Geochemical Journal, Vol. 36, pp. 209 to 217, 2002
Recycled noble gas and nitrogen
in the subcontinental lithospheric mantle:
Implications from N-He-Ar in fluid inclusions of SE Australian xenoliths
TAKUYA MATSUMOTO,1* DANIELE L. P INTI,2 JUN -ICHI MATSUDA1 and S USUMU UMINO 3
1
Department of Earth and Space Science, Graduate School of Science, Faculty of Science, Osaka University,
Toyonaka, Osaka 560-0043, Japan
2
Laboratoire de Géochronologie Multitechniques, CNRS UMR 8616 et UMR 7577, Université Paris SUD,
Bat. 504, 91405, Orsay Cedex, France
3
Department of Biology & Geosciences, Shizuoka University, Ohya 836, Shizuoka 422-8529, Japan
(Received December 28, 2001; Accepted February 8, 2002)
To elucidate the source of an air-like component in fluid inclusions of xenoliths from the subcontinental mantle, we measured N, He and Ar elemental and isotopic composition in gases released by crushing
from spinel-lherzolites of the Newer Volcanics, south-eastern Australia. Gas released from fluid inclusions in olivine separates shows δ15N ranging from –6.0 ± 1.2‰ to +2.0 ± 1.7‰. The range of measured
δ15N values are in contrast with a remarkably uniform 3He/4He ratio of 10.1 ± 0.2 × 10 –6. The lightest δ15N
value of –6.0 ± 1.2‰ is consistent with the measured MORB-like 3He/4He ratio of 10.1 ± 0.2 × 10–6 and
suggests that gases in xenoliths of southeast Australia are derived from a well-mixed upper mantle reservoir. The heavier nitrogen isotopic signatures (from ~0 to +2‰) and elemental ratio of argon to nitrogen
could be explained by the addition of 30% to 40% of a recycled sedimentary component. Nitrogen is
indeed recycled more efficiently in the mantle than helium, preserving the trace of present or past subduction. The heavy N component has been observed in xenoliths from the eastern side of the Newer Volcanic
province. Sedimentary nitrogen may result from subduction along the eastern margin of Australia, during
Paleozoic time. The present nitrogen results, together with the relatively low 40Ar/36Ar ratios and apparently correlated 3He and 36Ar contents in those xenoliths, suggest the long-term preservation of recycled
surface volatiles in the continental lithospheric mantle.
et al., 1999; Mohapatra and Murty, 2000a, 2002;
Matsumoto et al., 2001). Noble gases in ultramafic
rocks (=xenoliths and orogenic peridotites) are
mostly trapped in confined fluid inclusions. These
are probably metasomatic fluids infiltrated
through the mantle wall rock and their helium isotopic signatures clearly indicate mantle origin
(Matsumoto et al., 1997, 1998, 2000, 2001). As
will be shown later in this paper, Matsumoto et
al. (2001) found a clear correlation between primordial 3He and atmospheric 36Ar in former mantle wedge rocks, indicating that source for the
metasomatic fluids should also have air-Ar in the
I NTRODUCTION
Presence of isotopically air-like heavy noble
gas component is almost ubiquitous in mantlederived rocks. Origin of this air-like heavy noble
gas component is commonly regarded as a result
of atmospheric contamination. Indeed, this is in
many cases true especially for basaltic samples
erupted suboceanically and subaerially (e.g.,
Patterson et al., 1990). However, some recent studies pointed out that there is an occurrence of airlike heavy noble gases that is not consistent with
a secondary contamination hypothesis (e.g., Sarda
*Corresponding author (e-mail: [email protected])
209
210
T. Matsumoto et al.
mantle. Because a similar correlation can also be
recognized in xenoliths from the subcontinental
mantle, recycled air-Ar might have been accumulated in the subcontinental lithosphere as the continents evolve. One way to clarify this hypothesis
is to check elemental and isotopic compositions
of other volatiles in the xenoliths. We take here
the nitrogen because elemental and isotopic composition of nitrogen, as well as relative abundance
of argon to nitrogen, in the subducting material
should be greatly different from those in the mantle and in the atmosphere, which is quite helpful
to identify the recycled surface-related volatile
components in deep-seated xenoliths.
