Nitrogen-isotope compositions of metasedimentary ... - Science Direct

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Geochimica n Cosmochimica Aclo Vol. 56. pp. 2839-2849 Copyright Q 1992 Pergamon Press Ltd. Printed in U.S.A.

Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history GRAY E. BEBOUT”* and MARILYNL. FOGEL’ ‘Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 200 15 (Received May 16, I99

1; accepted in revised form April 23, 1992 )

Abstract-In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in 6 15Nai,with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 + 185 ppm N with 615N = + 1.9 f 0.60/w,whereas highgrade amphibolite equivalents contain 138 + 76 ppm N with 6r5N = +4.3 + 0.8%. Loss of N accompanied devolatilization reactions that evolved HzO-rich C-O-H-S-N fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite, biotite, garnet, and kyanite. Whole-rock N concentrations of up to 200 ppm in veins and pegmatites produced during highP/T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations ( A15N = G”NsUid - 6”NrmL) of - 1.5 + 1% were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K20 concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH: exchange at the temperature range of 350-6OO’C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25%0). The N systematics appear to be explained by N2-NH: exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and br5N cannot be explained by NHs-NH: exchange at these temperatures using the equilibrium models. The distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization (e.g., B, Cs, U). Similarity of the calculated fluid 615N ( - - 1.5 to +5.5%) with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using N as a tracer of large-scale volatile transport. INTRODUCIION THE N ISOTOPESYSTEMshows great potential for elucidating processes such as fluid-rock exchange, magma provenance and crystallization, and crust-mantle interaction (see JAVOY et al., 1986; ZHANG, 1988). High concentrations of NH: in igneous and metamorphic minerals ( HONMA and ITIHARA, 1981; DUIT et al., 1986) suggest that N may in some cases be an important fluid constituent. Nitrogen species (particularly N2) are commonly found in fluid inclusions ( KREULEN and SCHUILING, 1982; CASQUET, 1986; BOTTRELL et al., 1988; ANDERSENet al., 1989; SAMSONand WILLIAMS-JONES, 199 1) . Nitrogen is a common constituent of volcanic gases (e.g., WHITE and WARING, 1963), is abundant in some magmatic/hydrothennal deposits ( JUNGE et al., 1989; HALL, 1989), and occurs in trace amounts in various mantle-derived rocks (see JAVOYet al., 1986; HALL, 1988; ZHANG, 1988). It is an important constituent of natural gases from some sedimentary basins ( JENDENet al., 1988) and deep crystalline rock environments (SHERWOODet al., 1988). In silicate minerals, N resides as NH:, which substitutes readily for K+ (HONMA and ITIHARA, 1981). Significant amounts of N may also reside in fluid inclusions (e.g., ANDERSENet al., 1989). In metamorphic and igneous rocks, NH: is most strongly concentrated into micas, particularly * Present address: Department of Earth and Environmental Sciences, Williams Hall 3 1,Lehigh University, Bethlehem,Pennsylvania 18015. 2839

biotite. In such rocks, biotite, followed by muscovite, K-feldspar, and plagioclase, has the highest NH: content ( HONMA and ITIHARA, 198 1). Biotite in igneous and metamorphic rocks commonly contains >250 ppm NH: (HONMA and ITIHARA, 1981; DUIT et al., 1986) and may contain up to 5000 ppm NH: ( DARIMONTet al., 1988). Although the first N-isotope measurements on silicate minerals and whole-rock samples were done in 1955 (HOERING, 1955 ), N isotopes have not yet been exploited extensively in petrologic studies. Previous N extraction techniques for isotopic analysis of rocks and minerals have involved timeconsuming wet chemical techniques (dissolution/distillation) or fusions of samples (SCALAN, 1958; HAENDELet al., 1986; ZHANG, 1988). Kjeldahl distillation techniques involve numerous chemical steps and may not provide complete yields (MINAGAWAet al., 1984; RIGBYand BATTS, 1986), resulting in possible isotope fractionation and relatively poor precision ( RIGBY and BATTS, 1986; HAENDEL et al., 1986). Fusion techniques described by ZHANG ( 1988) allow extremely high sensitivity but are time-consuming. Laser microprobe techniques and static mass spectrometry show great potential for N-isotope analysis ( FRANCHI et al., 1989). In this study, combustion techniques used routinely in biogeochemical studies ( MACKO, 1981; MINAGAWAet al., 1984; RIGBY and BATTS, 1986; VELINSKYet al., 1989; KENDALLand GRIM, 1990) have been applied to the analysis of N isotopes in silicate minerals and whole-rock metamorphic samples. These techniques involve heating of mineral separate and whole-

2840

G. E. Bebout and M. L. Fogel

rock samples with CuO wire and Cu metal, with or without CaO (to remove Hz0 and CO2 ; KENDALL and GRIM, 1990)) in sealed quartz tubes for several hours at 910°C. Previous studies of N distribution in metasedimentary rock suites have demonstrated that N may be strongly depleted during metamorphism, presumably due to its speciation in metamorphic fluids during devolatilization reactions and fluid-rock exchange (see MILOVSKIYand VOLYNETS,1966; DUIT et al., 1986; HAENDELet al., 1986; COOPERand BRADLEY, 1990). HAENDELet al. ( 1986)) in a study of N concentration and stable isotope composition in regionally and contact-metamorphosed metasedimentary rocks, observed shifts toward higher 6 r5N which accompanied decrease in N concentration with increasing metamorphic grade. They interpreted these coupled trends as the result of Rayleigh distillation behavior during prograde metamorphism. We have further tested the Rayleigh distillation model (and other models) for explaining N isotope variation in progressively metamorphosed sedimentary rocks of the Catalina Schist, a subduction-related metamorphic terrane in southern California. We discuss the N data for these rocks in the context of the model of progressive devolatilization derived through comprehensive field, petrologic, and geochemical studies ( BEBOUTand BARTON, 1989; BEBOUT,199 1a). We consider mechanisms of N release into fluids during metamorphism and their potential impact on the mobilities of other trace elements during progressive volatile loss. ANALYTICAL TECHNIQUES Samples ranging from 15-4000 mg were loaded into quartz tubes ( 10 mm outside diameter, 23 cm long) with varying amounts of CuO wire and granular Cu metal reagents and CaO. CuO and CaO were precombusted at 850°C for 2 h and 1000°C for 1 h, respectively. Samples varied in average grain size, but relatively coarse powders (50-200 mesh) yielded the most precise results. After loading, samples were evacuated while being warmed intermittently (to ~50°C) with a heat gun and were sealed off under vacuum. The reagents and samples were homogenized in the sealed tubes with a Vortex mixer. Sealed sample tubes were heated in a programmed muffle furnace to 9 10°C for up to 3 h, then cooled slowly to 700°C. For the cooling interval of 700-5OO”C, a cooling rate of 0.6”C/min was used to maximize conversion of CO to CO2 and NO, to Nz. The calculated COJCO over this temperature interval ranges from IO5 to lo6 at the oxygen fugacity of the Cu-CuzO buffer, a conservative estimate of oxygen fugacity in the experiments. Two different gas purification methods were applied: a standard cryogenic technique using liquid nitrogen and a molecular sieve (e.g., MACKO,1981 ), and a gas expansion technique. For the cryogenic/ molecular sieve technique, sample tubes were cracked, and Hz0 and CO2 were first trapped in a cold finger for I2 min with liquid nitrogen. The Nz was then absorbed onto molecular sieve for 10 min with liquid nitrogen in collection vessels that were then placed on the inlet of the mass spectrometer. For the gas expansion technique, the N2 gas was released into a small-volume glass fitting, then expanded into collection vessels and allowed to equilibrate for 1 min. Liquid nitrogen was placed on the cracked sample tube for 30-60 set to collect HZ0 and CO* not removed by the CaO. This technique resulted in a transfer of about 30% of the Nr gas to the inlet system of the mass spectrometer. A Hg ram system, similar to a Toepler pump in design, improved recovery to -90%. Samples were analyzed on a modified, double inlet, double-focussing, double collector DuPont 49 I mass spectrometer. Scans of mass spectra were obtained to measure intensities of Hz0 and COz peaks, which indicate incomplete removal during extraction or contamination from molecular sieves. Argon peaks were examined for evidence of atmospheric contamination (i.e., leakage) during sample

loading or extraction. Variations in the isotopic composition of N in unknown samples (spl) are defined as follows: 615N

