Earth and Planetary Science Letters 194 (2001) 241^257
www.elsevier.com/locate/epsl
Volatile (C, N, Ar) variability in MORB and the respective
roles of mantle source heterogeneity and degassing:
the case of the Southwest Indian Ridge
P. Cartigny *, N. Jendrzejewski, F. Pineau, E. Petit, M. Javoy
Laboratoire de Gëochimie des Isotopes Stables, CNRS UMR 7047, Universitë Paris 7 et Institut de Physique du Globe, T54-64,
1er ëtage, 2 Place Jussieu, F-75251 Paris Cedex 05, France
Received 12 June 2001; received in revised form 26 September 2001; accepted 2 October 2001
Abstract
In order to better constrain the origin of volatile variability of mid ocean ridge basalt (MORB), we have performed
crushing analyses on 33 fresh basaltic glasses. Most samples originate from the Southwest Indian Ridge (i.e. 78^49³E,
EDUL cruise, August 97), the Southeast Indian Ridge, the Central Indian Ridge and the Rodriguez Triple Junction.
N13 C and N18 O of CO2 , N15 N of N2 together with C, N and Ar contents were determined. N15 N values vary between
35.9x and +2.1x while N13 C values range between 311.4 and 34.3x. C/N2 ratios vary by one order of magnitude
(316^3900). Most N2 /Ar ratios fall within a narrow range of values (48^90) but four samples yield higher values up to
220. Overall, the data for N13 C, N18 O, N15 N, C/N2 and N2 /Ar are within the range of those previously reported for
Pacific and Atlantic oceans. No volatile DUPAL anomaly has been detected in the present study. The correlations
between N2 /Ar, C/Ar and C/N2 together with N13 C and N15 N show that major volatile signatures, including N15 N, are
more influenced by degassing-induced fractionation than by mantle heterogeneity and/or late atmospheric
contamination. A two stage degassing model (a closed-system degassing followed by a Rayleigh distillation) can be
used to explain the data set. This model gives initial Indian MORB values similar to those for the Atlantic and Pacific
oceanic basalts N13 C0 V34.5x, N15 N0 V36.0x, (C/N2 )0 V130, (C/Ar)0 V14 000 and (N2 /Ar)0 V110 and C0 between
1100 and 5000 ppm C. This large range of possible initial carbon concentrations (due to the lack of constraints on the
extent of degassing under closed-system conditions) results in a large range of mantle flux estimates (i.e. from 0.4 to
1.8U1013 mol/yr). The correlations induced by these degassing processes permit an estimation of the relative solubilities
of C, N and Ar: SC /SN2 V5 and SAr /SN2 V1.2 as well as an evaluation of the fractionation of nitrogen isotopes between
the vesicles and the melt: vN = 31.6x. The present study proposes a coherent data set for volatile mantle fluxes with
PC V1.3 þ 0.1U102 PN2 V1.4 þ 0.1U104 PAr (molar). ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: Southwest Indian Ridge; carbon; nitrogen; stable isotopes; degassing
1. Introduction
* Corresponding author. Fax: +33-1-44-27-28-30.
E-mail address: [email protected] (P. Cartigny).
Estimating the £ux of volatile species such as
CO2 , N2 , Ar and He degassed along mid ocean
ridges (MORs) requires a good knowledge of (1)
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 4 0 - 4
EPSL 6038 21-12-01
242
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
the initial volatile characteristics in terms of abundance and isotopic composition of MOR magmas, (2) the type(s) and the intensity of degassing
process(es) which tend to a¡ect volatile concentrations and volatile to volatile ratios according
to their di¡erent solubilities in the melt, (3) the
solubility of each volatile species considered and
(4) for the most sensitive volatiles (i.e. Ar and N2 )
the degree of atmospheric contamination.
Key information concerning the type of degassing processes and their extent can be obtained
from a variety of tracers such as: the N13 C value
and abundance of carbon (both within vesicles
and dissolved in the melt; e.g. [1,2]), the 4 He/
40
Ar* (e.g. [3], where 40 Ar* refers to the amount
of mantle-derived 40 Ar assuming that all 36 Ar is
atmosphere-derived) coupled or not with 3 He/
22
Ne* ratios [4] can constrain the mode of degassing and its extent. From nitrogen isotope studies
[5^9], it has often been suggested that nitrogen
isotopic compositions, as opposed to carbon, are
little in£uenced by shallow level degassing processes providing a powerful tracer of source heterogeneities (e.g. [7^10]). Parallel studies were devoted to the determination of volatile solubilities
in basaltic magmas (e.g. [3,11^20]). The amount
of atmospheric contamination can be estimated
using argon or neon isotopes (e.g. [4]).
However, estimates of mantle volatile £uxes
found in literature vary typically within one order
of magnitude, this being particularly the case for
carbon (e.g. [21,22]). This is ¢rstly because natural
basaltic melts can undergo variable types of degassing processes (e.g. closed- or open-system
conditions [2,4,23]) which remain di¤cult to
quantify and also because solubilities (in particular those for N and He, [4,24] for discussion) are
not known precisely. Moreover, in MOR basalts
(MORB), carbon behavior controls the degassing
of other volatile species. Consequently, the understanding of magma degassing requires to know at
which depth of its ascent the magma became saturated with respect to carbon. The estimate of
mantle volatile £uxes and the correct detection
of volatile source heterogeneities on MOR are
thus intimately linked to the accuracy of degassing corrections.
Previous radiogenic isotope studies have demonstrated the existence in the southern Indian
Ocean of a distinct mantle domain compared to
the North Atlantic mantle (e.g. [25^29]), the maximum anomaly (referred to as DUPAL) being
centered on the Southwest Indian Ridge
(SWIR). Accordingly, the SWIR is an ideal place
where volatile source heterogeneities may exist
and could be detected. Moreover, the SWIR is
one of the least productive spreading centers on
the Earth (half spreading rate of 7^8 mm/yr [30])
and the associated large variations in depth, decreasing from 5000 m at the Rodriguez Triple
Junction (RTJ) to 3000 m in the south^western
part (V49³E), make this location an ideal area
to test pressure-induced degassing e¡ects. Despite
its great geological interest, this ultra-slow spreading ridge remains understudied. Most studies of
C, Ar and/or N in MORB have focused on samples from Atlantic and Paci¢c ridges [1,2,5^8,22^
24,31^38], the characterization of DUPAL volatiles (C, N, H2 O, noble gases) is mainly restricted
to the Southeast (SEIR) and Central Indian
Ridges (CIR) [8,9,37,39]. The French EDUL
(Eètude d'une Dorsale Ultra-Lente) sampling
cruise (August 1997) covered the axis of the
SWIR between RTJ and 49³E and now provides
a comprehensive sample set [40]. This sample set
was used, in the present study, to constrain the
composition of the MORB source in terms of its
volatile (C, N, Ar) contents and to investigate
what controls carbon, nitrogen and argon contents as well as carbon and nitrogen isotopic compositions in erupted basalts.
