wAbstract
@The Mt. Stafford region, central Australia, provides well-exposed
metasedimentary rocks whose protoliths were deposited at ca. 1.84-1.81
Ga, and that were metamorphosed at greenschist to granulite facies
pressure-temperature conditions at 1.8 Ga. The higher-grade rocks
show evidence of low-pressure partial melting (2-4 kbars) over
a temperature range of 650K to 850. These exposures afford evaluation
of the extent to which N is retained through devolatilization
and partial melting reactions experienced in the shallow- to mid-continental
crust.
@In the Mt. Stafford metapelitic/metapsammitic suite, whole-rock,
ย15Nair values of the lowest-grade,
greenschist facies rocks (Zone 1) range from +2.3 to +6.2๑ (mean
= +3.6๑; 1ะ = 1.5๑; n = 7) and values for the highest-grade granulite
facies rocks (Zone 4) range from +2.1 to +8.5๑ (mean = +5.2๑;
1ะ = 2.1๑; n = 27). Six of the twenty samples at the highest grade
have values higher than those of the greenschist rocks regarded
as likely protoliths. All but a few of the Zone 4 rocks have N
concentrations lower than the mean N concentration for the lowest
grade rocks (ranges of 107-346 ppm, mean 254 ppm, for Zone 1 rocks,
and 43-361 ppm, mean 185 ppm, for Zone 4 rocks). For the higher
grade rocks, which experienced multiple partial melting reactions,
varying retention of N, as NH4+
in K-feldspar or as N2 in cordierite, may
indicate that N is held in peritectic products of partial melting
rather than in the melt.
@Although assessments of element loss in the Mt. Stafford suite
are greatly complicated by the large degree of protolith-related
heterogeneity, some elements do show modest decline in concentration,
with increasing grade, consistent with their loss during devolatilization
and partial melting. nitrogen appears most similar in behavior
to Rb (the LILE most similar in ionic radius to NH4+)
but, of these two elements, N shows somewhat greater evidence
for whole-rock loss. K2O shows only very
subtle hints of whole-rock loss, in some samples, and Cs shows
considerably greater loss relative to N and Rb (resulting, for
example, in increases in Rb/Cs at the higher grades). Increased
whole-rock Ba/Rb for some higher-grade samples, relative to that
for lower-grade equivalents, likely reflects the formation of
a major Ba reservoir in K-feldspar at the expense of biotite (resulting
in no obvious whole-rock Ba loss), with Rb liberated from biotite
breakdown partitioning into the melts that at least partly left
the system. Reduced whole-rock B concentrations at the higher
grades resulted from the breakdown of tourmaline during the melting
reactions. These varying degrees of reduction in whole-rock N,
Rb, Cs, and B (and possibly K2O) concentration
are accompanied by decreased concentrations in H2O
(as indicated by LOI), Li, and U, the loss of U resulting in increased
Th/U in the highest-grade rocks. Apparent differential loss of
these elements, based on the whole-rock data, could reflect the
amounts of melt produced and the extents to which the melts were
removed at the scales sampled by the hand-specimens that were
analyzed. Six of the twenty Zone 4 samples showing greater shift
in ย15N, to values of +7.4 to +8.4๑, tend to be among
the samples showing the largest changes in other major and trace
element concentrations and ratios believed to reflect melt removal
(e.g., K2O, Cs/TiO2,
B/Al2O3, Th/U, and ratios
among Rb, Sr, Ba, and Cs). The metasedimentary rocks at Mount
Stafford also contain 217 to 1186 ppm C, likely as reduced C (graphite)
but, at the higher grades, probably also as CO2
in the channels of cordierite. At greenschist grade, whole-rock
ย13CV-PDB values are relatively
uniform, with organic compositions (mean = -25.1๑; 1ะ = 1.6๑;
n = 7), but at the highest grade (Zone 4) values are as high as
-17.7๑ (mean = -22.2๑; 1ะ = 2.3๑; n = 8). Overall, C and N, apparently
largely of organic origin, show surprising retention in rocks
having experienced peak temperatures of 800, demonstrating their
compatibility in mineral phases in fluid and melt residues (for
N, as NH4+ in K-feldspar and as
N2 in cordierite; for C, as graphite and
as CO2 in cordierite). These data have implications
for deep-Earth N cycling, bearing on the size of the N reservoir
in the mid-continental crust (and thus long-term storage of n
previously residing in the atmosphere and biosphere), and for
the use of the ย15N of upper-crustal intrusive rocks
as a tracer of their deep crustal sources.
Keywords: Nitrogen isotopes; Migmatite; Mount Stafford; Partial
melting; Isotope fractionationx
1. Introduction
2. Geologic setting and metamorphic history
3. Analytical methods
@3.1. Whole-rock geochemistry
@3.2. Stable isotope analyses
@@3.2.1. nitrogen - whole-rock samples
@@3.2.2. Nitrogen - cordierite separates
@@3.2.3. Whole-rock carbon isotope analyses
4. Results
@4.1. Mineralogy and textures
@4.2. Nitrogen and carbon isotope data
5. Discussion
@5.1. Reaction history as a function of bulk-rock chemistry
(and pressure/depth)
@5.2. Nitrogen behavior in the context of other major and trace
element geochemistry
@@5.2.1. Regarding element enrichment due to passive enrichment
during melt loss
@5.3. Isotopic fractionation of nitrogen and carbon during devolatilization
and melting
@@5.3.1. Nitrogen isotopic compositions of cordierite
@5.4. Significance of the size of the continental crustal N and
C reservoirs and Early Proterozoic organic N and C isotopic compositions
@5.5. Implications for the ย15N of granites
6. Conclusions
Acknowledgements
References