Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history

Gray Edward Bebout, Marilyn L. Fogel

Research output: Contribution to journalArticle

231 Citations (Scopus)

Abstract

In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in δ15Nair with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with δ15N = +1.9 ± 0.6‰, whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with δ15N = +4.3 ± 0.8‰. Loss of N accompanied devolatilization reactions that evolved H2O-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 high- P T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations (Δ15N = δ15Nfluid - δ15Nrock) of -1.5 ± 1‰ were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K2O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH4+ exchange at the temperature range of 350-600°C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25‰). The N systematics appear to be explained by N2-NH4+ exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and δ15N cannot be explained by NH3-NH4+ 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 δ15N( ~ -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.

Original languageEnglish
Pages (from-to)2839-2849
Number of pages11
JournalGeochimica et Cosmochimica Acta
Volume56
Issue number7
DOIs
Publication statusPublished - 1992
Externally publishedYes

Fingerprint

Nitrogen Isotopes
nitrogen isotope
metasedimentary rock
schist
History
distillation
Rocks
fluid
fractionation
Fractionation
history
Chemical analysis
Distillation
rock
Fluids
lawsonite
kyanite
amphibolite
albite
volatilization

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

@article{53b50756b307454e84205d670c1e8588,
title = "Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history",
abstract = "In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in δ15Nair with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with δ15N = +1.9 ± 0.6‰, whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with δ15N = +4.3 ± 0.8‰. Loss of N accompanied devolatilization reactions that evolved H2O-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 high- P T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations (Δ15N = δ15Nfluid - δ15Nrock) of -1.5 ± 1‰ were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K2O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH4+ exchange at the temperature range of 350-600°C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25‰). The N systematics appear to be explained by N2-NH4+ exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and δ15N cannot be explained by NH3-NH4+ 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 δ15N( ~ -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.",
author = "{Edward Bebout}, Gray and Fogel, {Marilyn L.}",
year = "1992",
doi = "10.1016/0016-7037(92)90363-N",
language = "English",
volume = "56",
pages = "2839--2849",
journal = "Geochmica et Cosmochimica Acta",
issn = "0016-7037",
publisher = "Elsevier Limited",
number = "7",

}

TY - JOUR

T1 - Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California

T2 - Implications for metamorphic devolatilization history

AU - Edward Bebout, Gray

AU - Fogel, Marilyn L.

PY - 1992

Y1 - 1992

N2 - In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in δ15Nair with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with δ15N = +1.9 ± 0.6‰, whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with δ15N = +4.3 ± 0.8‰. Loss of N accompanied devolatilization reactions that evolved H2O-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 high- P T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations (Δ15N = δ15Nfluid - δ15Nrock) of -1.5 ± 1‰ were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K2O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH4+ exchange at the temperature range of 350-600°C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25‰). The N systematics appear to be explained by N2-NH4+ exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and δ15N cannot be explained by NH3-NH4+ 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 δ15N( ~ -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.

AB - In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in δ15Nair with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with δ15N = +1.9 ± 0.6‰, whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with δ15N = +4.3 ± 0.8‰. Loss of N accompanied devolatilization reactions that evolved H2O-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 high- P T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations (Δ15N = δ15Nfluid - δ15Nrock) of -1.5 ± 1‰ were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K2O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH4+ exchange at the temperature range of 350-600°C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25‰). The N systematics appear to be explained by N2-NH4+ exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and δ15N cannot be explained by NH3-NH4+ 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 δ15N( ~ -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.

UR - http://www.scopus.com/inward/record.url?scp=0026488085&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0026488085&partnerID=8YFLogxK

U2 - 10.1016/0016-7037(92)90363-N

DO - 10.1016/0016-7037(92)90363-N

M3 - Article

AN - SCOPUS:0026488085

VL - 56

SP - 2839

EP - 2849

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

IS - 7

ER -