=

(‘5N/‘4N)sp, - (“N/14N)

(‘5N/‘4Nhti

1

nd lo’,

where the standard (std) is atmospheric Nz. Blanks were determined for varying amounts of each reagent and for molecular sieves. Blanks had 6 “N of -4 to +8%0 and were of sizes corresponding to about 0.1-0.2 micromoles of NZ (2.8-5.6 pg N). When molecular sieves were contaminated with Hz0 and CO*, as determined by mass scans, blanks were larger (up to 10 rg N) and had higher 815Nvalues (mean = +3.7%0, std. dev. = 3.5%0).Analyses of standards of sizes less than 50-75 pg N tended to have 6 15Nhigher than accepted values by up to 3 or 4L. After heating the molecular sieves at 4OO’C for 3 h, blanks were smaller (3-5 rg N) and had 6’jN nearer zero (mean = -0.4%0, std. dev. = 4.26); and 6’jN of small samples of standards was near accepted values. With the gas expansion technique, ten analyses of laboratory reference compound TCH ( NH.,(S04)z) yielded a mean value of -0. I& (std. dev. = 0.2%~)for samples corresponding to 50-400 pg N. Similar precision was obtained in this laboratory for analyses of the TCH standard by the cryogenic techniques ( VELINSKY et al., 1989;this study). Nitrogen

concentration was estimated from the ion intensity of N2 observed in a calibrated volume of the mass spectrometer. Analyses of various N standards ( NH4( SO,)z, phenylalanine, NaNOs) allowed construction of pg N-voltage calibration lines (r* > 0.98). In this study, 615N of samples with high C/N (up to 250) was commonly deflected to higher isotope values if gas samples were smaller than 50 pg. At larger sample sizes, isotope values reached a common value, indicating the effect of a background for small samples. Although analysis of larger samples generally alleviated the blank problem, sample sizes necessary for those experiments are prohibitively large for mineral separation work ( up to 4 g) . Experimentation with molecular seives, using different freezing and degassing procedures, indicated that, for small samples, contamination by other gases or residual Nz from sieves may alter the isotope values. The molecular sieves released significant Hz0 and CO2 after routine use with large, high-C/N samples for several months. After heating of the molecular sieves at 400°C for 3 h, and when CaO was added to the quartz tubes to remove Hz0 and COz, the background problems for the cryogenic technique were reduced. However, expansion of the gas into a collection vessel, without the use of a molecular sieve, seemed to yield the best results for small samples and was considerably more rapid. In experiments with CaO reagent, the quantities of CaO were chosen following the recommendations of KENDALL and GRIM ( 1990). It appears that for some whole-rock samples with unusually high C concentration and C/N (up to 237 on a weight basis), the reaction with CaO does not result in complete removal of COZ. Because of the relatively low N concentrations in such samples, the resulting &15Nvalues may be affected. Extractions by the gas expansion technique, with and without placement of liquid nitrogen on the sample tube, resulted in systematic variations in 615N.An extraction for one high-N sample without this last liquid nitrogen step resulted in a 615N value of +2.4%0, higher than those obtained through analysis of larger samples (mean 615N = +2.0%0, std. dev. = 0.1%0, n = 6) by the cryogenic methods. A mass scan of this sample showed significant CO* peaks. After freezing with liquid nitrogen, CO* peaks in a mass scan of a separate extraction of that sample were at background levels; and a 6 “N of + 1.9%0was obtained, similar to those obtained through analysis of large samples. For one low-N sample, a value of +4.84bowas obtained when liquid nitrogen was not used: four extractions of the same sample using liquid nitrogen yielded a mean 6”N of +4.2k with a standard deviation of 0.2k. Several lines of evidence indicate that complete yields were obtained through the heating techniques. First, linear relationships ( r2 FZ0.95 ) between sample size and voltage readings for most samples indicate either that release was complete or that the same fraction of the N was obtained in each reaction. Reaction products were reheated for various lengths of time and at 9 10 and 1000°C to test for incomplete reaction. In general, the amounts of N2 were similar to or only slightly higher (~8.4 Kg N, ~0.3 @moles NZ) than those of blanks. Reaction products were analyzed using a Carlo-Erba CNS analyzer; no ap-

2841

Isotope geochemistry of N in metamorphic rocks preciable N was detected. One buddingtonite sample (NH:-rich feldspar from Sulfur Bank, Lake County, California; see ERDet al., 1964;VONCKENet al., 1988) from the collection at the Smithsonian Institution of Washington, DC (ID#: 11775 l-6), for which an independent measure of N concentration was available, was analyzed for N concentration and isotopic composition. Determinations of N concentration using the voltage readings are within 0.05 wtl of the previously reported concentration value ( 1.69 f 0.05 wt% N) . The buddingtonite sample yielded a linear relationship between sample size and voltage ( rz 2 0.99) and precise isotope values (mean 615N = -1.2%, std. dev. = 0.1460,n = 4). Finally, our analyses of N concentration and 6”N of whole-rock samples provided by K. Brauer and D. K. Haendel compare favorably with their analyses obtained through a dissolution/distillation technique (see HAENDELet al., 1986, for description of techniques). We obtained a mean d lSN value of +2.9% (n = 9, std. dev. = 0.1960)and a mean N concentration of 560 ppm (std. dev. = 25 ppm) for one sample (sample WSl ); these data are very similar to their mean values of +2.9%0 (std. dev. = 0.1%) and 550 ppm (std. dev. = 30 ppm). For five analyses of another samde f samnle NST- I 1, we obtained 6 “N and N concentration values of +7.8%0 (std. dev. = 0.3%) and 80 ppm (std. dev. = 10 ppm), respectively, similar to mean values of +8.0’% (std. dev. = 0.5k) and 76 ppm (std. dev. = 10 ppm) reported by the other laboratory. Hz0 concentrations were obtained by isotope extraction techniques (combustion and reduction of Hz0 to H2 by use of a uranium furnace; BIGELEISEN et al., 1952). THE CATALINA SCHIST The Catalina Schist contains lawsonite-albiteto amphibolite-grade metasedimentary rocks that represent metamorphism at temperatures of 350-75O’C and pressures corresponding to 15-45 km depths (PLATT, 1975; SORENSEN,

1986; SORENSENand BARTON,1987; BEBOUTand BARTON, 1989). The three major metamorphic/tectonic units (lawsonite-albitelblueschist, glaucophanic greenschist/epidoteamphibolite, and amphibolite) represent packets of sedimentary, ma&, and ultramafic rocks under-plated and metamorphosed during early Cretaceous subduction (PLATT, 1975; BEBOUT,199la; GROVE,1991). Bach unit contains a similar assemblage of metamorphosed mafic, sedimentary, and ultramatic rocks. The samples of metasedimentary rocks analyzed in this study were collected from widespread exposures within each unit. Metasedimentary rocks from the three major units are similar in major and trace element composition (see SORENSEN,1986; BEBOUT,1989 ) . In each of the units, the metasedimentary rocks range in lithology from metapelites to metagraywackes; average grain size increases with increasing metamorphic grade (from several pm to several mm). The wide range of metamorphic grade in the Catalina Schist allows examination of rocks representing different stages of progressive devolatilization. Trends in HZ0 content with increasing metamorphic grade, together with prograde reaction histories inferred from mineral modes and mineral chemistry, indicate that devolatilization of the mafic and sedimentary rocks was particularly linked to chlorite breakdown over the approximate temperature interval of 350-600°C ( BEBOUT, 199la,b). Hz0 content in the metasedimentary rocks decreases from between 3-5.5 wt% in lowest-grade rocks to between 2-3 w-t%(O-3.5 wt% Hz0 loss) in highest-grade, amphibolite-facies rocks (see BEBOUT,199 la). Breakdown of chlorite and celadonitic white-mica stabilized muscovite-, biotite-, garnet-, and kyanite-bearing mineral assemblages through reactions of the following general types:

2 Phengite

+ Chlorite = Muscovite

+ Biotite + Quartz

2 Chlorite

+ 4 Quartz

= 3 Garnet

3 Chlorite

+ 7 Muscovite

+ 8 Hz0

( 1) (2)

+ Quartz

= AlzSiOs( Kyanite) Chlorite

+ 4 Hz0

+ 7 Biotite + 12 Hz0

( 3)

+ Quartz + 8 HsO.