In terms of nitrogen in the subcontinental mantle, nitrogen composition is so far available only
for the San Carlos xenoliths (Mohapatra and
Murty, 2000b). Mohapatra and Murty (2000b) reported a range of δ15N values from –10‰ to +10‰
in gases released by step-heating; the positive δ15N
values can be interpreted as a presence of recycled sedimentary nitrogen in the subcontinental
mantle. However, there is no direct measurement
on nitrogen and noble gases trapped in fluid inclusions of xenoliths from the subcontinental mantle. Therefore, in the present work, we attempt to
extract and analyse nitrogen and associated noble
gases in the subcontinental mantle xenoliths from
SE Australia by vacuum crushing technique. The
advantage of crushing compared to heating is that
external contamination (possibly with an
isotopically heavier organic nitrogen) is minimized. Note that the xenoliths from the same locality yielded mixture of MORB-type heavy noble gases and atmospheric component in gases
released by crushing extraction (Matsumoto et al.,
1998), suggesting that the metasomatic fluids
might have acquired both components in the mantle (Matsumoto et al., 2001).
SAMPLES DESCRIPTION AND
EXPERIMENTAL PROCEDURE
The Newer Volcanics in south-eastern Australia comprise a monogenetic volcanic field covering ca. 25000 km 2; the volcanism is of Plio-
Fig. 1. Map of southeastern Australia with xenolith
localities. The Delamerian-Lachlan fold belt boundary
is from Gibson and Nihil (1992).
Pleistocene age (Fig. 1). Volcanological features
of the Newer Volcanics include scoria cones,
maars, tuff rings and major valley lava flows from
small eruption points involving mainly alkali basalt magmas (Nicholls and Joyce, 1989). Six anhydrous lherzolites collected at three separate
eruptive centres, Mt. Leura, Mt. Shadwell and Mt.
Gambier, were selected for nitrogen and noble gas
isotope analyses. Mt. Leura and Shadwell are located on the Palaeozoic Lachlan Fold Belt,
whereas Mt. Gambier is about 200 km to the west
and on an older Delamerian Fold Belt. These
xenoliths are considered fragments of a shallow
mantle part of the continental lithosphere, brought
to the surface by a relatively recent activity of the
Newer Basalts (Griffin et al., 1984).
The samples were coarsely crushed, and clear
olivine grains were handpicked. After washing
three times with analytical grade acetone and ethanol, the samples were dried overnight and loaded
into a stainless-steel gas extraction line. Nitrogen
and argon were extracted from the olivine separates by vacuum crushing to selectively extract
mantle-derived component from the abundant
CO2-rich fluid inclusions of the samples. The extracted gases were analysed by using a quadrupole
mass spectrometer. Helium isotopic ratios were
also determined for different aliquots of the same
sample by using a noble gas mass spectrometer
Micromass ® 5400, after gas extraction by stepped
Recycled noble gas and nitrogen in the subcontinental mantle
211
Table 1. Nitrogen, argon and helium compositions in olivines from the anhydrous lherzolites of the
newer volcanics, SE Australia
*Duplicated analyses for different aliquots of the same sample.
**Uncertainties (2 σ) for N2 and Ar concentrations are 5% of the measured value.
***Results from Matsumoto et al. (1998).
pyrolysis (see Wada and Matsuda, 1998 for experimental details).
For the nitrogen and argon analyses, about 1
to 2 grams of olivine grains (1–3 mm in size) were
loaded into the sample holder. After baking overnight at 150°C, the samples were crushed about
300 strokes by a newly developed sample crusher.
This stainless-steel crusher has an internal volume
of 3.5 cm3, with a piston moved by compressed
air (Pinti et al., 1999; Matsumoto et al., 2001).