2. Samples studied and analytical techniques
From a total of 33 samples from 27 dredges, 27
originated from the SWIR, (26 being dredged
during the EDUL cruise, 1997 and one during
MD 34, 1983), one from the SEIR (MD 37 cruise,
1983), two from the RTJ (MD 23, 1980 and Jean
Charcot cruise, 1984), and three from the CIR
(MD 57 cruise, 1984). Their locations are reported
in Table 1 and shown in Fig. 1. Their eruption
depths vary between 2700 and 5300 m (Table 1).
EPSL 6038 21-12-01
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
Major and trace element analyses of samples
dredged during the EDUL cruise show typical
N-MORB signatures [41]. The characteristics of
the SEIR, CIR and RTJ samples can be found
in [26,42^44] (see also Table 1).
Large rounded and compact pieces of fresh basaltic glasses (V1^4 g) were hand-picked using a
binocular microscope. After cleaning and drying,
samples were loaded into an ultra-high vacuum
piston crusher and degassed for 12 h at 100³C.
The crushing and extraction experiments were
performed in one or several steps with a total of
at least 100 piston impacts. Incondensable gases
such as N2 , Ar, CO or CH4 ( 6 1 vol% [6,8]) were
separated cryogenically from the condensable
gases (mostly CO2 , trace amounts of H2 O, H2 S
and SO2 ). N2 was separated from any carbon
bearing species (i.e. remaining traces of CO2 ^
CO^CH4 ; e.g. [45^48]) to avoid any isobaric interference by passing the N2 ^Ar^CO^CH4 mixture over CuO at 450³C. Traces of CO2 and
H2 O formed from the oxidation of CO and CH4
were trapped cryogenically before being discarded. The e¤ciency of the oxidation process
was tested on N2 ^CO and N2 ^CH4 standard mixtures. N2 and Ar (less than 2 vol% of the total
gas) remain unmodi¢ed and were quanti¢ed with
a capacitance manometer with a precision better
than 5% [47]. Due to the low amounts of nitrogen
present (0^100U1039 mol), N15 N values
(N15 N = [(15 N/14 Nsample )/(15 N/14 Nair )31]U1000)
were measured using a triple collector static vacuum mass spectrometer with an accuracy of
þ 0.5x (2c) as established relative to two international standards (IAEA-N1 and N2 [47]). The
N2 /Ar ion ratio was measured with a precision
better than 10% (2c) in the mass spectrometer
by measuring the m/z = 28 and m/z = 40 ratio.
Blanks of nitrogen were below 5U10312 mol N2 .
In several experiments, the crushing of samples
containing virtually no vesicles yielded an amount
of gas even below this blank level. Blanks for
carbon were below detection levels. CO2 was puri¢ed, quanti¢ed with a precision of 5% and analyzed for N13 C and N18 O with a precision better
than 0.03x using conventional techniques [23].
The N values are expressed relative to the interna-
243
Fig. 1. Localization of basaltic glasses selected for the
present study. Most of the samples were dredged along the
SWIR during the EDUL cruise on R/V Marion Dufresne
(August 1997, mission MD 107). Additional samples come
from the CIR and SEIR.
tional standards PDB for carbon, SMOW for
oxygen and air for nitrogen.
Dissolved carbon contents were determined using Fourier transformed micro-infrared (FTIR)
spectroscopy on double-polished sections according to the methods and absorption coe¤cients
reported by [13,49].
3. Results
For a typical 2 g sample, 45 þ 15% of the crushing residue fell in the 6 1 mm fraction, 15 þ 2 and
40 þ 15% falling within the 1^2 mm and s 2 mm
fractions respectively. Consequently, some gas
probably remained in unopened vesicles (see also
[50] for similar discussion on xenoliths). This
could lead to underestimated total volatile abundances, hence only abundance ratios will be considered.
Results of the crushing experiments and of the
FTIR measurements are reported in Table 1.
They represent the largest existing database on
MORB volatiles for the Indian Ocean and for
the SWIR in particular. As shown in Fig. 1 and
Table 1, the high sampling density of the SWIR
associated with a large range of eruption depths
EPSL 6038 21-12-01
total
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
MFZ
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
EDUL
10-3-16
11-1-1
12-1-2
16
26-1-2
26
29-1-1
29-1-1
29-1-1
29-3-2
32-4-3
34-1-2
34-2-1
38-1-1
41-1-2
43-1-3
49
52
52
57-1-5
60
66
66
73
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
DR
RTJ
RTJ-JC 2-17-D1
MD 23 Site 4
SWIR
EDUL DR 1-1-7
EDUL DR 2
EDUL DR 6-4-1
EDUL DR 7-2-5
EDUL DR 8-1-2
EDUL DR 9-2-3
EDUL DR 9-2-3
EPSL 6038 21-12-01
3.4961
1.9732
3.2642
2.2825
1.4528
1.6460
1.8552
1.8195
3.3362
2.3872
2.6794
2.6111
3.8304
1.3533
60.50
60.16
60.16
59.34
59.16
58.85
57.74
56.13
56.13
55.63
55.27
54.37
54.37
49.85
65.98
65.53
65.36
63.91
61.93
61.93
61.32
61.32
61.32
61.32
68.73
68.44
67.22
66.64
66.34
66.26
66.26
1.3175
1.7990
2.3033
2.7427
2.3986
3.5418
1.8044
2.1065
1.8771
1.9582
2.0494
2.3103
2.9471
1.4407
1.0800
2.7018
2.2575
70.02
70.32
Longitude
(³S)
2.2136
1.9317
Weight
(g)
30.26
30.44
30.44
30.91
30.92
31.14
31.73
33.79
33.79
34.14
34.37
34.75
34.75
37.69
27.57
27.63
27.74
27.85
28.80
28.80
28.95
28.95
28.95
28.95
26.09
26.23
26.98
27.36
27.34
27.43
27.43
25.58
25.68
Latitude
(³E)
4 550
4 500
4 500
3 950
4 300
4 300
4 050
3 600
3 600
4 300
3 700
3 800
3 800
2 800
4 320
3 500
5 500
2 800
4 600
4 600
3 600
3 600
3 600
3 600
4 850
3 780
4 900
3 850
4 550
4 500
4 500
5 000
4 150
Depth
(m)
76
n.d.
93
70
71
87
71
95
95
43
n.d.
89
89
60
n.d.
59
n.d.
