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Geochemistry Geophysics Geosystems

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Research Letter

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Volume 5, Number 1 3 January 2004 Q01001, doi:10.1029/2003GC000612

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

ISSN: 1525-2027

Nitrogen isotopic composition of the MORB mantle: A reevaluation Ratan K. Mohapatra Department of Earth Sciences, University of Manchester, Manchester, M13 9PL, UK ([email protected])

S. V. S. Murty Planetary and Geosciences Division, Physical Research Laboratory, Ahmedabad, 38000 India ([email protected])

[1] Nitrogen in the mantle source of mid oceanic ridge basalts (MORBs) is commonly believed to have d15N5%. This is based on a (selective) statistical average drawn from nitrogen isotopic data obtained simultaneously with argon from MORBs. However, on critical evaluation, using the accompanying 40 Ar/36Ar data, we show this approach to be somewhat ambiguous. Considering the fact that MORBs can have a number of geochemical components in them, as has been shown by several recent studies, the approach of statistical averaging may actually point to a mixture rather than the true mantle end-member. A more reasonable approach, also used for deriving the mantle noble gas signatures, suggests nitrogen in the MORB mantle to be much lighter (15%) in its isotopic composition. Such a signature is consistent with nitrogen data from MORBs as well as from other mantle derived samples that are believed to have sampled volatiles from the MORB mantle. Components: 4600 words, 3 figures, 1 table. Keywords: Nitrogen; isotopes; MORB-mantle; argon; recycled-component. Index Terms: 1025 Geochemistry: Composition of the mantle; 1040 Geochemistry: Isotopic composition/chemistry; 1060 Geochemistry: Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008); 4820 Oceanography: Biological and Chemical: Gases. Received 29 July 2003; Revised 15 October 2003; Accepted 13 November 2003; Published 3 January 2004. Mohapatra, R. K., and S. V. S. Murty (2004), Nitrogen isotopic composition of the MORB mantle: A reevaluation, Geochem. Geophys. Geosyst., 5, Q01001, doi:10.1029/2003GC000612.

1. Introduction [2] Nitrogen isotopic composition (d15N) of the Earth’s mantle has implications for the planet’s precursors [Javoy, 1995]. It is defined by the per mil (%) deviation of 15N/14N ratio (of the two stable nitrogen isotopes) of a sample from that in air. [3] MORBs have been studied to understand nitrogen in the Earth’s mantle. Data from these studies show a wide variation in d15N from 15 Copyright 2004 by the American Geophysical Union

to +18% with a mean not much different from the air signature (0%), which is explained by invoking contributions from different end-members such as MORB mantle, air or air saturated water (ASW) and recycled materials [Exley et al., 1987; Marty and Humbert, 1997; Sano et al., 1998; Marty and Zimmermann, 1999; Mohapatra and Murty, 2000a]. With only two isotopes, it is impossible to resolve different components in MORBs by analyzing nitrogen alone. In this context, the study of nitrogen simultaneously with noble gases by static mass spectrometry [Frick and Pepin, 1 of 9


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mohapatra and murty: nitrogen isotopic composition

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Table 1. Statistics of the Literature MORB Nitrogen and Argon Data From Vacuum Crushing Experiments Alla d15N Mean 2s errorc Size (n) Minimum Maximum

1.8 0.6 74 8.9 ± 0.5 +6.5 ± 0.6

40

40

Ar/36Ar

N2/40Ar

d15N

140 55 74 19 1829

2.8 0.6 38 8.1 ± 0.6 +1.0 ± 0.9

3701 1517 74 299.4 ± 0.3 42366 ± 9713

a Data source [Marty and Humbert, 1997; Sano et al., b Filter as used by Marty and Zimmermann [1999]. c

Ar/36Ar > 1000 (‘‘filtered’’)b 40

Ar/36Ar

6662 2672 38 1237 ± 57 42366 ± 9713

N2/40Ar 129 21 38 37 709

1998; Marty and Zimmermann, 1999; Nishio et al., 1999].

Refers to the statistical error only.