About 50% (by weight) of the sample was crushed
into <#60 mesh, which we regarded as the crushed
yield (the weights shown in Table 1). The gas was
split into two fractions for nitrogen and argon
analyses. Hydrocarbons were removed from the
nitrogen fraction by exposing to a CuO/Cu trap
and to cold fingers. The noble gas fractions were
exposed to two stage Ti-Zr getters to remove active gases. Purified nitrogen and argon were introduced into a quadrupole mass spectrometer
(type QMA 420, Balzers®) operated under a static
vacuum for precise isotope analyses. Procedural
blank levels were measured prior to each sample
run. The N 2 and 40 Ar blanks were typically
10 –8~–9 and 10 –9~–10 cm3STP, respectively. These
values are quite low compared to N2 and 40Ar signals measured during sample analyses (~10 –6
cm3STP for optimum N isotope measurements and
212
T. Matsumoto et al.
10 –8 cm3STP for 40Ar measurements). Elemental
and isotopic compositions of N and Ar are corrected for instrumental and procedural blanks as
well as isotope discrimination factors determined
from repeated analyses on the known amounts of
air standard. Full descriptions of the analytical
method and data processing are given in
Yamamoto et al. (1998).
RESULTS AND DISCUSSION
Helium
Several studies have shown evidence of
MORB-type noble gases in the Australian SCLM
(Porcelli et al., 1986; Matsumoto et al., 1997,
1998, 2000). The MORB-type component is confirmed by our helium analyses (Table 1), which
show a remarkably uniform 3 He/ 4 He ratio of
10.1 ± 0.2 × 10–6, that is within the range of the
upper mantle helium isotopic ratio measured in
MORBs (e.g., Craig and Lupton, 1981).
N2/Ar ratios
Upper mantle nitrogen is characterized by a
striking N2/36Ar versus 40Ar/ 36Ar correlation, as
observed in MORB vesicles (Marty, 1995). This
correlation suggests that the mantle source for
MORBs has a N2/36Ar and a 40Ar/ 36Ar ratio higher
than that of the atmosphere, by more than two orders of magnitude (N 2/ 36ArAIR = 2.5 × 104 and
40
Ar/36ArAIR = 295.5; N2/36ArMORB > 3.2 × 107 and
40
Ar/36ArMORB > 4 × 104). As shown in Fig. 2, the
N 2 / 36 Ar and 40 Ar/ 36 Ar ratios in Australian
xenoliths seem to be correlated each other and plot
roughly in the same region of N-MORBs. The
straightforward interpretation of this apparent correlation would be a binary mixing between an atmospheric and a mantle component, the latter having uniform 40Ar/36Ar and N2/36Ar ratios, both in
sub-oceanic and sub-continental reservoirs. However, even though the present data form a quasilinear trend in a plot of 40Ar/36Ar versus N 2/ 36Ar
ratios, a simple binary mixing between the man-
Fig. 2. The 40Ar/36Ar versus N2/36Ar ratios in olivines of the SE Australian anhydrous lherzolites. Small open
squares, Australian lherzolites. Small filled circles, N-MORBs (Marty, 1995). Solid and dotted lines are mixing
lines between air and a hypothetical end member which derives from the mixture of a mantle (N 2/40Ar = 80; 40Ar/
36
Ar = 40000; Marty, 1995) and a sedimentary component (N 2/36Ar = 6 × 106; 40Ar/36Ar = 300; Sano et al., 1998)
in different proportions. Numbers indicate the proportions of the MORB component in this hypothetical end member. Arc-related samples have been reported for comparison (data from Sano et al., 1998).
Recycled noble gas and nitrogen in the subcontinental mantle
tle and the atmosphere is not sufficient to explain
the variable nitrogen isotopic ratios observed in
the Australian xenoliths. This requires an additional N component.
Nitrogen isotopic components in the sub-continental mantle
The nitrogen isotopic composition of Australian xenoliths is shown in Table 1. The present results are the first report on nitrogen isotopes measured in gases released by crushing the ultramafic
xenoliths. Therefore, they should be an important
Fig. 3. N 2/36Ar ratios as a function of δ15N values.