53
113
113
n.d.
n.d.
n.d.
76
96
81
96
108
86
n.d.
n.d.
n.d.
n.d.
ppm C
0.7
2.8
6.5
0.1
0.9
2.3
18.9
0.5
0.5
0.4
1.8
12.6
14.7
10.1
6.7
13.7
3.2
1.9
5.4
51.9
5.8
6.4
13.8
25.9
22.5
8.0
1.2
8.8
0.2
2.3
21.6
19.3
17.1
24.1
4.8
20.1
30.7
34.5
35.9
n.d.
32.3
33.6
35.5
33.0
31.8
1.6
31.6
33.7
33.9
30.1
32.4
32.0
0.1
30.4
30.2
31.4
32.0
31.2
31.7
31.6
32.4
30.2
30.4
31.1
n.d.
33.7
34.0
31.9
34.1
34.7
31.0
32.3
N15 N
N2
(nmol) (x vs.
air)
0.3
1.8
11.7
0.0
1.9
1.9
8.0
n.d.
n.d.
n.d.
2.6
6.5
n.d.
3.2
5.5
n.d.
4.2
3.0
12.7
35.5
5.0
5.9
12.1
22.9
16.5
20.1
0.7
13.6
n.d.
n.d.
18.6
11.1
14.6
23.1
3.2
10.5
CO2
(Wmol)
311.40
36.64
35.93
n.d.
37.52
36.19
36.69
n.d.
n.d.
n.d.
36.51
34.96
35.13
35.99
35.58
35.88
36.71
37.07
36.77
36.06
36.61
36.34
35.90
36.17
35.52
36.57
38.42
36.30
n.d.
n.d.
35.79
35.97
35.96
35.85
36.42
35.62
N13 C
(x vs.
PDB)
13.23
13.10
15.23
n.d.
n.d.
n.d.
8.92
9.14
8.09
12.48
16.02
10.45
8.42
9.15
9.51
10.78
9.32
n.d.
n.d.
10.20
9.42
8.74
8.57
10.09
9.06
10.95
12.98
10.08
10.11
10.13
9.90
9.27
11.10
11.72
N18 O
(x vs.
SMOW)
52
80
101
55
59
71
90
213
84
66
66
86
86
159
88
86
64
48
58
90
80
79
82
81
83
62
59
66
83
87
78
82
79
81
75
142
403
649
1 805
n.d.
2 182
814
421
n.d.
n.d.
n.d.
1 448
518
n.d.
316
818
n.d.
1 333
1 584
2 342
685
860
916
881
885
733
2 497
616
1 550
n.d.
n.d.
861
576
853
959
665
523
20 867
52 119
182 496
n.d.
129 052
57 872
37 722
n.d.
n.d.
n.d.
96 253
44 386
n.d.
50 343
71 650
n.d.
85 508
75 348
135 132
61 648
68 922
71 974
72 507
71 579
61 003
154 312
36 271
101 592
n.d.
n.d.
67 446
47 049
67 054
77 570
49 890
74 477
N2 /Ar C/N2 C/Ar
Table 1
Location, isotopic compositions of N13 C and N18 O of CO2 , N15 N of N2 and C, N and Ar abundances in vesicles of SWIR basaltic glasses
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Type
244
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
N
Eruption depths are mean values. EDUL samples correspond to mission MD 107 of the R/V Marion Dufresne. n.d. = not determined.
2 682 198 204
74
12.09
35.85
5.3
2.1
2.0
n.d.
2 700
77.62
2.1196
32.67
E
N
N
3 887 227 357
n.d. n.d.
1 110 72 127
58
76
65
10.61
n.d.
10.81
36.64
n.d.
34.36
7.8
n.d.
6.1
30.8
34.0
32.7
2.0
5.6
5.5
n.d.
n.d.
n.d.
2 700
3 700
3 700
6.22
15.87
15.87
1.1570
1.8934
2.7091
68.25
67.28
67.28
N
54 583
345
158
12.72
35.91
6.7
30.4
19.4
114
3 260
37.71
49.88
1.9077
MD 34 D3
CIR
MD 57 D 10P4
MD 57 D 7-2
MD 57 D 7-5
SEIR
MD 37-07-04-D1
Table 1 (Continued)
Weight
(g)
Longitude
(³S)
Latitude
(³E)
Depth
(m)
ppm C
N2
N15 N
(nmol) (x vs.
air)
CO2
(Wmol)
N13 C
(x vs.
PDB)
N18 O
(x vs.
SMOW)
N2 /Ar C/N2 C/Ar
Type
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
245
allows a ¢ne analysis of the variations of volatile
characteristics along this ridge.
Overall, we found no relation between N13 C,
15
N N, N18 O, C/N2 , C/Ar, N2 /Ar and the depth of
eruption of the samples, even when the sample set
is divided into subgroups according, for example,
to their location or chemistry. However, although
mean isotopic values and elemental ratios are
rather similar, samples collected west of Melville
Fracture Zone (hereafter referred to as W-MFZ)
appear to show more scattered values and less
coherent variations than samples from east of
MFZ (E-MFZ) (Figs. 3^9). The distinction between samples east and west of MFZ is not only
driven by the scattering in volatile ratios and isotopic compositions. Several other geophysical [51^
54] and geochemical [41] contributions also led to
treat separately the western and eastern parts of
the SWIR. Following these studies, the eastern
samples are likely to result from a simple, or at
least simpler, type of magma emplacement. It
may, in some instances, be linked to a colder
mantle structure and slower magma rise on the
eastern part [52]. As a consequence of these observations, in the following discussion, samples
from the SWIR will be separated into two groups
delimited by the MFZ (Fig. 1). For E-MFZ samples (EDUL dredges 1^29), the CO2 shows N13 C
values from 38.42 to 35.52x associated with
N18 OSMOW values from +8.57 to +12.98x.
N15 NAIR values of N2 range from 34.7 to
+0.1x together with C/N2 , N2 /Ar and C/Ar
(all molar ratios) from 576 to 2497, 36 to 90
and 33 to 154U103 respectively. W-MFZ samples
(EDUL dredges 32^73 and MD 34 D3) have N13 C
values ranging from 311.40 to 34.96x and N18 O
from +8.09 to +16.02x, N15 N from 35.9 to
+1.6, C/N2 , N2 /Ar and C/Ar from 316 to 2182,
from 52 to 213 and from 21 to 182U103 respectively.
The six samples from the SEIR, CIR and RTJ
investigated in this contribution have N13 C values
ranging from 36.64 to 34.36x, N18 O from
+10.61 to +12.09x, N15 N from 34.0 to
+2.1x, C/N2 from 523 to 3887, N2 /Ar between
58 and 76 and C/Ar from 0.49 to 227U103 .