1981] provides a useful approach. Although this (approach) allows one to analyze all the noble gases along with nitrogen in a sample, because of experimental difficulties (low concentrations and/ or mass spectrometric interferences) the noble gas data are mostly restricted to He and Ar. The isotopic ratios of He (3He/4He) and Ar (40Ar/36Ar) are excellent tracers of contributions from mantle and air [Ozima, 1994]. However, it should be noted here that in contrast to the case of argon, the concentrations of helium (3He and 4He) in air and ASW are much smaller than that in MORBs. Therefore contributions from the atmospheric endmembers affect only the indigenous argon isotopic signatures of MORBs and not 3He/4He. This fact is reflected in the narrow range of 3He/4He (8 times the air ratio) as compared to that in 40Ar/36Ar observed in MORBs [Ozima, 1994]. The rest of the discussion is therefore based on simultaneous nitrogen and argon isotopic data on MORBs. [4] Two types of experimental procedures have been adopted for simultaneous nitrogen and argon isotopic analysis in MORBs, which differ in their gas extraction techniques. The large range of d15N observed in MORBs is due to data from experiments adopting stepped heating technique of gas extraction, which are a few at the moment [Exley et al., 1987; Mohapatra and Murty, 2000a]. The MORB nitrogen data base, on the other hand, is dominated by data from experiments that extract gases by crushing in vacuum [e.g., Marty and Humbert, 1997; Marty and Zimmermann, 1999], henceforward referred to as the Nancy group), which present a narrower range (9 to +6%) of d15N. We postpone a discussion on the stepped

heating data to a later point in the paper as such data do not have much statistical weight. On the other hand, an unbiased statistical average for the literature MORB data from vacuum crushing experiments [Marty and Humbert, 1997; Sano et al., 1998; Marty and Zimmermann, 1999; Nishio et al., 1999] would suggest d15N1.8% for the MORB mantle (Table 1). Considering the estimates of equilibrium nitrogen isotopic fractionation (up to 2% [e.g., Marty and Humbert, 1997]), the above mean d15N is indistinguishable from the atmospheric signature. However, it should be realized that the simple statistical average derived above may not be a true representative of the MORB mantle d15N because of the possibility that measured nitrogen in MORBs may be a mixture of contributions from a number of components. In that case it is necessary to isolate the MORB mantle component from the effects of those superimposed on it so that one can derive its isotopic signatures, which is somewhat tricky. A popular approach to this problem is that adopted by the Nancy group, which interprets the MORB data as a two component mixture of MORB mantle and ‘‘surface-derived’’ (atmospheric and organic) contamination. It derives a MORB mantle d15N of 3.3 (±1.0)% [Marty and Zimmermann, 1999] from (vacuum crushing MORB) data chosen by the accompanying 40Ar/36Ar signatures. In this paper we present a critical evaluation of this approach and hence of the MORB mantle d15N derived by it. We organize the presentation the following way. A short introduction about the approach (which we refer to as the ‘‘conventional’’) is followed by a discussion on some of its flaws, which is followed by the proposition of an 2 of 9


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Figure 1. Histograms of (a) 40Ar/36Ar and (b) d15N for the literature MORB data from vacuum crushing experiments [Marty and Humbert, 1997; Sano et al., 1998; Marty and Zimmermann, 1999; Nishio et al., 1999]. The hatched region (in 1a) represents (51% of the literature) data with 40Ar/36Ar > 1000, which qualify to be considered for the derivation of d15N for MORB mantle by the ‘‘conventional’’ approach. The frequency distribution of d15N for such ‘‘filtered’’ data is shown by the curve in Figure 1b (see text for discussion).

alternate approach to derive the MORB mantle d15N, and the summary.

2. D15N of MORB Mantle: The ‘‘Conventional’’ Approach [5] 40Ar/36Ar in MORBs (Figure 1a) varies from the near atmospheric signature (295.5 [Ozima, 1994]) by about two orders of magnitude, with a mode at 1250. The variation in 40Ar/36Ar may be interpreted as a result of mixing between mantle (40Ar/36Ar 40,000 [Burnard et al., 1997]) and an end-member(s) with air-like 40Ar/36Ar. The Nancy group adopts a filter ‘‘40Ar/36Ar = 1000’’ to remove effects of the air-like 40Ar/36Ar (and d15N) component(s) in its MORB d15N data and derives the MORB mantle d15N signature from the statistical average of the ‘‘filtered’’ data. While the choice of this particular cutoff limit for 40Ar/36Ar is not clear, it turns out that the above isotopic filter

removes 49% of the literature argon and nitrogen data (Figure 1) from the consideration for deriving the isotopic composition of MORB mantle nitrogen. The filtered data (under the hatched region of Figure 1a and represented by the frequency distribution curve in Figure 1b) yield a mean d15N of 2.8 (±0.6, 2s error, Table 1)%, which according to the ‘‘conventional’’ approach should represent the MORB mantle nitrogen.