Symbols are the same as those in Fig. 2 (MORB isotopic data are from Marty and Humbert, 1997). Endmember composition of the MORB, SEDIMENTS and
AIR components is also shown. Mixing curves between
the MORB and AIR and SEDIMENTS and AIR have
been also reported. Values for each end member are
from Sano et al. (1998): δ15NMORB = –5‰; δ15NAIR =
0‰; δ15NSEDIMENT = +7‰; N2/36ArMORB = 6 × 106; N2/
36
ArAIR = 1.8 × 104; N2/36ArSEDIMENTS ≥ 6 × 106. Contribution of each of the three components noted in the
text is calculated by solving a simple system of equations:
(15N/14N)MEASURED = k( 15N/14N)MORB
+ l(15N/14N)AIR + m(15N/14N)SEDIMENT
(36Ar/14N)MEASURED = k( 36Ar/14N)MORB
+ l(36Ar/14N)AIR + m(36Ar/14N)SEDIMENT
1=k+l+m
where k, l and m denotes the fractions of nitrogen derived from MORB, AIR and SEDIMENTS, respectively.
213
addition to the relatively small nitrogen database
currently available for ultramafic xenoliths from
continental settings (Mohapatra and Murty,
2000b). As shown in Table 1, the observed δ 15N
values vary from –6.0 ± 1.2‰ to +2.0 ± 1.7‰.
The variability in the nitrogen isotopic composition contrasts with the constant 3He/ 4He ratios
measured in the same samples. As the gases were
extracted by vacuum crushing, the observed nitrogen isotope variation should represent that of
nitrogen trapped in ubiquitous CO2-rich fluid inclusions.
The lightest nitrogen composition of –6.0 ±
1.2‰ in Australian xenoliths has been measured
in a lherzolite from Mt. Gambier (Table 1). This
value is within the range of nitrogen in the MORBsource mantle (δ 15N ~ –5 ± 2‰) (e.g., Marty and
Humbert, 1997; Sano et al., 1998). As shown in
Fig. 3, the δ15N value and the N/36Ar ratio for samples GAMVL3 and VIC53C plot close to the mixing curve between a MORB-like mantle source
and air. At least these two samples appear to have
a nitrogen isotopic component similar to that in
the mantle, as also suggested by the noble gas isotopic signatures (Matsumoto et al., 1998). The
direct inference from this observation would be
that part of the sub-continental mantle shares the
same nitrogen reservoir as that feeding MORBs.
Although the δ15N values in all the other samples are either similar to or slightly heavier than
atmospheric nitrogen, it is clear that the composition of these samples cannot be explained by a
simple binary mixing between a MORB-like component and an atmospheric component. A third
component, which we assume to have a sedimentary origin, must be taken into account. This component is characterized by isotopically heavier N
(δ15N ≥ 7‰) and it is introduced together with
the air component into the mantle by subduction
of oceanic sediments (Sano et al., 1998). The occurrence of a third “sedimentary” component is
shown in Fig. 3, where the δ15N values are plotted versus the N2/36Ar ratios. Xenoliths are plotted in an area defined by mixing lines between a
MORB, a sedimentary and an atmospheric source.
Samples VIC51 and VIC51A have both δ15N = 0‰
214
T. Matsumoto et al.
within 1σ uncertainty. However, their 40Ar/ 36Ar,
N2/36Ar ratios are clearly not atmospheric. VIF54F
and SHD6 are plotted near the Air-Sediment mixing line as it was observed in some arc-related
samples (Sano et al., 1998).
It is possible to estimate the relative contribution of the atmospheric, the mantle and the sedimentary component in our samples by fixing the
δ15N values and N2/36Ar ratios of each component
and using simple ternary mixing equations (Sano
et al., 1998). If the end-member composition suggested by Sano et al. (1998) is applied
( δ 15 N MORB = –5‰; δ 15 N AIR = 0‰;
δ15NSEDIMENT = +7‰; N2/36ArMORB = 6 × 106; N2/
36
ArAIR = 1.8 × 104; N 2/ 36ArSEDIMENTS ≥ 6 × 106),
the contribution of the N sedimentary component
in the Australian xenoliths is estimated to range
from 30% to 40% of the total. The MORB-like
component should contribute from 5% to 40% of
total nitrogen. It should be noted that such estimates must be regarded as a first order approximation. For example, if a higher δ 15N SEDIMENT
value of +15‰ is chosen for a sedimentary component (as observed in metasediments), then its
contribution to the nitrogen composition is lowered to 20% of the total.