EPSL 6038 21-12-01
246
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
Fig. 2. Comparison of N13 C and N15 N values, C/N2 and N2 /Ar ratios from vesicles measured after release by crushing under vacuum of MORB vesicles from the SWIR (this study) and from the Atlantic and Paci¢c ridges [1,2,5^8,22^24,31^33,35^38,55] and
the CIR and SEIR [8,9,37,39]. Gray and white colors stand for N- and E-MORB respectively.
4. Discussion
4.1. Comparison of volatile characteristics of
MORB from the SWIR and other ridges
In order to identify any ¢rst order volatile
anomaly of the SWIR relative to other ridges,
the present results are compared to previous published data (Fig. 2). The literature data include
Fig. 3. C/Ar vs. C/N2 variations in MORB vesicles from the
SWIR, i.e. samples east and west of MFZ, samples from
RTJ, CIR and SEIR. The di¡erent step crushing results are
considered separately. Degassing path for dissolved volatiles
(¢ne gray curve) and for vesicles (heavy black curve) under
open system (i.e. Rayleigh distillation) and evolution path
for vesicles under closed system (heavy dashed black curve)
are indicated. Previous published data, volatile characteristics
of the MORB at stage A, C-enriched MORB end-member;
air and ASW end-members (as de¢ned in Table 2) are also
included.
some from the Atlantic and Paci¢c [1,2,5^8,22^
24,31^33,35^38,55] and from the CIR and SEIR
[9,39]. In Fig. 2, they are compared to all our
results (considering separately samples west and
east of MFZ would not change the following conclusions). In terms of ranges, distributions and
mean values of either N13 C, N15 N, C/N2 , C/Ar or
N2 /Ar, it can be seen that the vesicle gas from the
Indian MORB (SWIR with or without CIR,
SEIR and RTJ samples) falls in the same range
as vesicle volatiles from other worldwide basaltic
glasses (Fig. 2). Accordingly, no obvious anomaly
can be detected from C, N and Ar volatile signatures of Indian Ocean vesicles. This conclusion
had previously been reached from volatile studies
of the CIR and SEIR basalts [8,9,39]; our results
Fig. 4. N2 /Ar vs. N15 N variations in MORB vesicles from the
SWIR. Same legend as in Fig. 3.
EPSL 6038 21-12-01
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
Fig. 5. N15 N vs. C/N2 variations in MORB vesicles from the
SWIR. Same legend as in Fig. 3. Evolution path for residual
volatiles under closed system (¢ne dashed gray curve) is also
indicated. In this ¢gure, vesicles in equilibrium with the residual melt are depleted by 1.6x in 15 N isotopes relative to
the melt and have a C/N2 ratio about six times lower than
the C/N2 ratio of the melt. Error bars for N15 N values from
the literature are about þ 1x (2c).
extend this conclusion to the western part of the
Indian Ocean where the DUPAL anomaly as de¢ned by radiogenic isotopes is the most pronounced [25,29].
4.2. Origin of the C/N2 , C/Ar, N2 /Ar, N13 C, N18 O
and N15 N variations on the SWIR
As shown by Figs. 3^9, both sets of E-MFZ
and W-MFZ samples overlap and de¢ne similar
trends, the correlations being more striking for
Fig. 6. N2 /Ar vs. C/Ar variations in MORB vesicles. Same
symbols as in Fig. 3. The decrease in N2 /Ar with increasing
C/N2 or C/Ar demonstrates that, in basaltic magma, argon
solubility is higher than nitrogen solubility.
247
Fig. 7. N13 C vs. C/N2 variations in MORB vesicles from the
SWIR. Same symbols as in Fig. 3.
samples E-MFZ. Based on the multi-correlations
de¢ned by E-MFZ samples (and most W-MFZ
samples), we shall show that all the covariations
observed can be mainly described by a degassing
process whereas atmospheric contamination or
source heterogeneity cannot account for (or are
even in contradiction with) some of the correlations.
In discussing the respective roles of atmospheric
contamination, source heterogeneity and degassing, the C/Ar vs. C/N2 relationship (Fig. 3) is of
particular interest. The linear trend of Fig. 3
means that whereas C/Ar and C/N2 ratios vary
by over one order of magnitude, the N2 /Ar ratio
is rather constant, varying only by a factor of four
(Table 1). The origin of the C/Ar vs. C/N2 trend is
`ambiguous' since it can be the result of at least
Fig. 8. N2 /Ar vs. N13 C variations in MORB vesicles. Same
symbols as in Fig. 3.
EPSL 6038 21-12-01
248
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
Table 2
Summary of the di¡erent mixing end-members ^ air and
ASW, normal MORB and C-enriched MORB identi¢ed in
basaltic glasses, volatile (C, N, Ar) solubilities and fractionation factors for N and C from the literature
Air
Fig. 9. N13 C vs. N15 N variations in MORB vesicles. Same
symbols as in Fig. 3. The inverse correlation between N13 C
and N15 N demonstrates that nitrogen isotopes fractionate
upon degassing and that the sign of the fractionation is opposed to that of carbon isotopes.
three processes which will be considered separately in the following sections: (1) an atmospheric contamination of the MORB during (or after)
their emplacement on the sea£oor, (2) a binary
mixing at the source of the MORB, (3) a degassing process during the MORB ascent. It will be
shown that, considering all parameters together
(i.e. N13 C, N15 N, C/N2 , C/Ar, N2 /Ar), the main
process that can account for all the covariations
shown in Figs. 3^9 is degassing.
4.2.1. Atmospheric contamination
MORB N2 /Ar ratios (mostly between 60 and
120 with a mode at V80; see Fig. 2) are very
close to values for atmosphere or air-saturated
seawater (83.6 and 38 respectively, see Table 2).
In contrast, C/Ar and C/N2 ratios of air and airsaturated water (ASW) ( 6 100 and 6 5 respectively) are very di¡erent from C/Ar and C/N2 ratios of an average `undegassed' MORB ( s 20 000
and s 200 respectively, e.g. [6^8,24], Table 2).
The trend observed in the C/Ar vs. C/N2 diagram
(Fig. 3) could theoretically be the result of a mixing between an atmospheric component and a
MORB end-member.
