GJ 36-3-02_02520_ by goodbaby


									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

     Department of Earth and Space Science, Graduate School of Science, Faculty of Science, Osaka University,
                                       Toyonaka, Osaka 560-0043, Japan
       Laboratoire de Géochronologie Multitechniques, CNRS UMR 8616 et UMR 7577, Université Paris SUD,
                                      Bat. 504, 91405, Orsay Cedex, France
           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 subcontinen-
      tal 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 inclu-
      sions 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 reser-
      voir. 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 subduc-
      tion. 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 appar-
      ently 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;
                    I NTRODUCTION
                                                                Matsumoto et al., 2001). Noble gases in ultramafic
    Presence of isotopically air-like heavy noble               rocks (=xenoliths and orogenic peridotites) are
gas component is almost ubiquitous in mantle-                   mostly trapped in confined fluid inclusions. These
derived rocks. Origin of this air-like heavy noble              are probably metasomatic fluids infiltrated
gas component is commonly regarded as a result                  through the mantle wall rock and their helium iso-
of atmospheric contamination. Indeed, this is in                topic signatures clearly indicate mantle origin
many cases true especially for basaltic samples                 (Matsumoto et al., 1997, 1998, 2000, 2001). As
erupted suboceanically and subaerially (e.g.,                   will be shown later in this paper, Matsumoto et
Patterson et al., 1990). However, some recent stud-             al. (2001) found a clear correlation between pri-
ies pointed out that there is an occurrence of air-             mordial 3He and atmospheric 36Ar in former man-
like heavy noble gases that is not consistent with              tle wedge rocks, indicating that source for the
a secondary contamination hypothesis (e.g., Sarda               metasomatic fluids should also have air-Ar in the

*Corresponding author (e-mail: matsumoto@ess.sci.osaka-u.ac.jp)

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 accumu-
lated in the subcontinental lithosphere as the con-
tinents 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 com-
position of nitrogen, as well as relative abundance
of argon to nitrogen, in the subducting material
should be greatly different from those in the man-
                                                       Fig. 1. Map of southeastern Australia with xenolith
tle and in the atmosphere, which is quite helpful
                                                       localities. The Delamerian-Lachlan fold belt boundary
to identify the recycled surface-related volatile      is from Gibson and Nihil (1992).
components in deep-seated xenoliths.
    In terms of nitrogen in the subcontinental man-
tle, nitrogen composition is so far available only
for the San Carlos xenoliths (Mohapatra and            Pleistocene age (Fig. 1). Volcanological features
Murty, 2000b). Mohapatra and Murty (2000b) re-         of the Newer Volcanics include scoria cones,
ported a range of δ15N values from –10‰ to +10‰        maars, tuff rings and major valley lava flows from
in gases released by step-heating; the positive δ15N   small eruption points involving mainly alkali ba-
values can be interpreted as a presence of recy-       salt magmas (Nicholls and Joyce, 1989). Six an-
cled sedimentary nitrogen in the subcontinental        hydrous lherzolites collected at three separate
mantle. However, there is no direct measurement        eruptive centres, Mt. Leura, Mt. Shadwell and Mt.
on nitrogen and noble gases trapped in fluid in-       Gambier, were selected for nitrogen and noble gas
clusions of xenoliths from the subcontinental man-     isotope analyses. Mt. Leura and Shadwell are lo-
tle. Therefore, in the present work, we attempt to     cated on the Palaeozoic Lachlan Fold Belt,
extract and analyse nitrogen and associated noble      whereas Mt. Gambier is about 200 km to the west
gases in the subcontinental mantle xenoliths from      and on an older Delamerian Fold Belt. These
SE Australia by vacuum crushing technique. The         xenoliths are considered fragments of a shallow
advantage of crushing compared to heating is that      mantle part of the continental lithosphere, brought
external contamination (possibly with an               to the surface by a relatively recent activity of the
isotopically heavier organic nitrogen) is mini-        Newer Basalts (Griffin et al., 1984).