3. Drawbacks 3.1. Adopting Different Ways to Derive the MORB Mantle D15N and 40Ar//36Ar [6] As we are dealing with simultaneous nitrogen and argon data it should be possible to extend the above statistical procedure to derive the MORB mantle 40Ar/36Ar. However, a mean 40Ar/36Ar of 6662 (±2672, 2s error, Table 1) thus derived is about an order of magnitude lower than the gener3 of 9


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ally accepted mantle signature [Burnard et al., 1997]. The Nancy group adopts a compromising approach. Instead of adopting the procedure of ‘‘statistical average’’ (as for d15N), it derives the MORB mantle 40Ar/36Ar from the maximum measured MORB value (42,366 ± 9713 [Marty and Humbert, 1997]) apparently similar to that predicted by the noble gas studies. Now if one cannot use a statistical average for the argon data why should one adopt it for the nitrogen data (acquired simultaneously)? [7] The ‘‘filtered’’ data in Figure 1 still show wide ranges (Table 1) of d15N (from 8 to +1%) and 40 Ar/36Ar (from 1200 to 40000) signatures. One may invoke various processes of high temperature isotopic fractionation to explain the variation in d15N data. However, a variation in 40Ar/36Ar by over an order of magnitude, which is not possible to explain by any of such processes, requires contributions from more than one component. This suggests that the isotopic filter of the Nancy group is perhaps unable to isolate the MORB mantle component from the superimposing component(s).

3.2. In Relation to D15N Observed in Other Mantle Derived Materials [8] The MORB mantle d15N derived by the Nancy group is often justified by pointing at its similarity with the ‘‘mantle signature’’ (d15N5% [e.g., Cartigny et al., 1998]) proposed from the mode of the frequency distribution of d15N data from diamonds, which apparently have trapped the MORB volatiles too. A discussion of nitrogen in diamonds lies beyond the scope of the present paper. However, we must not forget the fact that diamonds do exhibit a wide range of d15N (25 [Cartigny et al., 1997] to +16% [Boyd and Pillinger, 1994]), which has so far not been completely understood. For example, there is no agreement yet as to whether the range of d15N is a result of mixing between different components or because of isotopic fractionation. If the diamond data really are a result of mixing, the mantle signature inferred from them by considering the (statistical) ‘‘central tendency’’ of d15N is as questionable as that derived from the MORB data. It is useful to note here that mantle xenoliths,

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which have also trapped the upper (MORB) mantle nitrogen, do show a d15 N signature 15% [Mohapatra and Murty, 2002], much lighter than the ‘‘conventional’’ MORB mantle signature.

3.3. Case of Stepped Heating Data [9] The MORB mantle d15N of 5% derived from the ‘‘conventional’’ approach leaves the stepped heating data (with d15N varying from 15 to +18%) with the following explanations. One may discard the stepped heating data by arguing for isotopic fractionation due to the stepped heating process or by attributing the extreme signals to various (organic?) contaminants capable of producing both the extreme light and heavy d15N signatures. However, it should be realized that the stepped heating technique adopted for the study of MORBs is well accepted in the study of meteorites, most of which are petrographically not much different from the terrestrial mantle derived rocks. For example, simultaneous nitrogen and noble gas isotopic analysis of Martian meteorites (basalts and ultramafic rocks) has provided many useful information regarding nitrogen and noble gases in Mars [Murty and Mohapatra, 1997]. These experiments adopt low temperature combustion, in addition to the routine overnight baking, to remove contamination from air and other low temperature components. Thus Mathew and Marti [2001] have been able to isolate the trapped Martian nitrogen and noble gas component in ALH 84001 and Chassigny at a temperature as low as 200C. All the stepped heating data considered in the present paper have been acquired by stepped heating MORB samples already cleaned by a combustion step at 400C [Mohapatra and Murty, 2000a]. Further it should be noted that ALH 84001, which spent a considerable period of time in the Antarctic ice, may perhaps be more contaminated (by the atmospheric gases) as compared to the MORB glass samples chipped from inside a lava flow. [ 10 ] Fractionation predicts that one should observe light d15N in the low temperature extractions, while heavier signatures in the higher temperature ones. However, it should be realized here that a MORB sample may release gases 4 of 9


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Figure 2. A plot of 40Ar/36Ar (in units of 1000) against N2/36Ar for the MORB data complied in Figure 1. The straight line represents a linear regression computed using the procedure of Williamson [1968] and from data with 40 Ar/36Ar > 1000. It however ignores data with improper errors [Sano et al., 1998; Nishio et al., 1999], as the ‘‘Williamson regression’’ is very sensitive to errors in both variables. The summary of results from the regression analysis is compiled in the inset of Figure 2. The dashed line represents a two component mixture of the MORB mantle and Air/ASW as proposed by Marty [1995] (see text for discussion).

trapped in vesicles also in the low temperature extractions, which may not be resolvable from the former (effect of fractionation). Similarly, it is difficult to explain any stepped-heating-induced nitrogen-isotopic-fractionation by the observation often of heavier d15N in the low temperature steps and lighter signatures in the high (even at the melting) temperature steps [Mohapatra and Murty, 2000b, 2002]. On the other hand, the variation in d15N observed in the stepped heating MORB data may be explained by contributions from different nitrogen components.