(4)

+ Muscovite

= Biotite + AlzSiOs( Kyanite)

Devolatilization resulted in the production of HzO-rich fluids (mole fraction Hz0 > 0.95) with uniform 0 and H isotope compositions ( BEBOUT and BARTON, 1989; BEBOUT, 199 la). Oxygen isotope data indicate that the fluids were derived from relatively low-temperature (350-6OO”C), sed-

iment-rich parts of the subduction zone; this conclusion is compatible with the petrologic inference that maximum devolatilization in the Catalina Schist occurred at these temperatures during reactions involving the breakdown of chlorite ( BEBOUT,199 la). NITROGEN CONCENTRATION AND ISOTOPIC COMPOSITION IN METASEDIMENTARY ROCKS OF THE CATALINA SCHIST The metasedimentary rocks show a general decrease in N concentration and a corresponding increase in 615N with increasing metamorphic grade (Fig. 1). The low-grade rocks contain - 100 to 1100 ppm N, whereas the highest-grade rocks contain only -30 to 250 ppm N (Fig. 1). The 6 “N of the lowest-grade rocks (lawsonite-albiteand blueschistfacies combined; Table 1) ranges from + 1.O to +3.1 L (mean

= +2.2k, std. dev. = 0.6%0,n = 27). 6t5N ofthe metasedimentary rocks increases with increasing metamorphic grade to values > 4% (mean = +4.3%0, std. dev. = 0.8s) for the highest-grade amphibolite rocks. Most of the decrease in N concentration and increase in 6 15Noccurs between the fields for lawsonite-albite/ blueschist and the epidote-amphibolite grade samples. For two pegmatites from amphibolite-grade 8 Lawsoniie-Albite Blueschist Greenschist 8 Eddote-Amohibolite awsonite-Albite

800

N (w-4600 400

0 +l

+2

+3

+4

+5

+6

815Nair FIG. 1. 6”N vs. N concentration for metasedimentary rocks in the Catalina Schist. Note trend of homogenization and decrease in N concentration and trend of increasing 6 “N with increasing metamorphic grade. Greenschist data include those for glaucophanic greenschist samples.

2842

G. E. Bebout and M. L. Fogel Table

1.

Nitrogen

Sample Technique

concentration

n

and isotope

Mean @N

data for metasedimentary

Std. Dew.

(n = 13; ‘2.9 f 0.6460:+6232 185 ppm) 0.10 +1.9 0.06 3 +1.3 : &3-2A +l.l :. 0.18 63-2D 1 +1.0 6.3-2F : 2 +2.0 0.06 b-3-26 +2.0 6-3-28 cc 1 +1.3 0.05 6-3-2Kt +2.1 0.14 : 6-3-2K’lt : 1 +2.7 6-3-2K’3t C 1 +2.4 6-3-2K”fgrt C +2.3 0.05 6-3-3 ; +2.7 0.13 7-2-132 g 1 +1.2 7-3-70 C ~_lue&sl (n; 14; +2.4 f 02% 431 f 3,‘z,$m) 0.13 +3.0 2 0.04 6-5-23~ C 1 +2.4 6-5-24a +2.1 0.04 6.5-24b : +2.4 : 0.25 6-5-688 2 +2.5 0.13 6-5-72 : 2 +2.3 0.15 l-2-92 2 +2.1 : 0.20 l-2-97 +2.2 1 7-3-l C +2.0 8.2.4E C 0.09 E 22 +2.3 0.06 8-3-90 2 +2.7 9-l-33 0.08 1 +2.7 I-3-1 : 2 +2.4 1987-7-3 C 0.21 Glaucophanic GreenschisUGreenschist (n = 5; +2.9 f 1.B. 8-2-24 1 +3.3 +2.6 8-2-28 : 0.22 +3.7 8-2-33 : 0.12 6-2-24 +4.0 : 0.02 ; +l.l 6-2-27a C 0.12 F_;tM&te-Amyibdite (n = 7; +4.3 f 0.57;,2;‘2 f 233 ppm)

rocks.

ppm N (wt. 5%KzO)

Law&t+A;ite

6-3-41’ 6-3-41” 6-3-53’ 6-3-54 l-3-43 7-3-45 Amphibdite 6-3-25’

C C

: 1

735 (2.96) zzz (2.72) 410 (2.35) 540 (1.54) 790 805 900 810 710 z (1.96) 360 130 120 z 315 (1.63) 710 150 (0.75) 780 1075 335 540 200 (1.26) 100 (0.28) 720 267 f 307 ppm) 65 110 170 810 180

+4.0 0.09 +4.6 +4.4 0.07 cc f +3.8 0.11 +3.9 3 0.03 1 +4.4 : (n = 7: +4.3 f 0.8%. 138 f 76 ppm) 4 +3.6 0.19 3 +3.6 0.45 2 2 6-5-63 +3.6 0.21 4 7-2-21 +4.5 ; 0.14 4 +4.2 0.15 1 +3.6 7-3-65 E +4.7 8-l-3 0.11 cc 22 +5.9 CAMS2 0.39 1 +4.3 CAMS C *Techniques for collection of N2 samples; C (cryogenic) and E (expansion).

tinterlayers in lawsonife-albite facies metasediintary

metasedimentary exposures (seeBEBOUT and BARTON, 1988, for description), 815N of separated muscovite (+4.5%0 with 280 ppm N and +4.3%0 with 305 ppm N) is similar to wholerock values for nearby metasedimentary hosts (see Table 1) and for muscovite from one metasedimentary rock (+4.4%0 for muscovite in sample 7-2-2 1). The whole-rock metasedimentary N concentrations correlate with the bulk major element composition of the rocks; metasandstones (metagraywackes) tend to have lower N concentrations than more pelitic compositions, which are generally finer-grained and have higher wt% I&O, A1203, and C (metamorphosed organic matter). Nitrogen concentrations correspond in general to the modal proportion of white-mica and, in higher-grade rocks, biotite. This is consistent with the data of HONMA and ITIHARA ( 198 1 ), which indicate that NH: is strongly partitioned into micas in igneous and metamorphic rocks. Accordingly, for both the low- and high-grade rocks, N is positively correlated with K20 content (Fig. 2; see Table 1 for K,O concentrations on anhydrous basis). In the Catalina metasedimentary rocks, micas constitute the only

165 740 90 265 180 230 30 65

(1.50)

110 (1.58) 150 (2.68) 185 160 (2.84) 35 (0.78) 260 (1.98)

exposure.

significant K reservoir as K-feldspar is not present in any of the rocks. Albitic plagioclase is present in most of the rocks but it is unlikely to contain significant amounts of NH: (see intermineral partitioning data of HONMA and ITIHARA, 1981).