mized. Note that the xenoliths from the same lo-           The samples were coarsely crushed, and clear
cality yielded mixture of MORB-type heavy no-          olivine grains were handpicked. After washing
ble gases and atmospheric component in gases           three times with analytical grade acetone and etha-
released by crushing extraction (Matsumoto et al.,     nol, the samples were dried overnight and loaded
1998), suggesting that the metasomatic fluids          into a stainless-steel gas extraction line. Nitrogen
might have acquired both components in the man-        and argon were extracted from the olivine sepa-
tle (Matsumoto et al., 2001).                          rates by vacuum crushing to selectively extract
                                                       mantle-derived component from the abundant
                                                       CO2-rich fluid inclusions of the samples. The ex-
          SAMPLES DESCRIPTION AND                      tracted gases were analysed by using a quadrupole
          EXPERIMENTAL PROCEDURE                       mass spectrometer. Helium isotopic ratios were
    The Newer Volcanics in south-eastern Aus-          also determined for different aliquots of the same
tralia comprise a monogenetic volcanic field cov-      sample by using a noble gas mass spectrometer
ering ca. 25000 km 2; the volcanism is of Plio-        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 ex-                 analyses. Hydrocarbons were removed from the
perimental details).                                          nitrogen fraction by exposing to a CuO/Cu trap
    For the nitrogen and argon analyses, about 1              and to cold fingers. The noble gas fractions were
to 2 grams of olivine grains (1–3 mm in size) were            exposed to two stage Ti-Zr getters to remove ac-
loaded into the sample holder. After baking over-             tive gases. Purified nitrogen and argon were in-
night at 150°C, the samples were crushed about                troduced into a quadrupole mass spectrometer
300 strokes by a newly developed sample crusher.              (type QMA 420, Balzers®) operated under a static
This stainless-steel crusher has an internal volume           vacuum for precise isotope analyses. Procedural
of 3.5 cm3, with a piston moved by compressed                 blank levels were measured prior to each sample
air (Pinti et al., 1999; Matsumoto et al., 2001).             run. The N 2 and 40 Ar blanks were typically
About 50% (by weight) of the sample was crushed               10 –8~–9 and 10 –9~–10 cm3STP, respectively. These
into <#60 mesh, which we regarded as the crushed              values are quite low compared to N2 and 40Ar sig-
yield (the weights shown in Table 1). The gas was             nals measured during sample analyses (~10 –6
split into two fractions for nitrogen and argon               cm3STP for optimum N isotope measurements and
212                                           T. Matsumoto et al.

10 –8 cm3STP for 40Ar measurements). Elemental            N2/Ar ratios
and isotopic compositions of N and Ar are cor-                 Upper mantle nitrogen is characterized by a
rected for instrumental and procedural blanks as          striking N2/36Ar versus 40Ar/ 36Ar correlation, as
well as isotope discrimination factors determined         observed in MORB vesicles (Marty, 1995). This
from repeated analyses on the known amounts of            correlation suggests that the mantle source for
air standard. Full descriptions of the analytical         MORBs has a N2/36Ar and a 40Ar/ 36Ar ratio higher
method and data processing are given in                   than that of the atmosphere, by more than two or-
Yamamoto et al. (1998).                                   ders of magnitude (N 2/ 36ArAIR = 2.5 × 104 and
                                                             Ar/36ArAIR = 295.5; N2/36ArMORB > 3.2 × 107 and
                                                             Ar/36ArMORB > 4 × 104). As shown in Fig. 2, the
                                                          N 2 / 36 Ar and 40 Ar/ 36 Ar ratios in Australian
Helium                                                    xenoliths seem to be correlated each other and plot
   Several studies have shown evidence of                 roughly in the same region of N-MORBs. The
MORB-type noble gases in the Australian SCLM              straightforward interpretation of this apparent cor-
(Porcelli et al., 1986; Matsumoto et al., 1997,           relation would be a binary mixing between an at-
1998, 2000). The MORB-type component is con-              mospheric and a mantle component, the latter hav-
firmed by our helium analyses (Table 1), which            ing uniform 40Ar/36Ar and N2/36Ar ratios, both in
show a remarkably uniform 3 He/ 4 He ratio of             sub-oceanic and sub-continental reservoirs. How-
10.1 ± 0.2 × 10–6, that is within the range of the        ever, even though the present data form a quasi-
upper mantle helium isotopic ratio measured in            linear trend in a plot of 40Ar/36Ar versus N 2/ 36Ar
MORBs (e.g., Craig and Lupton, 1981).                     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/
   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 mem-
ber. Arc-related samples have been reported for comparison (data from Sano et al., 1998).
