N-isotope composition of the primitive mantle compared to diamonds

Lithos 233 (2015) 131–138

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N-isotope composition of the primitive mantle compared to diamonds Yiefei Jia a,⁎, Robert Kerrich b a b

CSIRO Exploration and Mining, School of Geosciences, P.O. Box 28E, Monash University, Victoria 3800, Australia Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada

a r t i c l e

i n f o

Article history: Received 28 July 2014 Accepted 10 February 2015 Available online 23 February 2015 Keywords: Nitrogen Isotopes Primitive mantle Crust Recycle Neoarchean

a b s t r a c t The nitrogen isotopic composition of the Earth's primitive mantle is controversial. Chromium-isotopic ratios of various terrestrial minerals and rocks, and chondritic meteorites are consistent with the silicate Earth being a mixture of enstatite and carbonaceous chondrites. From their relative proportions and N-isotope compositions we estimate that the bulk primitive mantle δ15N is −7 ± 3‰. The negative value, as also evidenced by mantle-derived oceanic basalts and diamonds, is an intrinsic long-term feature of Earth's mantle. Some enstatite chondrite-like δ15N values down to −24‰ measured in very rare diamonds could be interpreted as a heterogeneous mantle. δ15N values in oceanic island basalts derived from the deep mantle have three components: deep mantle of ~−9‰ consistent with estimate, recycled sediments of about 15‰, and atmospheric N incorporated from groundwater and/or subducted atmospheric N (Mohapatra and Murty, 2000a; Chem. Geol. 164, 305–320). Some enriched δ15N values in MORB and OIB result from degassing fractionation. Shift of the upper mantle from an initial −7‰ to −5‰ by the Neoarchean can be explained by a combination of sediment recycling through subduction and upper mantle magma degassing processes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The origin and evolution of nitrogen in the mantle have been controversial since the 1950's (Brown, 1953; Pepin and Porcelli, 2002). The isotopic composition of nitrogen shows large different variations in extraterrestrial samples (e.g., chondritic meteorites; Clayton, 1981; Wänke, 1981), which makes this element a useful tracer of mass exchange between the surface and deep-Earth reservoirs and of fluid/melt-rock interactions in the crust and mantle (Busigny and Bebout, 2013; Busigny et al., 2011; Cartigny, 2005; Cartigny and Marty, 2013; Halama et al., 2012; Philippot et al., 2007), and a potentially important tracer for the origin of the terrestrial silicates and volatiles (Busigny et al., 2011; Cartigny, 2005; Cartigny and Marty, 2013; Jia and Kerrich, 2004; Kerrich et al., 2006; Marty, 2012). Nitrogen isotopic composition is expressed by the δ15N parameter defined as δ15N = [(15N/14N)sample/(15N/14N)standard – 1] × 1000, where the standard is the atmospheric N2. Enstatite (E) chondrites have a total range of δ15N from −15‰ to −43‰, with most values between −20‰ and −30‰ (Injerd and Kaplan, 1974; Grady et al., 1986; Kung and Clayton, 1978). In contrast, carbonaceous (C) chondrites have a range from +16‰ to +52‰ (Fig. 1; Injerd and Kaplan, 1974; Kerridge, 1985; Kung and Clayton, 1978; Lewis et al., 1983; Robert and Epstein, 1982). ⁎ Corresponding author at: SRK Consulting (China) Ltd, B1205, COFCO Plaza, 8 Jianguomennei Dajie, Dongcheng District, Beijing 100005, China. Tel.: + 86 10 6511 1053; fax: +86 10 8512 0385. E-mail address: [email protected] (Y. Jia).

http://dx.doi.org/10.1016/j.lithos.2015.02.009 0024-4937/© 2015 Elsevier B.V. All rights reserved.