Air or ASW contamination (N15 N = 0x;
C/N2 6 5) of a typical degassed or undegassed
MORB (mean N15 N = 34 þ 2x; C/N2 from 200
to 2000; Table 2) predicts an increase in the
N13 C
(x)
N15 N
(x)
C/N2
N2 /Ar
C/Ar
ppm C
vC
vN
SC
SN2
SAr
Typical MORB
Enriched MORB
n.d.a n.d.a
ASW
35 þ 2b
6 37c
0
0
34 þ 2d
v2e
I1
83.6
I1
340
V3
400 þ 200f
38
152 þ 58f
V100 60 000 þ 40 000
400^4000h
4.5^2.2i s
0 (assumed)j
2.56U1034k
4.26U1035l
5.54U1035 m
s 3 700g
44 þ 7g
s 180 000
a
n.d. = not de¢ned because variable according to the di¡erent dissolved components.
b
[1,2,6,8,23,32,35,36,46,66].
c
A low N13 C value down to 325 has sometimes been quoted
to account for a lower than mantle-like N13 C value (e.g.
[57]).
d
[6^9].
e
[7,8,10,57].
f
[6^8,57].
g
[8,9,48,57].
h
The highest value corresponds to the MORB popping-rock
[6], the lower value to more common but degassed samples
(e.g. [1,8,34,35,37]).
i
[1,8,34,35,37].
j
[7^10].
k
[13,15].
l
[12,48].
m
[3,14].
MORB N15 N together with a decrease in its
C/N2 ratio. The SWIR samples do not show an
overall decrease of C/N2 ratios with increasing
N15 N (Fig. 5). On the contrary, the highest C/N2
values correspond to the highest N15 N values (Fig.
5). Moreover, air or ASW contamination would
cause samples with N15 N around 0x to have
N2 /Ar close to air or ASW values (i.e. 83.5 and
38). Fig. 4 shows that the N2 /Ar ratios of SWIR
samples with N15 N close to 0x do not plot on
the air and/or ASW end-members but are outside
(W-MFZ samples) or between the two zones
(W-MFZ and E-MFZ samples). The main process
responsible for the variations of C/N2 , N2 /Ar and
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P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
C/Ar ratios as well as the N15 N range of values
thus cannot be atmospheric or ASW contamination solely. Such a conclusion is also clearly
supported by N2 /Ar vs. C/Ar variations (Fig. 6).
Burnard [56] recently suggested that deeper samples are less in£uenced by air contamination. Our
observations, based on samples erupted at particularly great depths (V3600^4700 m for E-MFZ,
Table 1) support this conclusion. However, this
does not indicate if the atmospheric contamination is e¡ectively smaller for deep samples or if it
seems less in proportion because the samples are
less degassed and thus richer in volatiles.
4.2.2. Mixing
The C/Ar vs. C/N2 trend shown in Fig. 3 could
also be the result of a binary mixing between a
MORB end-member (low C/Ar and C/N2 ratios,
Table 2) and a C-enriched end-member (high
C/Ar and C/N2 ratios; N2 /Ar ratio V44 þ 7;
Table 2, values are from the literature [8,9]).
This second component, for example, could be a
mantle source contaminated by recycled sedimentary carbon, as often invoked to account for socalled abnormal N15 N, C/N2 , C/He ratios or for
lower than average N13 C values found in many
MORB studies (e.g. [7^10,16,32,37,57]).
If the C/Ar vs. C/N2 trend (Fig. 3) re£ects such
a mixing, this mixing should also be responsible
for the isotopic trends observed in Figs. 5, 7 and
9. For example, according to these ¢gures, the EMFZ samples presenting high C/N2 ratios should
also have high N15 N and low N13 C. The hyperbolic
mixing curve between a MORB end-member (Table 2) and a hypothetical C-enriched (recycled)
end-member (see Table 2) is represented in Fig.
10.
From Fig. 10, it is clear that a mixing relationship between a normal MORB and a C-enriched
end-member cannot account for the N13 C^N15 N
covariation and especially not for the trend observed for the E-MFZ samples. These samples
de¢ne a sub-linear trend, implying, in terms of a
hypothetical mixing, that both end-members
would have similar C/N2 ratios, which is of course
in contradiction with variations identi¢ed in Figs.
3, 5 and 7. It can be concluded that the contribution of a C-enriched source to a MORB end-
249
Fig. 10. N13 C vs. N15 N variations in MORB vesicles. The
trend expected from a mixing relationship between a typical
and C-enriched end-member (both de¢ned from the literature, see Table 2) is indicated. The ¢gure shows that N13 C vs.
N15 N variations are unlikely to result from such a mixing relationship.
member is not responsible for most of the measured volatile variations along the SWIR. Other
authors have also invoked three component mixing (e.g. [9]). Although an appropriate mixing between three or even four components could account for the measured values, it would not
predict the simple relationships that are identi¢ed
for samples between MFZ and RTJ.
4.2.3. Degassing
The C/Ar vs. C/N2 trend of Fig. 3 can also be
interpreted as the result of degassing processes.
From experimental data, carbon solubility (e.g.
[13,15]) is known to be much higher than that
of nitrogen [12,48] and argon [3,14]. Accordingly,
C/N2 and C/Ar ratios increase during degassing
(black arrow in Fig. 3).
Moreover it has clearly been established that
N13 C values of dissolved and exsolved carbon
vary upon degassing (e.g. [1,2]). An isotopic fractionation results from the change in carbon speciation when carbonate ions dissolved in the melt
exsolve as CO2 in the vesicles with a positive fractionation factor (vC , [1,23,37,58,59]). Consequently, the N13 C value of the residual carbon
decreases during degassing and one expects an
inverse correlation between C/N2 (or C/Ar) and
N13 C. Fig. 7 shows that, for E-MFZ samples,
such a relation between N13 C and C/N2 exists.
EPSL 6038 21-12-01
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P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
C/N2 increases by one order of magnitude when
N13 C decreases by more than 3x. The same observation can be made in a N13 C vs. C/Ar diagram
(not shown). The variations in C/N2 and C/Ar are
thus likely to result from the same degassing process(es) responsible for the variations in N13 C.
According to a degassing process, the slope of
the trend identi¢ed in the C/Ar^C/N2 diagram
(Fig. 3) depends on the solubility of nitrogen
over that of argon. Since the slope is very close
to 1 (Fig. 3), it implies that in basaltic magmas,
nitrogen and argon have similar solubilities, in
agreement with previous observations [7] and experimental determinations [12,19,20]. However, it
is important to emphasize that the value of the
slope corresponding to SAr /SN2 V1.2 þ 0.1 demonstrates that nitrogen is slightly less soluble than
Ar. The fact that SAr s SN2 is also required to
explain the inverse correlation between N2 /Ar
and C/Ar (Fig. 6) or C/N2 (not shown) and the
correlation between N2 /Ar and N13 C (Fig. 8). This
is important as nitrogen has often been assumed
to have the same [9] or slightly higher [8,48] solubility than argon. We conclude here that it is not
the case and that according to Figs. 3, 5 and 8,
the order of the solubilities is: SC s SAr s SN2 .
