.g., open- vs. closed-system behavior; sources,amounts, and compositions of fluid). However, the variousmass-balance considerations (see Fig. 5 and previous discus-sions) indicate that, even using the assumption of 25% Beloss, the D5/D&, of the removed fluid component is unlikelyto have been less than -5; because the estimate of 25% Beloss is considered to be a maximum, the D1/ DD, of the fluidis likely to have been significantly greater than 5. The cal-culations in Fig. 5c demonstrate that the B/Be of the fluidsis therefore likely to have exceeded -400, at least initially,and that it may have been significantly greater than 400, asexpected based on the results of previous studies of B andBe mobility in natural settings (e.g., concentrations in hy-drothermal fluids; enrichments during metasomatic processes;CAMPBELL et al., 1988; MORGAN and LONDON, 1989) andthe experiments on Be mobility by TATSUMI and ISOYAMA(1988). The mass balance calculations also indicate that sig-nificant evolution of the hydrous fluid B/Be toward lowervalues may occur during progressive devolatilization and/orfluid-rock interactions involving externally derived hydrousfluids (see curves in Fig. Sc).
In contrast with the devolatilization process, which pro-duces hydrous fluids with relatively high B/Be, partial melting(which appears not to fractionate B and Be significantly; seeRYAN and LANGMUIR, 1993) may produce silicate melts withlow B/Be inherited from source-rocks depleted in B by pre-vious devolatilization processes. The transfer of slab meltswith high B/Be (>30) to mantle source regions is thus un-likely because rock B/Be is reduced by subsolidus meta-morphic processes prior to partial melting. Fractionation inthe mantle wedge during metasomatic processes (e.g., by sta-bilization of B- and Be-bearing, mica- and amphibole-richmineral assemblages above the subducted slab) and chemicalfractionation related to large-scale transport of fluids (e.g.,hydrous fluids, silicate liquids) and fluid-rock interactions inthe mantle wedge (see NAVON and STOLPER, 1987; discussionin STERN et al., 1991) cannot be discounted. However, thevarying impacts of hydrous fluid (from variably devolatilizedslab) and silicate melt additions to arc source regions couldin part explain significant variations, among arcs, in the rangesof B/Be observed in front-rank volcanoes (e.g., for the Bis-marck arc, B/Be = 20 to 190; for the Cascades arc, B/Be< 5) and variations in B/ Be across individual arcs. Both theB/Be of the hydrous fluids (i.e., as related to the extent ofdevolatilization; see Fig. 5c) and the relative proportions ofhydrous fluid and silicate melt derived from the subductedrocks are presumably related to the thermal structure of thesubduction zone.
Boron and Be may serve as analogues for other incom-patible elements (e.g., B for Rb, Cs, Ba, U, Pb; and Be forZr and the other HFSE; see LEEMAN et al., 1990a) whichappear to behave similarly in hydrous fluid-solid and melt-
solid systems. These results indicate that, particularly for ele-ments such as B which are relatively mobile in hydrous fluids(or other C-O-H-S-N fluids), the retention of presubductionconcentrations in mafic and sedimentary rocks subducted todepths beneath arcs should not be assumed in geochemicalcycling models of convergent margins. Subsolidus devolatil-ization processes may be capable of homogenizing presub-duction variability in the concentrations of these elementsin rocks through selective removal by metamorphic fluids.Subduction-zone metamorphic processes may, through mix-ing, produce hydrous metamorphic fluids with relatively uni-form trace element (and isotopic) compositions. In normal,relatively cool arcs, these homogeneous hydrous fluids couldinfiltrate parts of the mantle wedge and result in the remark-ably regular mixing trends observed in arc volcanic rocks forB and Be, particularly in the arcs with high B/Be (see MORRISet al., 1990a). In hotter arcs, melting processes might alsobe expected to deliver relatively uniform slab B-Be signaturesbecause the B /Be of the source rocks is lowered to uniformlylow values (perhaps to <30) by previous devolatilizationprocesses.
Acknowledgments-We extend special thanks to J. D. Morris and F.Tera and the Department of Terrestrial Magnetism, Carnegie Insti-tution of Washington, in whose laboratories the B and Be separationswere performed. We thank J. D. Morris, F. Tera, A. E. Bebout, J.Selverstone, G. Morgan VI, J. Baker, M. Grove, and C. Edwards forhelpful discussions and reviews and the Santa Catalina Island Con-servancy for its support of the field research.
Editorial handling: J. D. Morris
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