                         Recycled noble gas and nitrogen in the subcontinental mantle                    213

tle and the atmosphere is not sufficient to explain      addition to the relatively small nitrogen database
the variable nitrogen isotopic ratios observed in        currently available for ultramafic xenoliths from
the Australian xenoliths. This requires an addi-         continental settings (Mohapatra and Murty,
tional N component.                                      2000b). As shown in Table 1, the observed δ 15N
                                                         values vary from –6.0 ± 1.2‰ to +2.0 ± 1.7‰.
Nitrogen isotopic components in the sub-continen-        The variability in the nitrogen isotopic composi-
tal mantle                                               tion contrasts with the constant 3He/ 4He ratios
    The nitrogen isotopic composition of Austral-        measured in the same samples. As the gases were
ian xenoliths is shown in Table 1. The present re-       extracted by vacuum crushing, the observed ni-
sults are the first report on nitrogen isotopes mea-     trogen isotope variation should represent that of
sured in gases released by crushing the ultramafic       nitrogen trapped in ubiquitous CO2-rich fluid in-
xenoliths. Therefore, they should be an important        clusions.
                                                             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 MORB-
                                                         source 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 sam-
                                                         ples GAMVL3 and VIC53C plot close to the mix-
                                                         ing 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 iso-
                                                         topic signatures (Matsumoto et al., 1998). The
                                                         direct inference from this observation would be
Fig. 3. N 2/36Ar ratios as a function of δ15N values.    that part of the sub-continental mantle shares the
Symbols are the same as those in Fig. 2 (MORB iso-       same nitrogen reservoir as that feeding MORBs.
topic data are from Marty and Humbert, 1997). End-
member composition of the MORB, SEDIMENTS and
                                                             Although the δ15N values in all the other sam-
AIR components is also shown. Mixing curves between      ples are either similar to or slightly heavier than
the MORB and AIR and SEDIMENTS and AIR have              atmospheric nitrogen, it is clear that the composi-
been also reported. Values for each end member are       tion of these samples cannot be explained by a
from Sano et al. (1998): δ15NMORB = –5‰; δ15NAIR =       simple binary mixing between a MORB-like com-
0‰; δ15NSEDIMENT = +7‰; N2/36ArMORB = 6 × 106; N2/
                                                         ponent and an atmospheric component. A third
   ArAIR = 1.8 × 104; N2/36ArSEDIMENTS ≥ 6 × 106. Con-
tribution of each of the three components noted in the   component, which we assume to have a sedimen-
text is calculated by solving a simple system of equa-   tary origin, must be taken into account. This com-
tions:                                                   ponent is characterized by isotopically heavier N
                                                         (δ15N ≥ 7‰) and it is introduced together with
(15N/14N)MEASURED = k( 15N/14N)MORB                      the air component into the mantle by subduction
              + l(15N/14N)AIR + m(15N/14N)SEDIMENT
                                                         of oceanic sediments (Sano et al., 1998). The oc-
(36Ar/14N)MEASURED = k( 36Ar/14N)MORB                    currence of a third “sedimentary” component is
              + l(36Ar/14N)AIR + m(36Ar/14N)SEDIMENT     shown in Fig. 3, where the δ15N values are plot-
                                                         ted versus the N2/36Ar ratios. Xenoliths are plot-
1=k+l+m                                                  ted in an area defined by mixing lines between a
                                                         MORB, a sedimentary and an atmospheric source.