Similarly, for the E-MFZ samples, N13 C and
15
N N de¢ne an inverse linear correlation (Fig. 9).
Contrasting with assumptions made in [7^10], the
relationship between N13 C and N15 N demonstrates
that N15 N values are in£uenced by degassing. The
negative slope implies that nitrogen stable isotopes fractionate during degassing and that this
fractionation is opposite to that of carbon isotopes (i.e. degassing yields higher N15 N in the
melt, [46]). A fractionation of nitrogen isotopes
between the dissolved and gas phase is also required to explain the inverse correlation between
N15 N and N2 /Ar observed in Fig. 4 or the rough
correlation between N15 N and C/N2 (Fig. 5) that is
de¢ned by E-MFZ samples.
The hypothesis that the range of N13 C values
(311.40 to 34.36x) originated from a fractionation process is supported by the positive linear
relationship observed between the N13 C and the
N18 O of the vesicles CO2 (not shown; Table 1).
As previously suggested [2], this correlative increase likely corresponds to the reequilibration
of O-isotopes, at ordinary temperatures, between
exsolved CO2 and H2 O, the amount of the latter
increasing in vesicles with degassing. Furthermore, the in£uence of degassing processes bu¡ering concentrations, speciations and isotopic compositions of carbon is con¢rmed by the observed
relationships between dissolved carbon concentrations in the glasses and the depth of eruption for
these samples [60].
4.3. Modelling the degassing
4.3.1. Rational of the model
One can approximate magma degassing as a
two main step process. At the stage of MOR magma formation, the volatile initial characteristics
are : N13 C0 , N15 N0 , (C/N2 )0 , (C/Ar)0 , (N2 /Ar)0 . As
the magma rises, it becomes ¢rstly supersaturated
with respect to carbon. The ¢rst vesicles formed
remain in equilibrium with the volatiles dissolved
in the magma. As the magma further rises, and
until the vesicles escape, the dissolved volatiles
di¡use to concentrate into the gas phase and still
remain in equilibrium with the magma. The equations driving degassing at depths are thus those
under closed-system conditions. When the magma
enters the oceanic crust, its velocity most of the
time stays low whereas the bubble size increases.
The bubbles acquire a higher velocity and leave
the portion of magma which produced them. At
this stage, hereafter referred to as `stage A', it will
be considered that all bubbles have been lost, the
volatiles still dissolved in the residual magma have
the following characteristics: N13 CA , N15 NA ,
(C/N2 )A , (C/Ar)A , (N2 /Ar)A . The magma continues to rise toward the surface and becomes again
C-supersaturated. This induces the nucleation of a
new generation of bubbles, the newly formed
vesicles being continuously lost. The equations
driving this second step of degassing are thus
those under open-system conditions (i.e. Rayleigh
distillation).
The present degassing scenario has been deduced from both carbon (e.g. [2] see also
[61,62]) and noble gas studies [4]. The late stage
of Rayleigh distillation is well accepted [2,7,8,22^
24,46,55,63]. The variations (e.g. in N13 C or in
4
He/40 Ar*) are indeed too large to be accounted
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P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
by a closed-system solely. However, the ¢rst degassing stage is not always considered as it is
more di¤cult to constrain. This is the case, for
example, in studies using only the 4 He/40 Ar* ratio
as an index of degassing. As such a degassing
index cannot decompose the respective importance of the two degassing steps, the ¢rst step of
degassing is generally ignored (contrasting with
conclusions brought when more noble gases are
considered together [4]). This point is important
since the consideration of a single degassing stage
under open-system conditions implies lower estimates of initial volatile concentrations and thus
lower mantle volatile £uxes relative to a twostep model.
The evolution of the concentration of i (C, N,
Ar) in the magma or in the vesicles can be derived
for both closed and open (i.e. Rayleigh distillation) degassing systems from the following equations (see also [3,24,63]) : the Henry's law predicts
that:
C melt
b S i WT C =T 0
fi ˆ i 0 ˆ
…1†
V ‡ b S i WT C =T 0
Ci
where fi is the remaining fraction of the volatile
species i in the melt, Ci melt is the concentration in
the melt and Ci 0 its initial concentration, Si is its
solubility (cm3 STP/g/bar), V* (cm3 /cm3 ) is the
vesicularity, b is the melt density taken as 2.8
g/cm3 , TC and T0 are closure (V1000 K) and
standard (273 K) temperatures respectively.
During closed-system degassing, the ratio of the
concentrations of two volatile species i and j, in
the melt or in the vesicles, is given by the following equations:
C melt
C 0i S i V ‡ b S j WT C =T 0
i
…2†
ˆ 0U
C j S j V ‡ b Si WT C =T 0
C melt
j
and:
V ‡ b S j WT C =T 0
C ves
C 0i
i
ˆ
U
C ves
V ‡ b S i WT C =T 0
C 0j
j
…3†
The equations for open-system degassing can be
derived from Eqs. 2 and 3 by replacing the initial
volatile concentrations by the residual volatile
251
concentrations remaining after the previous degassing incremental step.
In a closed system, the isotopic composition (Ni )
of i in the magma is related to its remaining fraction f in the melt as follows [1,23]:
N i;melt ˆ N i;0 ‡ vi …f i 31†
…4†
and in an open system:
N i;melt ˆ N i;0 ‡ vi Uln f i
…5†
where vi is the equilibrium fractionation factor of
i between the vesicles and the melt:
vi ˆ N i;ves 3N i;melt
…6†
The present model relies on a series of assumptions, that can be adjusted but which are widely
used in other studies. In particular, it is assumed
that,
during
degassing,
solubility
ratios
[4,8,9,18,24,33,55], vC and vN remain constant
[1,2,23]. The open-system degassing is simulated
by removing the gas phase in 0.01% increments
[4,63,64]. The degassing is modelled by ¢xing only
SC and vC . A carbon solubility value of 0.137
ppm C/bar (2.56U1034 cm3 /g/bar up to 2 kbar
[13]) was taken. Although no consensus exists
for vC , estimations ranging from +2.3 [16] to
+4.5x [59], a vC value of +3.5x was adopted,
as recently redetermined experimentally [65] and
observed in natural samples containing homogeneous vesicles in terms of N13 C [23]. Rather than
assumed, SN2 , SAr and vN (not very well constrained) were determined using the correlations
identi¢ed in Figs. 3^9.
4.3.2. Application of a Rayleigh distillation model
A late Rayleigh distillation process is required
on the basis of three observations: (1) whatever
the parameters are, it is impossible to model the
variations shown in Figs. 3^9 according to a
closed-system model solely, (2) volatile to volatile
ratios and carbon isotopic variations exceed volatile solubility ratios (from what we roughly
know) and carbon isotopic fractionation factor
respectively and (3) the variations can indeed be
modelled under open-system conditions.