An enstatite (E) chondrite model for the Earth has been proposed by several workers (Javoy, 1997, 1998; Tolstikin and Marty, 1998) based on two principal lines of evidence: similarity of oxygen isotope compositions of the Earth–Moon system with E-chondrites (Javoy and Pineau, 1983; Javoy et al., 1986), and rare 15N-depleted diamonds down to − 21‰ (Shatskii et al., 2011) or − 24‰ (Cartigny, 2005; Cartigny et al., 1997, 1998). The most depleted nitrogen isotope compositions of −40‰ are also found in a few diamonds (Palot et al., 2012), although both the size and location of this reservoir are unknown. According to Javoy (1998), 99.8% of the planet is of E-chondrite composition, contributing to the nitrogen budget of the Upper Earth (UE: upper mantle + crust + ocean–atmosphere) after upper–lower mantle stratification. A late veneer of C1 chondrite and/or comets of 0.14% Earth mass brought in 30% of the UE budget. In this model, mantle δ15N would have evolved from an initial state characterized by E-chondrite type material having δ15N values of less than − 25‰, matching those of some rare diamonds (Cartigny, 2005; Cartigny et al., 1997, 1998; Palot et al., 2012; Shatskii et al., 2011) to about − 5‰ by the Archean (Javoy, 1997, 1998). This model was questioned based on no direct evidence or record of a secular change in mantle δ15N values (Cartigny and Marty, 2013). They rather propose that the marked nitrogen isotopic contrast between the mantle and the Earth's surface could reflect origins of the nitrogen disequilibrium. This proposal might provide insights into mantle–surface interactions over geological time including recycling of surface sediments into the deep mantle, although the cause of such disequilibrium is not fully understood (Cartigny and Marty, 2013).

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Estastite chondrites

Carbonaceous chondrites

A

-45

B

-40

-35

-30

-25

-20

-15

15

δ15N (‰)

20

25

30

35

40

45

50

55

δ15N (‰)

Fig. 1. δ15N values for enstatite chondrites (A) and carbonaceous chondrites (B). Enstatite chondrites from Injerd and Kaplan (1974), Kung and Clayton (1978), and Grady et al. (1986). Carbonaceous chondrites from Injerd and Kaplan (1974), Kung and Clayton (1978), Robert and Epstein (1982), and Kerridge (1985).

Tolstikin and Marty (1998) also suggested that the Earth accreted from E-chondritic material having a δ15N of − 30‰. In their model, due to a major phase of accretion, impact(s), core segregation, and mantle layering, the upper mantle underwent extensive degassing from 4.9 × 1023 mol N at 4.5 Ga, at ~ 60 Ma post accretion, to 2 × 1017 mol at 4.36 Ga accompanied by a shift of +30‰ stemming from hydrodynamic fractionation in the upper atmosphere. That shift generates the present value of − 5‰ observed in mid-ocean ridge basalts (cf. Marty and Humbert, 1997). Their model also leads to an atmospheric δ15N of + 2.5‰ at 4.3 Ga, and further degassing of mantle N ~− 5‰ allowed this element to reach its present-day value of 0‰. A pure E-chondritic mantle hypothesis has been questioned based on various lines of evidence: (1) the chemical (Si/Mg) and isotopic (187Os/186Os) compositions of the bulk earth are not consistent with enstatite chondrites or C1 chondrites alone (Allègre et al., 1995a; Drake and Righter, 2002); and (2) modern plumes, such as the Icelandic plume, which include a deep mantle component as indicated by noble gas isotopes and Nb/Th ratios (Breddam et al., 2000; Moreira et al., 2001), do not have δ15N of −25‰; rather they are characterized by δ15N of −10.5‰ to −0.5‰ (Marty et al., 1991). More recently, Marty and Dauphas (2003) proposed a new sediment-recycling model for the isotopic composition of nitrogen in different mantle reservoirs. Nitrogen in the upper mantle, as sampled by mid-oceanic ridge basalts and diamonds having a mean δ15N of −5‰, originated from recycling of 15N-depleted Archean chert-like N into the upper mantle, whereas nitrogen in the lower mantle, as sampled by modern mantle plumes such as ocean island basalts (OIB) with δ15N of 1 to 8‰, was derived from recycling of 15N-enriched post-Archean sediments into the lower mantle. The recycling model developed by Marty and Dauphas (2003) relies heavily both on selected data for Archean cherts of Beaumont and Robert (1999), and on a sequence of assumptions: (1) that organic N in cherts is representative of recycled Archean sediment/crust; and (2) that there has been a switch from recycling of sediments into upper mantle in the Archean, but into the lower mantle post-Archean. A curious feature of their model is an unspecified primitive mantle δ15N (see Fig. 3 of Marty and Dauphas (2003)). This is in contrast to initial ratios for Sr, Nd, Hf, Pb, and Os isotopes, which are used to constrain models of mantle evolution (Dickin, 1997). Hence their model was seriously questioned by Cartigny and Ader (2003) and Kerrich and Jia (2004). Recent studies on Cr-isotopes (53Cr/52Cr) of various terrestrial and extraterrestrial geological samples show great potential for an independent method to estimate the nitrogen isotope composition of the primitive mantle. The radionuclide 53Mn is unstable, decaying to 53Cr by beta emission with a half-life of 3.7 ± 0.4 Ma (Honda and Imamura, 1971). Given that the Cr-isotope composition of the bulk Earth, E-, and C1-chondrites are known (Lugmair and Shukolyukov, 1998; Shukoolyukov and Lugmair, 1998), it is possible to conduct a mass balance for N and N-isotopes.