where k, l and m denotes the fractions of nitrogen de-
rived from MORB, AIR and SEDIMENTS, respectively.        Samples VIC51 and VIC51A have both δ15N = 0‰
214                                           T. Matsumoto et al.

within 1σ uncertainty. However, their 40Ar/ 36Ar,        tionships strongly indicate that:
N2/36Ar ratios are clearly not atmospheric. VIF54F           1) Near-atmospheric δ15N in the continental
and SHD6 are plotted near the Air-Sediment mix-          xenoliths do not derive from an atmospheric con-
ing line as it was observed in some arc-related          tamination.
samples (Sano et al., 1998).                                 2) A sedimentary component characterized by
     It is possible to estimate the relative contribu-   elevated δ15N and N 2/40Ar ratio is required to ex-
tion of the atmospheric, the mantle and the sedi-        plain the observed elemental and isotopic compo-
mentary component in our samples by fixing the           sition of the studied xenoliths.
δ15N values and N2/36Ar ratios of each component
and using simple ternary mixing equations (Sano          Possible origin for the nitrogen components in the
et al., 1998). If the end-member composition sug-        SCLM
gested by Sano et al. (1998) is applied                      The presence of a sedimentary component in
( δ 15 N MORB = –5‰; δ 15 N AIR = 0‰;                    continental lherzolites has also been suggested by
δ15NSEDIMENT = +7‰; N2/36ArMORB = 6 × 106; N2/           the nitrogen isotopic composition of San Carlos
   ArAIR = 1.8 × 104; N 2/ 36ArSEDIMENTS ≥ 6 × 106),     lherzolites (Mohapatra and Murty, 2000b). These
the contribution of the N sedimentary component          samples showed δ 15N values ranging from –10‰
in the Australian xenoliths is estimated to range        to +10‰ even in gases released from a single
from 30% to 40% of the total. The MORB-like              specimen at different temperatures during stepwise
component should contribute from 5% to 40% of            pyrolysis. Present results significantly reinforce
total nitrogen. It should be noted that such esti-       the idea that this sedimentary component is a sig-
mates must be regarded as a first order approxi-         nature of the mantle. This is because we extracted
mation. For example, if a higher δ 15N SEDIMENT          gases from CO2-rich fluid inclusions, where man-
value of +15‰ is chosen for a sedimentary com-           tle-derived noble gases are preserved (e.g.,
ponent (as observed in metasediments), then its          Matsumoto et al., 1998, 2000). Therefore, the sedi-
contribution to the nitrogen composition is low-         mentary nitrogen and the MORB-like (nitrogen
ered to 20% of the total.                                and noble gas) component should have been mixed
     In the 40Ar/ 36Ar versus N2/36Ar plot (Fig. 2),     each other in the mantle to a variable extent prior
the contribution of a sedimentary component to a         to the entrapment of CO2-rich fluids as fluid in-
mantle sources result in a steeper slope of the mix-     clusions in mantle wall rocks.
ing line. The most obvious case is for the arc- and          It has previously been suggested that nitrogen
back arc-related samples whose 40Ar/36Ar and N2/         could be recycled into the mantle much more ef-
   Ar ratios requires an end-member significantly        ficiently than the light noble gases (Bebout, 1995).