EPSL 6038 21-12-01
252
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
Figs. 3^9 show the degassing paths modelled
under open-system conditions together with the
data obtained in the present study. From the covariations shown in Figs. 3 and 8, we deduce that
SAr /SN2 = 1.2 þ 0.1. From the computation and
Figs. 6 and 7, the SC /SN2 ratio is about 5‡2
31 .
The relatively low solubilities of both nitrogen
and argon compared with carbon imply that the
remaining fractions of N2 and Ar in the melt decrease dramatically relative to fC . A Rayleigh degassing process also predicts a straight line in a
N13 C^N15 N diagram (Fig. 9) with a slope depending on vC , vN , SC and SN2 values. Considering
vC = 3.5x, SC = 2.56U1034 cm3 /g/bar, and a deduced value of 3.65U1035 cm3 /g/bar for SN2 (i.e.
0.045 ppm/bar), the data in Figs. 3^9 can be explained using a value of 31.6 þ 0.3x for vN
(vesicles^melt). The corresponding MORB characteristics at stage A (i.e. before the beginning
of the Rayleigh distillation) are thus the following: N13 CA = 37.8x, N15 NA = 34.5x, (C/N2 )A =
600, (C/Ar)A = 57 000, (N2 /Ar)A = 95 and CA
V150 ppm C (Table 2).
Figs. 3^9 show that the model degassing paths
can indeed account for most of E-MFZ and WMFZ values. In Fig. 4, one can however notice
that four samples from the W-MFZ have N2 /Ar
ratios s 140 (and rather high N15 N; Table 1) and
clearly fall outside the degassing path. These values are also higher than that of air, ASW or the
so-called C-enriched end-member. It is also important to notice that the highest value was obtained on a second sample of DR 52 (i.e. same
dredge but not the same pillow) for which a normal N2 /Ar ratio was ¢rst measured. The anomalous N2 /Ar values are also not restricted to a speci¢c part of the ridge as they occur both on the
W-MFZ and on RTJ. The origin of such an
anomaly is unknown. It may be related to either
a non-totally incompatible behavior of nitrogen
(see [66,67]) or argon [68] during partial melting
(i.e. contrasting with the likely incompatible behavior of carbon) or to a real mantle volatile heterogeneity.
4.3.3. Closed-system degassing and the initial
MORB volatile characteristics
The MORB volatile characteristics at stage A
Fig. 11. Estimated MORB initial volatile characteristics as a
function of degassing importance under closed-system conditions typi¢ed, here, by a theoretical vesicularity. The considered range of vesicularity was derived from [4,23].
(at the beginning of the Rayleigh distillation) are
however not its initial characteristics. The initial
characteristics can be deduced after quanti¢cation
of the extent of the preceding degassing under
closed-system conditions. Fig. 11 shows how, according to Eq. 2, the initial volatile characteristics
can be deduced from volatile characteristics at
stage A and the magma theoretical vesicularity
just before vesicles loss.
From two previous studies [4,23], one can estimate the range of vesicularity of a MORB magma
degassing under closed system. Vesicularities between 1.5 and 40% are found in one case [4] and
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between 1.7 and 12% in the second case [23]. Considering that during the ¢rst stage of degassing
(under closed-system conditions), the SWIRMORB had a vesicularity (V*) ranging from 2
to 20% (gray ¢eld in Fig. 11) leads to modelled
initial volatile characteristics as follows: N13 C0 between 34.7 and 34.5, N15 N0 V36.0x, N2 /Ar
V110, C/N2 V130, C/ArV14 000 and C0 between 1100 and 5000 ppm C. Because carbon solubility increases at high pressures, the carbon solubility was indexed relative to pressure ([13] up to
2000 bar, [15] for higher pressures up to 0.220
ppm C/bar at 20 kbar).
The initial volatile characteristics, particularly
those of C0 , N13 C0 or (C/N2 )0 , are very dependent
on the vesicularity range that was deduced from
previous studies. They show however strong similarities with the MAR popping-rock values for
carbon content, N13 C values and nitrogen isotopic
composition (C0 s 4000 ppm C, N13 CV34.0x;
N15 NV36.0x; [6,23], unpublished data) ; the
deduced N2 /Ar and C/N2 ratios are however higher and lower respectively by a factor of 3 and 6
[6].
To check and strengthen the present conclusions, we have extended the modelling to C/He
and He/Ar ratios. With a value of SHe /SC V2.5
[3,13] and (He/Ar)A V8, one can reproduce the
range of C/He and He/Ar among MORB samples
(i.e. 4 He/40 Ar* between 10 and 100). In particular,
the estimated (He/Ar)0 falls within the well accepted range of MORB initial He/Ar ratio of
1.5 þ 0.5 (e.g. [4,6]).
4.4. Implications
4.4.1. Nitrogen isotopes fractionation during
degassing
In previous studies, the possible fractionation
of nitrogen isotopes between the dissolved and
exsolved phases during degassing has generally
been neglected. Variations in N15 N of MORB
were considered as direct evidence of source heterogeneity [7^10]. For example, N15 N values close
to that of atmosphere or slightly positive measured in some E-MORB [7,8] were used to suggest
that the source of E-MORB would contain recycled material, this being supported by high
253
C/N2 ratios considered to re£ect high initial
C/N2 ratios (i.e. s 4000) (e.g. [8]).
The present data show that nitrogen isotopes
do fractionate during degassing. The vesicles are
depleted in 15 N relative to the residual melt by
about 1.6x. Since the residual melt is enriched
in 15 N during degassing, positive N15 N values
measured in E-MORB [7,8] or carbonatites [69]
do not necessarily imply a source anomaly but
can be the result of degassing. From calculations
given in Section 4.2, one can discuss the evolution
of stable isotopic and abundance ratios of C, N
and Ar during degassing of an E-MORB. According to [4], E-MORB would be more a¡ected by
degassing under open system than N-MORB.
Considering that the initial characteristics of EMORB would be similar to N-MORB (as given
in Table 2), a Rayleigh degassing predicts the occurrence of vesicles with the following characteristics : N13 C 6 312% together with N15 N s +2%
and C/N2 s 4000, C/Ar s 400 000 and N2 /Ar 6
40. In particular, a degassing model could account
for the fact that E-MORB shows lower N2 /Ar
(mean = 44 þ 7 [8]) than N-MORB (mainly between 90 and 150, e.g. [8]), high C/N2 ratios and
slightly positive N15 N values without requiring any
source heterogeneity. In general, given the extremely large chemical and isotopic variability
that degassing processes can produce on volatiles
such as C, N and Ar, the generally accepted conclusion that E-MORB would bear strong volatile
anomaly is equivocal.