3.4. N//Ar Elemental Ratios [11] N2/36Ar and N2/40Ar elemental ratios from the vacuum crushing MORB data have been employed to derive many important conclusions about nitro-

gen and argon in the MORB mantle [e.g., Marty, 1995]. It is commonly believed that nitrogen and argon in such data show a good ‘‘correlation,’’ which has played a key role in deriving the MORB mantle d15N. Figure 2 is a plot of 40Ar/36Ar against N2/36Ar in which the MORB data from Figure 1 have been examined by a linear regression analysis using the procedure of Williamson [1968] that considers errors in both the variables. A regression through all the data resulted in a meaningless intercept (by yielding negative N2/36Ar value). We therefore consider data with ‘‘40Ar/36Ar > 1000’’ to obtain the regression line in Figure 2 (defined by the regression parameters shown inside the plot). It should however be mentioned here that data from Sano et al. [1998] and Nishio et al. [1999], which do not have any errors mentioned, have been ignored in the above exercise. This is 5 of 9


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because of the fact that the Williamson method of regression analysis is very sensitive to the errors in the variables, and using such data with no errors may introduce artificial influence on the regression parameters. [12] From the scatter of data with respect to the regression line in Figure 2, it is very difficult to understand any correlation in the MORB data. Marty [1995] interprets the vacuum crushing MORB data as a two component mixture of contributions from the ‘‘conventional’’ MORB mantle and the atmospheric reservoirs (Air and ASW). Represented by the dashed straight line in Figure 2, this mixing relationship is hardly supported by the MORB data. Considering the difficulties in fractionating the argon isotopes to produce the order magnitude variation in 40 Ar/ 36 Ar, one may attribute the scatter in Figure 2 to elemental fractionation between N2 and 36Ar such as loss or gain of nitrogen. However, it should be realized that any loss of nitrogen from a sample may also affect its d15N. A simple calculation (using the nitrogen and argon signatures of MORB mantle proposed by the Nancy group) would suggest that the process of nitrogen loss, in order to explain the observed range of N2/36Ar (3.5 106, Figure 2), would result in a variation in d15N much larger than that observed in MORBs. One may thus invoke two different types of fractionation processes to explain the MORB nitrogen and argon data: (1) those fractionating nitrogen from argon but not affecting the nitrogen isotopes and (2) those that fractionate nitrogen (alone). On the other hand, the MORB data in Figure 2 may also be explained by considering additional contributions from a third component (?), without invoking the above complicated fractionation processes. This in turn would suggest that while the isotopic filter of Nancy group is able to eliminate the Air/ASW contributions, it cannot do so for those from the third component. Therefore a statistical average from the filtered d15N data may represent nitrogen in the mixture rather than that from MORB mantle. [13] While Marty [1995] concludes that the MORB mantle has a N2/40Ar (80 ± 25) signature indistin-

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guishable from that of air, this (such a conclusion) is inadequate to explain the ranges of N2/40Ar signatures observed for the literature MORB (19 to 1829 for all and from 37 to 709 for the ‘‘filtered’’ data, Table 1) data compiled in Figures 1 and 2 and from stepped heating [Mohapatra and Murty, 2000a].

3.5. MORBs: A Multicomponent System [14] The multi component aspect of MORB N-Ar data (both stepped heating and vacuum crushing) may be understood in Figure 3, a plot of d15N against 40Ar/36Ar [after Mohapatra and Murty, 2000a]). Also shown in Figure 3 are different end-members: MORB mantle (a: d15N = 3.3%, 40 Ar/36Ar = 40,000, defined by the Nancy group), ASW (d15N = 0%, 40Ar/36Ar = 295.5) and recycled materials (R: d15N = +18%, 40Ar/36Ar = 375 [after Mohapatra and Murty, 2000b]) and the binary mixing curves among them. In the ‘‘conventional’’ approach, one explains the MORB 40Ar/36Ar data by invoking atmospheric contamination (from Air, ASW) of a. On the other hand the MORB d15N data are explained by invoking organic materials as an additional source of contamination, and isotopic fractionation (operative at temperatures both in the atmospheric and mantle environments). It should however be noted that, in Figure 3, 40Ar/36Ar signatures