rich in a sedimentary component with respect to a        This is because nitrogen is fixed in the form of
MORB component (Fig. 2). Among the Austral-              ammonium ions in the crystal lattice of K-bear-
ian xenoliths, VIC51 and VIC51A depart from the          ing minerals leading to a much stronger retention
mixing line between air and a pure MORB com-             of ammonium than of gaseous helium. Helium is
ponent, confirming the contribution of a third,          easily lost by devolatilization in the subduction
sedimentary component in these samples. On the           zone prior to incorporation of the subducting slab
other hand, sample GAMVL3, which was charac-             into the upper mantle (Staudacher and Allègre,
terized by a clear mantle δ15N value, plots on the       1988), whereas 10% to 20% of the N in sediments
Air-MORB mixing line.                                    could be preserved up to their melting tempera-
     Although definite proportions of each compo-        ture (Boyd et al., 1993; Hall, 1999). This selec-
nent in the samples cannot be determined without         tive recycling of nitrogen over helium may explain
ambiguity due to the difficulties in fixing the end-     why nitrogen isotopic ratios are heterogeneous in
member compositions, two independent arguments           the xenoliths, whereas helium has preserved only
from δ 15N–N 2/ 36Ar and N2/ 36Ar–40Ar/ 36Ar rela-       its mantle isotopic signature (Table 1). This is
                         Recycled noble gas and nitrogen in the subcontinental mantle                      215

                                                          Horoman complex, Northern Japan, can be re-
                                                          garded as an evidence for the presence of recy-
                                                          cled atmospheric argon in wedge mantle (Fig. 4).
                                                          It was also pointed out that the ultramafic xenoliths
                                                          from SE Australia and Europe have similar corre-
                                                          lations, that might also be explained by the pres-
                                                          ence 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 contain-
                                                          ing boninites and low-Ti andesites have been ob-
Fig. 4. Concentration of 3He (primordial isotope) and
36                                                        served (Chappell, 1984; Crawford et al., 1984).
   Ar in the SEA xenoliths reported in Matsumoto et al.
(1998). Concentration of 36Ar is that of air-Ar in each   In this respect, the absence of a clear recycled ni-
sample, calculated based on a binary mixing between       trogen signature in the Mt. Gambier sample
an atmospheric component and a MORB component             (GAMVL3), located hundreds kilometres west to
with 40Ar/36Ar = 295.5 and 40000, respectively. Results   Mts. Shadwell and Leura, might correspond to the
of Horoman ultramafics (Matsumoto et al., 2001) are       change in the heterogeneity of the lithospheric
also shown for comparison. Note that a simple atmos-
pheric contamination should not necessarily result in
                                                          mantle from east to west. Such a lateral heteroge-
such correlation between 3He and 36Ar.                    neity 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) re-
mainly due to the difficulty in distinguishing shal-      ported a small range in 87Sr/ 86Sr ratios (~0.7040)
low level contamination from addition of recycled         in western part of the Mt. Shadwell, then an abrupt
material, as both should have a near-atmospheric          change to a wide range, and much more radiogenic
noble gas isotopic composition. Reported high N2/         Sr in the basalts from Mt. Shadwell eastwards
   Ar ratios in sediments implicitly suggest that         (0.7040–0.7055). Note that this boundary roughly
recycling is more efficient for nitrogen than ar-         coincides with that between the Delamerian and
gon. However, the 40Ar/36Ar ratios observed in the        Lachlan Fold Belts, with the latter being built up
Australian lherzolites (<10000; Matsumoto et al.,         by a series of arcs (e.g., Powell, 1983; Collins and
1998) are always significantly lower than those           Vernon, 1994). Mt. Shadwell is likely to have been
reported for the gas-rich MORBs (~60000)                  located on the edge of the arc front and may have
(Burnard et al., 1997). This, and the present ni-         been affected to a larger degree by a recycled com-
trogen results, indicates that the systematically low     ponent than the Mt. Gambier lithosphere, 200 kilo-
   Ar/36Ar ratios in fluid inclusions of the xenoliths    metres to the west. The occurrence of a sedimen-
may not due to mere atmospheric contamination,            tary N component in the SE Australia SCLM is
but instead partly reflect the presence of recycled       consistent with the ancient tectonic history of the
argon in the SCLM. In regard to the possible heavy        area, and the hypothesis of an oceanic arc system
noble gas recycling, Matsumoto et al. (2001) re-          at the south-eastern margin of Australia during
cently suggested that correlated 3 He and 36Ar            Paleozoic (e.g., Powell, 1983; Collins and Vernon,
abundances in a series of orogenic lherzolites from       1994).