4.4.2. Implications for volatile £uxes: a consistent
set of parameters
There are probably as many estimates of the
amount of volatiles degassed on MOR than methods adopted and these typically vary within one
order of magnitude. Estimates for carbon £ux
(PC ) indeed vary from V2.2U1012 [22,55] to
V2.0U1013 mol/yr [6,21,70]. The former estimate
is deduced from C/3 He systematics and depends
on the estimate of 3 He £ux degassed on MOR, C
and He solubilities and Ar solubility when introduced in the degassing corrections. The latter is
deduced from C abundances and N13 C systematics
and depends on vC , carbon solubility, crust thickness and spreading rate. Both estimates strongly
EPSL 6038 21-12-01
254
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Table 3
Estimations of both MORB volatile characteristics at stage
A (i.e. before Rayleigh distillation but after closed-system degassing) and initial MORB characteristics
N13 C (x)
N15 N (x)
C/N2
N2 /Ar
C/Ar
ppm C
vC
vN
SC
SN2
SAr
MORB before Rayleigh
distillation stage (stage A)
Figs. 3^9
Initial MORB
characteristics
Fig. 11
37.8x
34.5x
600
95
57 000
150
3.5x (¢xed)
31.6x
2.56U1034 (¢xed)
5.12U1035
6.14U1035
34.5 þ 0.2x
V36.0x
V130 þ 10
V110
14 000 þ 1 000
1 100^5 000
depend, of course, on the chosen degassing model(s) (i.e. whether a closed-system step is considered or neglected).
From the initial volatile characteristics deduced
from our model (Table 3), one can propose a
consistent set of £ux estimates such as
PC V1.3 þ 0.1U102 PN2 and PC V1.4 þ 0.1U104
PAr . As mentioned above (Section 4.3.3), both
(C/N2 )0 and (C/Ar)0 are relatively low and thus
PN2 and PAr are higher than previous estimates
[6,33]. Considering a MORB initial carbon content between 1100 to 5000 ppm C, the deduced
carbon £ux varies from 0.4 to 1.8U1013 mol/yr
(computed in a similar way to [21,70]) overlapping the range of carbon £ux estimates found in
literature. However, it is clear that the initial carbon content and thus the carbon £ux estimate
strongly depend on the extent of closed-system
degassing which remains little constrained.
4.4.3. Implications for the origin of the
DUPAL anomaly
MORB samples from the SWIR can show
anomalous radiogenic isotopic ratios of e.g. strontium, neodymium and lead. The origin of this
anomaly remains unclear and several hypotheses
have invoked the reinjection of delaminated subcontinental lithosphere (e.g. [28]), sediments or
crust (e.g. [26,27]). The rigid subcontinental lithosphere, metasomatized by mantle-derived £uids or
not, remaining isolated for long periods can in-
deed develop the required isotopic compositions
for Sr, Nd, Pb, He or Ar. However, this process
would probably have only little e¡ect on the stable isotopic compositions of C and N or C/N2
ratios. The fact that MORB samples from the
Indian Ocean do not show any volatile anomaly
in terms of C/Ar, C/N2 , N2 /Ar, N13 C and N15 N
(this study, [8,9]) could support the delaminated
continental lithosphere hypothesis.
However, the present results do not completely
rule out a model calling for crust or sediment
recycling. In this case, the source characteristics
would depend on the relative amounts of volatile
recycled into the Indian Ocean mantle source relative to the amount of mantle volatiles. According
to [27], the MORB Indian source would contain,
as a maximum, about 0.5 wt% of recycled pelagic
sediments with most samples containing as little
as 0.05 wt%. Given the present restricted knowledge on nitrogen evolution during subduction and
thus of nitrogen recycling £uxes, the present discussion shall concentrate on carbon mass balance.
Depending also on the amount of organic matter
(mean N13 CV325x) and carbonates (at
N13 CV0x), the recycled end-member may not
bear any anomalous N13 C values. An appropriate
mixing of 20% organic matter and 80% carbonate
would for example lead to a recycled carbon isotopically very similar to mantle carbon
(N13 CV35x).
Assuming a mantle carbon content of about
400 ppm ([46,66], favoring thus the highest carbon
mantle £ux estimate), recycled sediments or crust
bearing 1 wt% of carbon would bring between 50
and 5 ppm according to the 0.5 and 0.05 wt%
mixing hypothesis respectively. Whereas the latter
mixing type probably has little in£uence on mantle source characteristics it is not the case for the
former since it may contribute more than 10% of
the total carbon content. Alternatively, if one assumes a mantle carbon content of about 30 ppm
[33,55], one can exclude the previous recycled endmembers since those would contribute more than
15% of the total carbon budget.
In summary, if the contribution of delaminated
continental lithosphere seems the simplest hypothesis from a volatile (C, N, Ar) point of view, the
recycled component has to be considered as well,
EPSL 6038 21-12-01
P. Cartigny et al. / Earth and Planetary Science Letters 194 (2001) 241^257
provided that it bears low enough (or any) volatile levels.
5. Conclusions
Volatile (C, N, Ar) data measured on vesicles
from basaltic glasses from the SWIR show that:
1. The observed elemental and isotopic covariations mainly depend upon degassing processes
as opposed to air contamination or source effects.
2. The covariations demonstrate that nitrogen
isotopes fractionate during degassing.
3. A degassing model allows relative solubilities
of carbon, nitrogen and argon to be constrained: SC /SN2 V5 and SAr /SN2 V1.2.
4. The SWIR samples show similar isotopic and
elemental ratios to basaltic glasses from other
oceans. In other words, no anomaly can be
de¢ned from a (C, N, Ar) volatile point of
view.
5. The present results reemphasize the fundamental role of degassing process(es) over source
heterogeneity for (C, N, Ar) volatile variability.
Acknowledgements
We gratefully acknowledge ¢nancial support
from INSU Geosciences Marines and the IFRTP.
M. Girard is thanked for his technical advice and
the maintenance of our static mass spectrometer.
Thanks are also due to E. Humler, C. Mevel, C.
Meyzen, M. Moreira and C. Gautheron for helpful discussions and personal communication of
their data and to M. Adams for English improvement of the manuscript. We also would like to
thank the captain and crew of the R/V Marion
Dufresne for their dedication to the success of
the mission. M. Carroll, D. Hilton and an anonymous reviewer are thanked for their very positive
and constructive comments. Contribution IPGP
1783 and CNRS 293. Our friend and colleague,
Stuart Boyd, left us for a better world. This contribution is dedicated to him.[AH]
255
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