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(Basu et al. 1997). Three Raniganj K-rich rocks range between 0.70493 and
0.71067 (Middlemost et al. 1988), and in seven Jharia ultrapotassic ultama
c rocks
87
Sr/
86
Sr ratios range between 0.70526 and 0.70595 (Rock et al. 1992). The Bokaro
samples thus encompass a much wider range of
87
Sr/
86
Sr ratios. These enriched Sr-
isotope data of the Jharia, Raniganj and Bokaro rocks are similar to those of
lamproites world wide (Bergman 1987). For a given suite
87
Sr/
86
Sr ratios do not co-
vary with SiO
2
. A plot of
87
Sr/
86
Sr versus 1/ Sr of the Raniganj and Jharia ultra-
potassic rocks do not generate a straight line for mixing between crustal and mantle
sources (Fig.
5.4
). Consequently, these K-rich rocks have not been affected by
crustal contamination, and are believed to re
ect mantle source characteristics
(Bergman 1987; Nelson et al. 1986; McCulloch et al. 1983).
Hall (1999) measured NH
4
+
, contents of up to 54 ppm in hydrothermally altered
oceanic basalts, and suggested that subduction of such rocks provides a mechanism
for recycling N into the mantle. Fyfe (1997) addressed recycling of volatiles,
including N, from surface reservoirs into the mantle. The subducted
flux of N is
10
12
g/yr, or
estimated at 2
flux Jia et al. (2001).
However, according to Marty and Humbert (1997), N is quantitatively released
from the slab at convergent margins by arc magmatism, consistent with the high N/
Ar ratios of volcanic gases in arcs. In contrast, Javoy (1998) argues that N may
accumulate in the mantle from progressive recycling, stored in a stable form such as
Osbornite (TiN).
Four leucite samples from tephritic rocks of Roccamon
×
five times the mantle outgassing
na (Italy) reported by
15
N values ranging from 2.7 to 4.6 % and
nitrogen contents ranging from 60 to 270 ppm (Fig.
5.1
). These results are also
consistent with nitrogen being added to the mantle by subduction of sediments or
continental crust.
Recycling of subducted sedimentary/ crustal N into the deep mantle has also
been found by Dauphas and Marty (1999), who analysed samples of the 370 Ma
ultrama
Gupta et al. (2002) also show enriched
δ
c
alkaline
carbonatite complex in the Kola peninsula, which have a range
-
-
15
N from +0.7 to +6.5 % (Fig.
5.2
; one outlier). Based on noble gas (He, Ne,
and Ar) isotopic compositions, and coupled Sr- and Nd-isotopic compositions,
which provide no evidence for crustal contamination, they interpreted the
15
N-
enriched signature of a Deep mantle material to be recycled sedimentary N through
subduction processes.
A recent study on N-rich
of
δ
fluids in the mantle provides evidence that molecular
nitrogen, as a major component in
fluids, was trapped by minerals in the upper
mantle xenoliths during a
fluid (melt) rock interaction process within the mantle
(Andersen et al. 1995). Ammonium-bearing phlogopite and amphibole breakdown
by an oxidation
-
dehydration reaction such as:
phl
þ
2NH
4
ð
Þ
Mg
3
AlSi
3
O
10
OH
ð
2
ð
Mg
2
SiO
4
Þ
Fo
þ
3
=
20
2
ð
15
:
1
Þ
¼ð
MgAl
2
O
4
Þ
Sp
þ
7 MgSiO
3
ð
Þ
En
þ
N
2
þ
6H
2
O
and
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