216                                              T. Matsumoto et al.

                    CONCLUSIONS                                granites in the Lachlan fold belt, southeastern Aus-
                                                               tralia. Phil. Trans. Roy. Soc. London Ser. A 310, 693–
    The N-Ar-He systematics in the SE Australian               707.
xenoliths requires the presence of at least two dis-         Collins, W. J. and Vernon, R. H. (1994) A rift-drift-
tinct sources of volatiles beneath SE Australia.               delamination model of continental evolution:
                                                               Palaeozoic tectonic development of eastern Australia.
One source seems to be the same of that for
                                                               Tectonophysics 235, 249–275.
MORBs (upper mantle) as suggested by measured                Craig, H. and Lupton, J. E. (1981) Helium-3 and man-
  He/4He ratios of 10 × 10–6 and δ15N values of                tle volatiles in the ocean and the oceanic crust. The
–6‰. The second source is sedimentary volatiles                Sea (Emiliani, C., ed.), 7, 391–428.
recycled into mantle through subduction. This                Crawford, A. J., Cameron, W. E. and Keays, R. R.
component is characterized by nitrogen having a                (1984) The association boninite low-Ti andesite-
                                                               tholeiite in the Heathcote Greenstone belt, Victoria;
heavier isotopic signature than that of upper man-             ensimatic setting for the early Lachlan fold belt.
tle and very high N2/40Ar ratios. Finally, nitrogen            Austr. J. Earth Sci. 31, 161–177.
in the xenoliths from the SCLM suggests that the             Gibson, G. M. and Nihil, D. N. (1992) Clenelg river
recycled component should have been stored in                  complex: Western margin of the Lachlan Fold Belt
the mantle for a significant period of time, with-             of extension of the Delamerian Orogen of southeast-
                                                               ern Australia. Tectonophysics 214, 69–91.
out being completely mixed with the nitrogen in
                                                             Griffin, W. L., Wass, S. Y. and Hollis, J. D. (1984)
the convective mantle. In this respect, the SCLM               Ultramafic xenoliths from Bullenmerri and Gnotuk
itself is a possible candidate for the storage site            maars, Victoria, Australia: Petrology of sub-continen-
as they are thought to have persisted in their                 tal crust-mantle transition. J. Petr. 25, 53–87.
present form over long time periods.                         Hall, A. (1999) Ammonium in granites and its
                                                               petrogenetic significance. Earth Sci. Rev. 45, 145–
Acknowledgments—The Mt. Gambier sample had
                                                             Marty, B. (1995) Nitrogen content of the mantle in-
kindly been provided by Prof. Sue O’Reilly at
                                                               ferred from N 2 -Ar correlation in oceanic basalts.
GEOMOC. Eleanor Dixon is also acknowledged for
                                                               Nature 377, 326–329.
reading a manuscript very carefully. Comments and
                                                             Marty, B. and Humbert, F. (1997) Nitrogen and argon
suggestions by R. Mohapatra on an earlier version of
                                                               isotopes in oceanic basalts. Earth Planet. Sci. Lett.
the ms were quite helpful. S.V.S. Murty is thanked for
                                                               152, 101–112.
his constructive and critical review. D.L.P. stay in Osaka
                                                             Matsumoto, T., Honda, M., McDougall, I., Yatsevich,
University was funded by EU Commission contract no.
                                                               I. and O’Reilly, S. Y. (1997) Plume-like neon in a
CIPI 940115 and JSPS P 96239. This work was partly
                                                               metasomatic apatite from the Australian lithospheric
supported by Grants-in-Aid from the Japan Society for
                                                               mantle. Nature 388, 162–164.
the Promotion of Science to T.M. (12740306).
                                                             Matsumoto, T., Honda, M., McDougall, I. and O’Reilly
                                                               S. Y. (1998) Noble gases in anhydrous lherzolites
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