Nitrogen-isotope record of fluid-rock interactions in the Skiddaw Aureole and granite, English Lake District
The Skiddaw Granite and its contact metamorphic aureole in the English Lake District provide an excellent opportunity to test the capability of the N-isotope system to trace devolatilization and large-scale transfer of crustal fluids. In the aureole, Skiddaw Group metasedimentary rocks with relatively uniform lithology and major-element compositions show a dramatic decrease in N content toward the granite contact (from {ge}800 ppm at distances >2.5 km from the contact, to <410 ppm {le}0.55 km from the contact). Far from the intrusive body (>1.5 km), these rocks have extremely uniform {delta}{sup 15}N{sub air} near +3.7%, whereas closer to the contact ({le}1 km) {delta}{sup 15}N is shifted to higher values (up to +8.7%). The coupled decreases in N content and increases in {delta}{sup 15}N are compatible with the removal of N having low {delta}{sup 15}N in fluids during continuous, prograde devolatilization reactions involving the breakdown of white mica and the stabilization of biotite-, cordierite-, and andalusite-bearing assemblages. In the same metasedimentary rocks, the lack of obvious trends in major-element concentrations (including SiO{sub 2}/TiO{sub 2}, SiO{sub 2}/Al{sub 2}O{sub 3}, and the ratios of other major oxides to TiO{sub 2} and Al{sub 2}O{sub 3}) with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism. Ratios of whole-rock N, B, Rb, and Ba concentrations to whole-rock K{sub 2}O content are believed to reflect the differing fluid-mica partitioning (and involving varying relative proportions of white mica and biotite) of these trace elements during devolatilization reactions.
- Research Organization:
- Lehigh Univ., Bethlehem, PA (US)
- OSTI ID:
- 20000951
- Journal Information:
- American Mineralogist, Vol. 84, Issue 10; Other Information: PBD: Oct 1999; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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