The lowest-grade rocks have high concentrations of N similar to many unmetamorphosed sediments (SWEENEYet al., 1978). The 6 “N of the lowest-grade rocks of the Catalina Schist (mean = + 1.9%0,std. dev. = 0.610, n = 13) is similar to that of unmetamorphosed sediments and porewater NH4 (e.g., RAU et al., 1987) and is similar to the value of +1.5%0 for a low-grade Franciscan Formation metagraywacke (ZHANG, 1988). The C/N (on a weight basis) ofthe lowestgrade metasedimentary rocks of the Catalina Schist ranges from 5-20, with a mean near 13. Such C/N values are characteristic of many unmetamorphosed sedimentary rocks, including those in trench and off-trench environments (see MULLER, 1977; PATIENCE et al., 1990). Higher-grade rocks have more variable C/N that range to higher values (C/N of 28-237 for five amphibolite-grade samples), presum-

Isotope geochemistry of N in metamorphic rocks

2843

800

Constraints on the Magnitude of Nitrogen Loss and Isotope Shifts

z

800

8 s

400

8 b -2

200

Estimates of the extent of N loss and the 6 “N shifts, both necessary for the batch and Rayleigh distillation calculations, are difficult to obtain due to the effect of variability in protolith compositions (e.g., see wide range of compositions for lawsonite-albite-facies samples; Table 1) . Protolith variability was taken into account in two ways. The first method involved the comparison of mean N concentration and 6 15N for the lowest- and highest-grade sample suites (see statistical data in Table 1) . This comparison results in an estimated F ( fmction of the initial N remaining in the rock) of 0.22 and an estimated isotopic shift of 2.4%~. A second method involved comparisons of N concentration and 6 15N in lower grade samples (lawsonite-albite and blueschist grade) with amphibolite-grade samples which have similar I&O concentrations. Considering only shifts between samples with similar KzO content (see nearly vertical lines connecting data for low- and high-grade samples on Fig. 2), the mean F is 0.26 with a standard deviation of 0.07; the ratio of the slopes of the lines fit through the high- and low-grade data is 0.26. A range of0.85-2.85% was used as a plausible 615N shift, based on the mean ( 1.85%) and one standard deviation ( l.OOL) of the differences in 6 15Nof low- and high-grade sample pairs with similar K20 content. For the following discussion, we have preferentially adopted the values inferred from the second method involving the use of KZO concentrations.

0

1

3

2

Kp (weight %) FIG. 2. Relationship of N and K20 concentrations in metasedimentary rocks of the Catalina Schist (see calculated lines, both with rz of -0.75, and data in Table 1). Nearly vertical lines connect samples with similar K20 content; for twelve sample pairs, highgrade samples contain 0.26 + 0.07 of the N in low-grade samples with similar K content. Calculated Fis shown for several of the sample pairs. The mean 6j5N shift between low- and high-grade samples in these same pairs is +1.85% (standard deviation of 1.0460).Symbols are the same as for Fig. 1.

ably due to the preferential volatilization reactions.

loss of N to fluids during de-

NITROGEN ISOTOPE FRACTIONATION DURING DEVOLATILIZATION OF THE CATALINA SCHIST

Batch Volatilization

Fluid-rock isotope fractionation of N during devolatilization of the me&sedimentary rocks of the Catalina Schist was evaluated using models of batch volatilization (see discussion by VALLEY,1986) and Rayleigh distillation (equation given by BROECKERand OVERSBY,197 1) and the observed trends of N loss and increasing 6 15N (see Fig. 1, Table 1) . Fractionation Factors for Isotope Exchange in the Nitrogen System Fractionations among species in the N system were calculated by HANSCHMANN( 1981); the results of those calculations for temperatures of 327-927°C are tabulated in HAENDELet al. ( 1986). RICHETet al. ( 1977),who calculated only fractionations among fluid species, presents calculations for NH3-N2 exchange which differ somewhat from those of HANSCHMANN( 1981) . Based on an interpolation of the data in HANSCHMANN( 1981) , the calculated A “Nnuid_mk( A 15N = d’SN”uid-6“Nmk) would range from approximately -3.4 to -2.25% at 350-600°C for the exchange reaction (after SCALAN, 1958): 15N14N+ 14NH: =

The batch volatilization model assumes that all of the fluid released during devolatilization equilibrates with the rock and is lost in a single batch (nonincremental release; see discussion by VALLEY, 1986). The dashed lines in Fig. 3 show rock 6 “N evolution due to batch volatilization for varying values of A’5NnUid_rWk appropriate for Nz-NH: and NHs-NH: exchange based on the calculations of HANSCHMANN( 1981). For the metasedimentary rocks of the Catalina Schist, values for F and the isotope shift are inferred by comparison of samples with similar K20 content (see Fig. 2 and the box in Fig. 3). A batch volatilization model involving Nz-NH: exchange at 350-600°C can explain the observed shifts in N concentration and isotopic composition (see box and dark shaded region between dashed lines labelled -3.4 and -2.25% on Fig. 3). Figure 3 demonstrates that batch devolatilization involving NH3-NH: exchange at these temperatures is incapable of explaining the shifts in N concentration and d 15N.Such a process would result in isotope shifts dramatically higher than those observed in the metasedimentary rocks of the Catalina Schist. Rayleigh Distillation

‘4&

+

‘sNH+

4.

(5)

Based on data in HANSCHMANN( 198 I), A ‘5Nnuid_rocl; would range from approximately - 11 to -6% in the same temperature range for the exchange reaction (,after SCALAN, 1958): “NH3 + 14NH: = 15NH; + 14NHs.

(6)

The Rayleigh distillation model, as applied to rock devolatilization (see review by VALLEY,1986)) implies sequential removal of infinitesimal aliquots of fluid, each equilibrated with the entire rock system. Figure 4 demonstrates the dependency of the calculated 6”N evolution on assumed shifts in 615N; estimates of the fraction of original N remaining (F); and fractionation factors, here expressed as

2844

G. E. Bebout and M. L. Fogel

A ‘5NRuid-rcck . When F of -0.26 + 0.07 (inferred from comparisons of samples with similar KzO; solid horizontal lines) and 6”N shifts of0.85-2.85% are used (curves labelled 0.85 and 2.85), a range in A “Nauid_rwkof -0.5 to -2.6’S ( - - 1.5 ? 1k ) is appropriate (shaded region). For comparison, the dashed lines in Fig. 4 indicate the calculation of a bulk fluidrock fractionation ( A’5Nn,id.,k) of approximately - 1.6%0 assuming 78% N loss (F = 0.22) and a 2.4%0 shift in 6”N of the rock based on differences in the mean 61SN and N content of lawsonite-albite- and amphibolite-grade metasedimentary rocks in the Catalina Schist (see statistical data in Table 1). The A ’5Nn”id.rmk of - 1.5 +- 1%Ocalculated in Fig. 4 is similar to but somewhat smaller than the calculated N,-NH: fractionations of HANSCHMANN( 198 1) for the appropriate tem-

_, 1 0.0

,

)(~)I

1.o 0.5 Fraction N Remaining (F)

FIG. 3. Batch volatilization calculations using N concentration and isotope data for metasedimentary rocks of the Catalina Schist (1S’~Ns~i,_,-6”N,klabelled on each line). The box indicates the range of N loss and isotope shift inferred from comparison of sample with similar KZO (see Fig. 2). The shaded regions are ranges of isotope effects expected for N2-NH: exchange (darker shading) and NHSNH: exchange (lighter shading) using the calculated ranges in fractionation factors from HANSCHMANN ( 198 1) for the temperature range of 350-6OO’C. The observed shifts in concentration and isotope composition are compatible with batch volatilization involving N1NH: exchange. Batch volatilization with a A’SN”.ld_mcrof -3. I %O can explain the shifts estimated from the differences in mean concentration and d 15Nfor the low- and high-grade rocks.