In the 40Ar/ 36Ar versus N2/36Ar plot (Fig. 2),
the contribution of a sedimentary component to a
mantle sources result in a steeper slope of the mixing line. The most obvious case is for the arc- and
back arc-related samples whose 40Ar/36Ar and N2/
36
Ar ratios requires an end-member significantly
rich in a sedimentary component with respect to a
MORB component (Fig. 2). Among the Australian xenoliths, VIC51 and VIC51A depart from the
mixing line between air and a pure MORB component, confirming the contribution of a third,
sedimentary component in these samples. On the
other hand, sample GAMVL3, which was characterized by a clear mantle δ15N value, plots on the
Air-MORB mixing line.
Although definite proportions of each component in the samples cannot be determined without
ambiguity due to the difficulties in fixing the endmember compositions, two independent arguments
from δ 15N–N 2/ 36Ar and N2/ 36Ar–40Ar/ 36Ar rela-
tionships strongly indicate that:
1) Near-atmospheric δ15N in the continental
xenoliths do not derive from an atmospheric contamination.
2) A sedimentary component characterized by
elevated δ15N and N 2/40Ar ratio is required to explain the observed elemental and isotopic composition of the studied xenoliths.
Possible origin for the nitrogen components in the
SCLM
The presence of a sedimentary component in
continental lherzolites has also been suggested by
the nitrogen isotopic composition of San Carlos
lherzolites (Mohapatra and Murty, 2000b). These
samples showed δ 15N values ranging from –10‰
to +10‰ even in gases released from a single
specimen at different temperatures during stepwise
pyrolysis. Present results significantly reinforce
the idea that this sedimentary component is a signature of the mantle. This is because we extracted
gases from CO2-rich fluid inclusions, where mantle-derived noble gases are preserved (e.g.,
Matsumoto et al., 1998, 2000). Therefore, the sedimentary nitrogen and the MORB-like (nitrogen
and noble gas) component should have been mixed
each other in the mantle to a variable extent prior
to the entrapment of CO2-rich fluids as fluid inclusions in mantle wall rocks.
It has previously been suggested that nitrogen
could be recycled into the mantle much more efficiently than the light noble gases (Bebout, 1995).
This is because nitrogen is fixed in the form of
ammonium ions in the crystal lattice of K-bearing minerals leading to a much stronger retention
of ammonium than of gaseous helium. Helium is
easily lost by devolatilization in the subduction
zone prior to incorporation of the subducting slab
into the upper mantle (Staudacher and Allègre,
1988), whereas 10% to 20% of the N in sediments
could be preserved up to their melting temperature (Boyd et al., 1993; Hall, 1999). This selective recycling of nitrogen over helium may explain
why nitrogen isotopic ratios are heterogeneous in
the xenoliths, whereas helium has preserved only
its mantle isotopic signature (Table 1). This is
Recycled noble gas and nitrogen in the subcontinental mantle
Fig. 4. Concentration of 3He (primordial isotope) and
36
Ar in the SEA xenoliths reported in Matsumoto et al.
(1998). Concentration of 36Ar is that of air-Ar in each
sample, calculated based on a binary mixing between
an atmospheric component and a MORB component
with 40Ar/36Ar = 295.5 and 40000, respectively. Results
of Horoman ultramafics (Matsumoto et al., 2001) are
also shown for comparison. Note that a simple atmospheric contamination should not necessarily result in
such correlation between 3He and 36Ar.