perature range of the Catalina rocks. In Fig. 5, the rock evolution in N concentration and 6 “N expected as the result of Rayleigh distillation with a A ‘5NnUid_mfk of - 1.5% is compared with the evolution predicted for Rayleigh distillation using the data of HANSCHMANN( 198 1) for N2-NH: exchange (shaded region). A sample calculation for a A “Nnuid_rmkof -6%0, the minimum value for NH3-NH: exchange in the temperature range of 350-6OO”C, is also shown. Calculated shifts in 6 “N for the observed N loss, using the Rayleigh distillation model and fractionation factors for NHS-NH; exchange (see HANSCHMANN,198 1), are considerably larger than those observed. The Applicability of Batch and Rayleigh Devohttiliition Models Endmember batch volatilization behavior is considered unlikely because rocks are probably incapable of retaining the large volumes of fluid produced during devolatilization without failing (fracturing; see discussions in GRAHAMet al.,

0.6

0.6

F

0.4 +6

A5N Fluid - Rock FIG. 4. Demonstration of the interdependencies of calculated shift in d “N due to Rayleigh distillation ( labelled curves) on calculated fluid-rock fractionation (A “Nnuid.mk;related to alpha, the fractionation factor, by b ‘5NflUid-8 15Nrmk= 10’ in alpha) for varying N loss. Indicated are estimates of N loss% based on differences in mean N content in the lawsonite-albite and amphibolite facies rocks (0.22; dashed line) and based on normalizations by use of the K20 data (Fig. 2; indicates an F or -0.26 + 0.07; see solid horizontal lines that indicate mean If: one standard deviation). Curves are for the mean of the 615N shifts for the pairs with similar K20 content (+ one standard deviation on lower and higher 6j5N sides; 1.85 f 1.07~; solid curves), and the difference in the mean 6”N of the lawsonite-albite and amphibolite samples (dashed curve labelled 2.4%0). The shaded region indicates the range of fractionations (- 1.5 & 1.O%O)compatible with the Catalina data based on these data for N loss and 6 15Nshift. Vertical arrows indicate fractionations calculated by use of differences in mean N and 615N of the low- and highgrade rocks (- -1.6460) and by use of the KzO-normalized data (- -1.4%0).

0.0

0.5

1.0

Fraction N Remaining (F) FIG. 5. Comparison of the expected Rayleigh distillation N-isotope inferred in Fig. 4 (see calculated curve evolution using A 15Ns,id_rocli forA15N flu,d ._rack = - 1.54bo)with that expected for Nz-NH: exchange (darker shading) and NH?-NH: exchange (lighter shading) using the &culated ran& in fmcti&ation factors-from HAN~CHM~NN ( 1%I ) for the temperature range of 350-600°C.

Isotope geochemistry of N in metamorphic rocks 1983; WALTHERand ORVILLE, 1982; VALLEY,1986). Metamorphic devolatilization processes might generally be expected to result in stable isotope signatures intermediate to those predicted by batch and Rayleigh distillation models (see RUMBLE, 1982; VALLEY,1986), as demonstrated in this study for the N-isotope system. Fluids produced by devolatilization reactions are likely to be released episodically in response to deformation and enhanced pore pressure. The application of the Rayleigh model assumes that the fluid is capable of efficiently escaping the rock system throughout devolatilization. WALTHERand ORVILLE( 1982) suggested the plausibility of Rayleigh distillation; given reasonable porosities and reaction rates, fluids should be able to escape, perhaps toward fractures. Additional assumptions are ( 1) that the system is closed to infiltration by fluids from external sources, particularly fluids out of N-isotopic equilibrium with the rocks (see discussion below); and (2) that bulk-rock fractionation factors do not vary during the course of the devolatilization due to temperature effects or due to changes in the modes of minerals which differentially fractionate N isotopes (see discussions of Rayleigh models in RUMBLE, 1982; VALLEY, 1986). Because the release of the water-rich fluid occurred over a temperature range ( -350 to 600°C; see BEBOUT, 1991a), the latter assumption is probably not strictly correct. However, for N2-NHa exchange, fractionation factors do not appear to vary substantially over the 350 to 600°C range (see HANSCHMANN,198 1). Several analyses of whole-rock samples and mineral separates from the same amphibolite-grade samples, which represent metamorphism at temperatures of 650-75O”C, indicate that no significant fractionations exist between coexisting minerals at these relatively high temperatures (see discussion above; ZHANG, 1988). In addition, sodic amphibole veins in blueschist-grade metasedimentary exposures have similar S “N to host-rocks (for four veins, mean 6 15N = +2.1960; std. dev. = 0.1 k), suggesting that fluid/sodic amphibole fractionation was similar to fluid/whole-rock fractionation. These findings are consistent with the probability that N resides in the same molecule (NH:) in the different minerals and the likelihood that N-isotope fractionations are thus strongly governed by N-H bonding. Thus, the estimation of a single bulk-rock Nisotope. fractionation factor may be at least roughly appropriate for the metasedimentary rocks of the Catalina Schist, despite the inferred range of metamorphic temperatures and the known variations in mineral modes. The Rayleigh calculations assume that isotopic equilibrium is maintained between the fluid phase and the remaining mineral phases during incremental loss of fluid. In the Catalina metasedimentary rocks, the mechanisms affording this continual reequilibration could presumably have involved diffusive exchange and dissolution-reprecipitation during grain coarsening. The increase in white mica grain size, a trend in white mica chemistry (decrease in celadonite substitution, [(Mg, Fe+‘) + Si = 2Al]; SORENSEN,1986; BEBOUT, 1989), and increases in the grain size and degree of crystallinity of carbonaceous matter ( BEBOUT, 1989) could reflect these processes. For other rocks, DUIT et al. ( 1986) reported coupled decreases in mica and whole-rock N concentration with increasing metamorphic grade. Such decreases are consistent with progressive partitioning of N into fluids equilibrated with the remaining micas, as opposed to selective re-

2845

lease of N due to mica breakdown (i.e., with the remaining mica retaining its original higher N content). Unless significant intermineral N-isotope fractionations exist, no trend in rock 6 “N would be expected if preferential breakdown of some minerals occurs without reequilibration of the whole rock with the fluid. The range in A “N~uid_~~ calculated in this study with the Rayleigh distillation equation (- 1.5 + 1960)overlaps but is somewhat smaller than the range of A 15Nauid_wkcalculated by HANSCHMANN( 198 1) for N2-NH$ exchange over the ap propriate temperature range. The values are also smaller than ranges of -3.0 to -4.0460 and -6.0 to -11.56 obtained by RREULEN et al. (1982) and BOTTRELLet al. ( 1988), respectively, for rock/mineral-fluid inclusion pairs in low-grade metasedimentary rocks. Our derivation of A ‘sNfl,id-mc~ values slightly lower than the calculated values of HANSCHMANN ( 198 1) is consistent with Rayleigh distillation behavior involving incomplete reequilibration of silicate phases (primarily the micas) with fluids during devolatilization. If parts of the rock system retained some N with lower 6 “N not equilibrated with the successive aliquots of fluid released by devolatilization reactions, the shift of the rock toward higher 615N values would be smaller than predicted for Rayleigh distillation. If a significant fraction of the N in the fluids was speciated as NHs, this lack of reequilibration would have been more dramatic to result in the observed relatively small shifts in 61SN (~3%). Major element zoning in micas in some metasedimentary samples of the Catalina Schist (Bebout, unpubl. data) and in samples from other metamorphic suites (e.g., JACOBSON, 1980) may reflect incomplete reequilibration of micas during progressive metamorphism. For other metasedimentary rocks, the preservation in garnets of gradients in 6 “0 produced by progressive closed-system devolatilization (CHAMBERLAINand CONRAD, 199 1) implies a similar lack of whole-rock reequilibration during prograde devolatilization reactions. Mixture of N fluid species (Nz-NHs) to produce bulk A15Nfhd._rock of the magnitude inferred in this study is considered unlikely. The A ‘5Nfiuid_rmk values calculated here are slightly lower than those for Nz-NH: exchange. If the calculations of HANSCHMANN( 198 1) are correct, significant speciation of N as NH3 would, in the case of complete reequilibration, have resulted in shifts greater than those predicted for N2-NH: exchange (see Fig. 5 ) . However, exchange involving NH: and NH3 with a smaller degree of reequilibration (i.e., less efficient equilibration of the fluid and rock during volatile loss) cannot be entirely discounted. Evidence for Nitrogen Speciation and Isotopic Composition in the Catalina Metamorphic Fluids

Several lines of evidence, other than the dramatic loss of N observed in the Catalina metasedimentary rocks, suggest the presence of N, probably as Nz, in the Catalina metamorphic fluids. The abundance of N in veins in the lowgrade units (up to 200 ppm in sodic amphibole veins) is direct evidence for at least local-scale redistribution of N in fluids. Quadrupole mass spectrometry of fluid inclusions in mafic blocks in the blueschist-grade melange in the Catalina Schist detected small amounts of NZ (probably < 1 mole%; T. C. Hoering, pers. comm., 1990). Calculations of N

2846

G. E. Bebout and M. L. Fogel

speciation in metamorphic fluids by FERRY and BAUMGARTNER(1987), DUIT et al. ( 1986), and BOTTRELLet al. ( 1988) suggest that NZ is the dominant N fluid species under most crustal metamorphic conditions (including those appropriate for the low-grade units of the Catalina Schist), except under exceptionally reduced conditions which may stabilize NH3 relative to N2 in HzO-rich C-O-H-N fluids. The Rayleigh distillation and batch volatilization calculations presented above indicate that the isotopic composition of N in the Catalina metamorphic fluids may have been governed by the Nz-NH: exchange reaction (reaction 5), which involved structurally bound NH 2 in silicate phases (particularly the micas) and Nz in the fluids. Phlogopite-vapor distribution coefficients for NH: partitioning (from Bos et al., 1986; KD (NH:/K) = 1.2 to 1.6 at 550 to 65O’C; see also HALLAM and EUGSTER, 1976) suggest that only very low fugacities of N-species in the fluids (NH3, N2) may be required to stabilize concentrations of NH: in micas observed in most metamorphosed sedimentary suites (see DUIT et al., 1986; generally > 100 ppm and 5500 ppm) . Veins and metasomatic rinds on mafic blocks in melange (see SORENSENand BARTON, 1987, and BEBOUTand BARTON, 1989, for description) contain minerals with 615N similar to that of the metasedimentary rocks (e.g., Na-amphibole veins in the blueschist unit). This agreement in 6 “N is compatible with the equilibration of veins and rinds with fluids derived during devolatilization of the metasedimentary rocks; 0, H, and C isotope data for veins and host-rocks are also indicative of a metasedimentary fluid source ( BEBOUTand BARTON, 1989;BEBOUT, 1991a).Arangeof6”Nof-1.5 to +5.5%0 is inferred for NP in the Catalina metamorphic fluids based on the A’5Na,,‘d_rock of - -1.5 + lo/w inferred from the Rayleigh distillation calculations and the present 615N of the rocks (see Fig . 6) . NITROGEN ISOTOPES AS TRACERS OF DEVOLAT’ILIZATION PROCESSES

Closed-system distillation behavior (fluid loss without infiltration by externally derived fluids), superimposed on premetamorphic variability, may be expected to produce a scatter in 6 “N and N concentration in single minerals (e.g., muscovite from adjacent layers) where bulk compositional variations result in varying fluid-loss histories. The equilibrium batch volatilization and Rayleigh distillation models applied in this study appear to satisfactorily explain local evolution in N isotope composition in the metasedimentary rocks of the Catalina Schist. However, the apparent applicability of these models does not preclude some infiltration by externally derived fluid, particularly for the case where the infiltrating fluid has previously achieved N-isotopic equilibrium within the nearby rock system. If fluids produced by devolatilization reactions diffuse toward fractures (channels for fluid removal) through rock of similar mineralogy and 6”N (on a mm to m scale; e.g., along lithologic layers of uniform composition ), then no isotopic (or concentration) shift is expected as a result of their transport. The fluids are previously equilibrated with the rocks and are presumably unreactive relative to the N-isotope system (see BEBOUT, 1991b). In contrast, fluid transport across lithologic layering may serve to homogenize interlayer variations in d15N and N concentrations of single minerals (e.g., muscovite in adjacent layers) along the fluid

Blueschist + Lawsonite-Albite Glaucophanic Greenschist

Amphibolite I

-3

1

’

I

I

I

I

+3

0

I

I

I

+6

615N,i, FIG. 6.6 15Nvs. metamorphic grade ( increasing toward bottom of figure) for Catalina metascdimentary rocks, showing the estimated range in fluid 615N (shaded area) based on trends in 6”N and N concentration and the A’SN”uid_Wtinferred from the Rayleigh calculations (range reflects A ‘5Nfluid_mck of - 1.5 f 1% without consid-

eration of temperature dependence). flow paths. The uniformity of 6 “N in some low-grade exposures may reflect this transport (e.g., compare 6 lSN of samples 7-2-97 and 7-2-92 and samples 6-5-23dk and 6-523grn). The preservation of gradients in 6”N ( 1.4%0variation) in one interlayered metamorphosed sandstone-shale exposure (see data for cm-scale interlayers in 6-3-2K in Table 1) indicates the local inefficiency of this reequilibration process. If N partitions selectively into certain minerals (e.g., micas), then the modal abundance of these minerals may dictate whole-rock N concentration in cases of open-system behavior (e.g., fluid-buffered mineral composition and flow paths cross-cutting layers of different compositions). The extreme result of such buffering would be a perfect straight-line fit of N and KZO content (see Fig. 2); deviations from straightline behavior could reflect failure to reequilibrate (i.e., preservation of local devolatilization history). Carbon and 0 isotope evidence supports the model of batch volatilization to Rayleigh distillation behavior indicated by the N data. The 613C of carbonaceous matter in the same rocks increases from values of -26 to -24%0 in the lowestgrade rocks to values of -2 1 to - 19%0in the highest-grade, amphibolite-facies rocks. Progressive fractionation of “C from carbonaceous matter in metasedimentary rocks into CHI in fluids by a Rayleigh distillation process could presumably explain the observed shift in 6 13C(cf. fractionation data in BOTTINGA, 1969 ) . Heating/ freezing experiments (SORENSENand BARTON, 1987) and analyses by quadrupole mass spectrometry indicate that CH4 is the dominant C species, ranging from 25 ppm). We are able to analyze -20 samples in one day. It appears that, for whole-rock metasedimentary samples, the combustion techniques described in this paper yield data with precision equal to or, in some cases, greater than that obtained by the dissolution/distillation techniques (e.g., see HAENDEL et al., 1986; JUNGE et al., 1989). Nitrogen is initially bound primarily in organic matter, but, during early diagenesis, the organic matter is remineralized, resulting in release of NH: which can then be incorporated by silicate phases, probably primarily authigenic clay minerals and low-grade metamorphic micas (MULLER, 1977; ROSENFELD,1979; JUSTER et al., 1987). Efficient retention of N by the rocks during diagenesis and low-grade metamorphism is indicated by similarities in N content and C/ N of the lowest-grade metasedimentary rocks in the Catalina Schist with unmetamorphosed seafloor sedimentary equivalents. Data for progressively metamorphosed sedimentary rocks of the Catalina Schist are consistent with other studies that have documented N loss with increasing metamorphism ( MILOVSKIYand VOLYNETS,1966; DUIT et al., 1986; HAENDEL et al., 1986; COOPER and BRADLEY, 1990). The data are explained by a devolatilization process approximating Rayleigh distillation; fluid-bulk rock N isotope fractionations inferred in this study are compatible with calculated fractionation factors of HAN~CHMANN( 198 1) . The shifts in 6 ’5N and N concentration in the Catalina Schist metasedimentary rocks are consistent with NI-NH: exchange as the dominant mechanism of N-isotope fractionation (cf. HAENDELet al., 1986). Reduction in N concentration accompanies decrease in Hz0 content in the Catalina metasedimentary rocks. This finding is consistent with the progressive partitioning of N into H1O-rich fluids produced by devolatilization reactions primarily involving breakdown of chlorite and celadonitic white mica to produce muscovite-, biotite-, garnet-, and kyanite-bearing mineral assemblages. The degree of heterogeneity in N concentration and isotopic composition in packets of metasedimentary rocks of similar grade is due to the following: I ) initial premetamorphic variability; 2) major element bulk composition, which controls the local devolatilization history and the abundances of particularly N-rich minerals; and 3) the extent to which open-system behavior occurs during devolatilization, particularly involving fluids out of N isotope equilibrium with the rocks. The N-isotope system may provide a valuable means of evaluating open- and closed-system behavior during metamorphic devolatilization and other fluid-rock interactions and may prove effective as a tracer of large-scale volatile transport. Acknowledgmenfs-Special thanks are due T. C. Hoering, D. Rumble III, D. Velinsky,and P. L. Koch for assistanceand helpful discussions;