mainly due to the difficulty in distinguishing shallow level contamination from addition of recycled
material, as both should have a near-atmospheric
noble gas isotopic composition. Reported high N2/
40
Ar ratios in sediments implicitly suggest that
recycling is more efficient for nitrogen than argon. However, the 40Ar/36Ar ratios observed in the
Australian lherzolites (<10000; Matsumoto et al.,
1998) are always significantly lower than those
reported for the gas-rich MORBs (~60000)
(Burnard et al., 1997). This, and the present nitrogen results, indicates that the systematically low
40
Ar/36Ar ratios in fluid inclusions of the xenoliths
may not due to mere atmospheric contamination,
but instead partly reflect the presence of recycled
argon in the SCLM. In regard to the possible heavy
noble gas recycling, Matsumoto et al. (2001) recently suggested that correlated 3 He and 36Ar
abundances in a series of orogenic lherzolites from
215
Horoman complex, Northern Japan, can be regarded as an evidence for the presence of recycled atmospheric argon in wedge mantle (Fig. 4).
It was also pointed out that the ultramafic xenoliths
from SE Australia and Europe have similar correlations, that might also be explained by the presence of recycled air-Ar in the subcontinental
lithosphere (Fig. 4) incorporated through the
growth of SCLM at arc settings. In fact, there are
several lines of evidence suggesting that the SE
Australian margin was a subduction zone similar
to a modern arc system in the early Paleozoic
(Middle Cambrian). In the Lachlan fold belt,
where Mts. Shadwell and Leura are located, large
granite outcrops and greenstone terranes containing boninites and low-Ti andesites have been observed (Chappell, 1984; Crawford et al., 1984).
In this respect, the absence of a clear recycled nitrogen signature in the Mt. Gambier sample
(GAMVL3), located hundreds kilometres west to
Mts. Shadwell and Leura, might correspond to the
change in the heterogeneity of the lithospheric
mantle from east to west. Such a lateral heterogeneity in the continental lithosphere beneath SE
Australia has previously been recognized in the
distribution of the Sr isotope ratios in the Newer
Basalts (Price et al., 1997). Price et al. (1997) reported a small range in 87Sr/ 86Sr ratios (~0.7040)
in western part of the Mt. Shadwell, then an abrupt
change to a wide range, and much more radiogenic
Sr in the basalts from Mt. Shadwell eastwards
(0.7040–0.7055). Note that this boundary roughly
coincides with that between the Delamerian and
Lachlan Fold Belts, with the latter being built up
by a series of arcs (e.g., Powell, 1983; Collins and
Vernon, 1994). Mt. Shadwell is likely to have been
located on the edge of the arc front and may have
been affected to a larger degree by a recycled component than the Mt. Gambier lithosphere, 200 kilometres to the west. The occurrence of a sedimentary N component in the SE Australia SCLM is
consistent with the ancient tectonic history of the
area, and the hypothesis of an oceanic arc system
at the south-eastern margin of Australia during
Paleozoic (e.g., Powell, 1983; Collins and Vernon,
1994).
216
T. Matsumoto et al.
CONCLUSIONS
The N-Ar-He systematics in the SE Australian
xenoliths requires the presence of at least two distinct sources of volatiles beneath SE Australia.
One source seems to be the same of that for
MORBs (upper mantle) as suggested by measured
3
He/4He ratios of 10 × 10–6 and δ15N values of
–6‰. The second source is sedimentary volatiles
recycled into mantle through subduction. This
component is characterized by nitrogen having a
heavier isotopic signature than that of upper mantle and very high N2/40Ar ratios. Finally, nitrogen
in the xenoliths from the SCLM suggests that the
recycled component should have been stored in
the mantle for a significant period of time, without being completely mixed with the nitrogen in
the convective mantle. In this respect, the SCLM
itself is a possible candidate for the storage site
as they are thought to have persisted in their
present form over long time periods.
Acknowledgments—The Mt. Gambier sample had
kindly been provided by Prof. Sue O’Reilly at
GEOMOC. Eleanor Dixon is also acknowledged for
reading a manuscript very carefully. Comments and
suggestions by R. Mohapatra on an earlier version of
the ms were quite helpful. S.V.S. Murty is thanked for
his constructive and critical review. D.L.P. stay in Osaka
University was funded by EU Commission contract no.
CIPI 940115 and JSPS P 96239. This work was partly
supported by Grants-in-Aid from the Japan Society for
the Promotion of Science to T.M. (12740306).
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