and to T. C. Hoering, P. L. Koch, S. Macko,A. E. Moran, J. Morrison, J. M. Palin, and two anonymous reviewersfor helpful critique of the manuscript. D. Velinsky analyzed some samples on the Carlo-Erba

CNS analyzer at the University of Delaware. The Smithsonian Institution (P. J. Dunn) provided the buddingtonite sample used as a secondary standard. S. S. Sorensen provided several samples of metasedimentary rocks. K. Brauer and D. K. Haendel (Akademie der Wissenschafien der DDR) kindly provided samples of metasedimentary rocks used for interlaboratory comparison. Hydrogen extractions were performed in the laboratmy of M. D. Barton at UCLA. GEB wishes to thank the Santa Catalina Island Conservancy for its continued support of the Catalina field research. Editorial handling: S. A. Macko

REFERENCES ANDERSEN T., BURKEE. A. J., and AUSTRHEIM H. ( 1989)Nitrogenbearing, aqueous fluid inclusions in some eclogitesfrom Western Gneiss Region of the Norwegian Caledonides. Contrib. Mineral. Petrol. 103, 153-165. E. ( 1989) Geological and geochemical investigations of fluid flow and mass transfer during subduction-zone metamorphism. Ph.D. thesis, Univ. California, Los Angeles. BEBOUTG. E. ( 1991a) Field-based evidence for devolatilization in subduction zones: Implications for arc magmatism. Science 251, 413-416. BEBOUT G.

BEBOUTG. E. ( 1991b) Stable isotope and trace element indicators of devolatilization history in metashalesand me&sandstones.Ann. Repc. Dir. Geophys. Lab., Carnegie Inst. Wash., 1990-1991. pp. 34-4 I. Geophys. Lab., Washington, DC.

BEBOUT G. E. and BARTONM. D. ( 1988) Field evidence for partial melting of sediment and oceanic crust in a subduction complex. Catalina Schist Terrane, California. Eos; Trans. Amer. Geophys. Union 69, 505.

BEBOUTG. E. and BARTONM. D. ( 1989) Fluid flow and metasomatism in a subduction zone hydrothermal system: Catalina Schist terrane, California. Geology 17,976-980.

BEBOUTG. E., RYANJ. G., and LEEMANW. P. ( 1991) Boron and beryllium concentrations in subduction-related metamorphic rocks of the Catalina Schist: Implications for subduction-zone recycling. Ann. Rept. Dir. Geophys. Lab., Carnegie Inst. Washington, 19901991, pp. 23-29. Geophys. Lab., Washington, DC. BIGELEISENJ., PERLMANM. L., and PROSSERH. C. ( 1952) Conversion of hydrogenic materials to hydrogen for isotopic analysis. Anal. Chem. 24, 1356- 1357. BOS A., DUIT W., VAN DER EERDEN AD M. J., and JANSEN J. B. H. ( 1986) Nitrogen storagein biotite: An experimental study of the ammonium and potassium partitioning between 1M-phlogopite and vapour at 2 kb. Geochim. Cosmochim. Acta 52, 12751283. BOT~INGAY. ( 1969) Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-COz-graphitemethane-hydrogen and water vapor. Geochim. Cosmochim. Acta 33,49-64. BOTTRELLS. H., CARR L. P., and DUBESSYJ. (1988) A nitrogenrich metamorphic fluid and coexisting minerals in slates from North Wales. Mineral. Mug. 52,45 l-457. BROECKERW. S. and OVERSBYV. M. ( 197 1) Chemical Equilibrium in the Earth. McGraw-Hill. CASQUET C. ( 1986) C-O-H-N fluids in quartz segregations from a major ductile shear zone: The Berzosa fault, Spanish Central System. J. Metam. Geol. 4, 117-130. CHAMBERLAINC. P. and CONRADM. E. ( 1991) Oxygen isotope zoning in garnet. Science 254, 403-406. COOPERD. C. and BRADLEYA. D. ( 1990) The ammonium content of granites in the English Lake District. Geol. Mag. 127, 579-586. DARIMONTA., BURKE E., and TOURET J. (1988) Nitrogen-rich metamorphic fluids in devonian metasediments from Bastogne, Belgium. Bull. Mineral. 111, 321-330. DUIT W., JANSENJ. B. H., VAN BREEMENA., and Bos A. (1986) Ammonium micas in metamorphic rocks as exemplified by Dome de L’Agout (France). Amer. .I. Sci. 286, 702-732. ERD R. C., WHITE D. E., FAHEYJ. J., and LEE D. E. ( 1964) Buddingtonite, an ammonium feldspar with zeolitic water. Amer. Mineral. 49, 83 I-850. FERRYJ. M. ( 198 I ) Petrology of graphitic sulfide-rich schists from

Isotope geochemistry of N in metamorphic rocks south-central Maine: An example of desulfidation during prograde regional metamorphism. Amer. Mineral. f&908-930. FERRY J. M. and BAUMGARTNER L. ( 1987) Thermodynamic models of molecular fluids at the elevated pressures and temperatures of crustal metamorphism. Rev. Mineral. 27, 743-753. FRANCHIL. A., WRIGHT I. P., GIBSON E. K., JR., and PILLINGER C. T. ( 1989) Applications of lasers in small-sample stable isotopic analysis. In New Frontiers in Stable Isotope Research: Laser Probes, Ion Probes, and Small-Sample Analysis (ed. W. C. SHANKSIll and R. E. CRISS); USGS Bull. 1890,S l-59. GRAHAMC. M., GRIEG K. M., SHEPPARDS. M. F., and TURI B. ( 1983 ) Genesis and mobility of the H20-CO2 fluid phase during regional greenschist and amphibolite facies metamorphism: A petrological and stable isotope study in the Scottish Dalradian. J. Geol. Sot. London 140,577-599. GROVE M. ( 199 1) Insight into the thermal history of the Catalina Schist: Implications of preliminary 40Ar/39Arresults (abstr.) GSA Abstr. Prog. 23, 445. HAENDELD., MUHLE K., NITZSCHEH.-M., STIEHLG., and WAND U. ( 1986) Isotopic variations of the fixed nitrogen in metamorphic rocks. Geochim. Cosmochim. Acta 50,749-758. HALLA. ( 1988) Crustal contamination of minette magmas: Evidence from their ammonium contents. N. Jb. Min. Mh. 1988, 137-143. HALL A. (1989) Ammonium in spilitized basalts of southwest England and its implications for the recycling of nitrogen. Geochem. J. 23, 19-23. HALLAMM. and EUGSTERH. P. ( 1976) Ammonium silicate stability relations. Contrib. Mineral. Petrol. 57, 227-244. HANSCHMANNG. ( 1981) Berechnung von lsotopieeffekten auf quantenchemischer Grundlage am Beispiel stickstof fhaltiger Molekule. Zfl.-Mitt.. 41, 19-39. HEIERK. S. ( 1973) Geochemistry of granulite facies rocks and problems of their origin. Phil. Trans. Roy. Sot. Lond. 273, 429-442. HOERING T. C. ( 1955 ) Variations of nitrogen-l 5 in naturally occurring substances. Science 122, 1233-1234. HONMAH. and ITIHARAY. ( 1981) Distribution of ammonium in minerals of metamorphic and granitic rocks. Geochim. Cosmochim. Acta 45,983-988. JACOBSONC. E. ( 1980) Deformation and metamorphism of the pclona Schist beneath the Vincent Thrust, San Gabriel Mountains, California. Ph.D. thesis, Univ. California, Los Angeles. JAVOYM., PINEAUF., and DELORMEH. ( 1986) Carbon and nitrogen isotopes in the mantle. Chem. Geol. 57,41-62. JENDENP. D., KAPLANI. R., POREDAR. J., and CRAIGH. ( 1988) Origin of nitrogen-rich natural gases in the California Great Valley: Evidence from helium, carbon, and nitrogen isotope ratios. Geochim. Cosmochim. Acta 52, 85 l-861. JUNGE F., SELTMANNR., and STIEHLG. ( 1989) Nitrogen isotope characteristics of breccias, granitoids, and greisens from eastern Erzgebirge tin ore deposits (Sadisdorf: Altenberg) , GDR. Proc. 5th Working Mtg., Isotopes in Nature, Leipzig, September,l989, pp. 321-332. JUSTERT. C., BROWNP. E., and BAILEYS. W. ( 1987) NH&earing illite in very low grade metamorphic rocks associated with coal, northeastern Pennsylvania. Amer. Mineral. 72, 555-565. KENDALLC. and GRIM E. ( 1990) Combustion tube method for measurement of nitrogen isotope ratios using calcium oxide for total removal of carbon dioxide and water. Anal. Chem. 62, 526529. KREULENR. and SCHUILINGR. D. ( 1982) NTCHcCQ2 fluids during formation of the Dome de LAgout, France. Geochim. Cosmochim. Acta 46, 193-203. KREULENR., VAN BREEMENA., and DUIT W. ( 1982) Nitrogen and carbon isotopes in metamorphic fluids from the Dome de L’Agout, France. Proc. 5th Intl. Co@ Geochronol. Cosmochronol. Isot. Geol., p. 191. LEEMANW. P., MORANA. E., and SIS~ONV. B. ( 1990) Compositional variations accompanying metamorphism of subducted oceanic lithosphere: Implications for genesis of arc magmas and mantle replenishment. Abstr. 7th ICOG Mtg., p. 58. MACKOS. ( 198 1) Stable nitrogen isotope ratios as tracers of organic geochemical processes. Ph.D. thesis, Univ. Texas at Austin. MILOVSKIYA. V. and VOLYNETSV. F. ( 1966) Nitrogen in metamorphic rocks. Geochem. Intl. 3, 752.

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MINAGAWAM., WINTERD. A., and KAPLAN I. R. ( 1984) Comparison of Kjeldahl and combustion methods for measurement of nitrogen isotope ratios in organic matter. Anal. Chem. 56, 18591861. MULLERP. J. ( 1977) C/N ratios in Pacific deepsea sediments: Effect of inorganic ammonium and organic nitrogen compounds sorbed by clays. Geochim. Cosmochim_Acta 41,?65-7761 OLIVERN. H. S.. HOERINGT. C.. JOHNSTONT. W.. SHANKS W. C.. and RUMBLED., Ill ( 1990) Sulfur isotope geochemistry of gra: phitic, sulfidic schists from the Waterville-Augusta area, Maine, using SO* and SF6 techniques. Ann. Rept. Dir. Geophys. Lab., Carneaie Inst. Washinaton 1989-l 990. DD. . . 30-33. Geonhvs. 1 _ Lab.. Washington, DC. PATIENCER. L., CLAYTONC. J., KEARSLEYA. T., ROWLANDS. J., BISHOPA. N., REES A. W. G., BIBBYK. G., and HOPPERA. C. ( 1990) An integrated biochemical, get-chemical, and sedimentological study of organic diagenesis in sediments from Leg 112. Proc. ODP; Sci. Results (E. SUESS, R. VON HUENE, et al.) 112, 135-153. PLATT J. P. ( 1975) Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrain. GSA Bull. 86, 1337-l 347. RAU G. H., ARTHURM. A., and DEAN W. E. ( 1987) “N/ r4N variations in Cretaceous Atlantic sedimentary sequences: Implication for past changes in marine nitrogen biogeochemistry. Earth Planet. Sci. Lett. 82, 269-279. RICHET P., BOTTINGA Y., and JAVOYM. (1977) A review of hydrogen, carbon, nitrogen, oxygen, sulphur, and chlorine stable isotope fractionation among gaseous molecules. Ann. Rev. Earth Planet. Sci. 5,65- 110. RIGBYD. and BAT-I-SB. D. ( 1986) The isotopic composition of nitrogen in Australian coals and oil shales. Chem. Geol. 58. 273282. ROSENFELD J. K. ( 1979) Ammonium adsorption in nearshore anoxic sediments. Limnol. Oceanonr. 24(2). 356-364. RUMBLED. ( 1982) Stable isotope f&_&nation during metamorphic volatilization. Rev. Mineral. i0, 327-353. SAMSONI. M. and WILLIAMS-JONES A. E. (. 1991), C-Q-H-N-salt fluids associated with contact metamorphism. McGerrigle Mountains, Quebec A Raman spectroscopic study. Geochim. Cosmochim. Acta 55, 169-177. SCALANR. S. ( 1958) The isotopic composition, concentration, and chemical state of the nitrogen in igneous rocks. Ph.D. diss., Univ. Arkansas. SHERWOODB., FRITZ P., FRAPES. K., MACKOS. A., WEISES. M., and WHELHANJ. A. ( 1988) Methane occurrences in the Canadian Shield. Chem. Geol. 71,223-236. SORENSENS. S. ( 1986) Petrologic and geochemical comparison of the blueschist and greenschist units of the Catalina Schist terrane, southern California. GSA Mem. 164, 59-75. SORENSEN S. S. and BARTONM. D. ( 1987) Metasomatism and partial melting in a subduction complex: Catalina Schist, southern California. Geology 15, 115-I 18. SWEENEYR. E., LIU K. K., and KAPLANI. R. ( 1978) Oceanic nitrogen isotopes and their uses in determining the source of sedimentary nitrogen. In Stable Isotopes in the Earth Sciences, (ed. B. W. ROBINSON); DSIR Bull. 220,9-26. VALLEYJ. W. ( 1986) Stable isotope geochemistry of metamorphic rocks. Rev. Mineral. 16,445-489. VELINSKYD. J., PENNOCKJ. R., SHARP J. H., CIFIJENTESL. A., and FOGEL M. L. ( 1989) Determination of the isotopic composition of ammonium-nitrogen at the natural abundance level from estuarine waters. Mar. Chem. 26, 35 l-36 1. VONCKENJ. H. L., KONINGSR. J. M., JANSENJ. B. H., and WOEN~DREGT C. F. ( 1988) Hydrothermally grown buddingtonite, an anhydrous ammonium feldspar ( NH,AlSi30,). Contrib. Mineral. Petrol. 115, 323-328. WALTHERJ. V. and ORVILLEP. M. ( 1982) Volatile production and transport in regional metamorphism. Contrib. Mineral. Petrol. 79, 252-257. WHITE D. E. and WARINGG. A. ( 1963) Volcanic emanations. In Data of Geochemistry; USGS Prof Pap. 440-K. ZHANGD. ( 1988) Nitrogen concentrations and isotopic compositions of some terrestrial rocks. Ph.D. diss., Univ. Chicago.