Consequently, the aims of this paper are: (1) to re-examine published mantle N-isotope and concentration data recorded by diamonds, mid-oceanic ridge basalts, oceanic island basalts, and mafic alkaline magmas such as lamproites; (2) to discuss the differences between them; (3) to estimate the δ15N of the primitive mantle utilizing mass balance calculations based on the Cr-isotope data; and (4) to discuss possible evolution of mantle δ15N based on mass balance calculations.

2. N-isotope characteristics of the upper mantle The nitrogen isotope composition of the upper mantle has been inferred from two sets of geological samples: mid-ocean ridge basalts (MORBs) and diamonds. Fig. 2 depicts a worldwide compilation of Nisotopic compositions of upper mantle-derived material, where initial results on fibrous diamonds in Zaire were reported by Javoy et al. (1984) and Boyd et al. (1987, 1992). The δ15N in fibrous diamonds show a very tight range of − 8.7 to − 1.7‰, with a mean of − 5‰. Eclogitic diamonds have a mean δ15N value of −5.5 ± 2.0‰ (n = 40), nearly identical to that of fibrous diamonds (Cartigny, 2005; Cartigny et al., 1998). The mean δ15N value of peridotitic diamonds is also negative (−8 ± 5.2‰; n = 42), although the range of this type of diamonds is larger than fibrous counterparts (see Fig. 2; Cartigny et al. (1997, 2009) and references therein). The diamonds are thought to grow in the upper mantle from carbonate melts reacting with continental lithospheric mantle (CLM), and are mostly Archean in age (Richardson et al., 1984; Richardson et al., 2001). The N-isotope data on mid-ocean ridge basalts having 40Ar/36Ar N1000 also show 15N-depleted values between − 8.1 and − 0.5‰, with a median of −3.4 ± 1.6‰. Samples with very high 40Ar/36Ar exhibit δ15N close to − 5‰ (Fig. 2; Marty and Humbert, 1997; Marty and Zimmermann, 1999). For some mid-ocean ridge basalt samples showing a relatively positive δ15N values, they generally associated with low 40Ar/36Ar ratios, which might reflect contamination by surface material or source heterogeneity (Cartigny and Marty, 2013). Collectively, the δ15N signature of MORB and diamonds are comparable, albeit with MORB extending to less negative values. According to Marty and Zimmermann (1999), the difference is attributed to distinct mantle sources for diamond forming and MORB magmas: diamond is thought to represent the nitrogen isotopic ratio of deeper asthenospheric mantle liquids, whereas the latter is characterized by shallow-level mantle N. However, Marty and Dauphas (2003) conclude that the MORB and diamond sample common volatile sources after correction for fractional degassing and fractionation during diamond growth respectively. Alternatively, the 15N-depleted isotopic composition of both diamond and MORB is a long-term feature of the Earth's upper mantle (Cartigny et al., 1997, 1998), and the relatively higher or positive δ15N values in some MORB samples compared with diamonds is attributed to nitrogen isotope fractionation during shallow degassing (Cartigny and Ader, 2003).

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such as δ13C and N2/Ar ratios, demonstrating that nitrogen isotopes do fractionate up to 8‰ during degassing, variations being incompatible with mixing relationships (see Fig. 9 of Cartigny et al. (2001a)). Consequently, on the basis of a δ15N value of −5‰ with 40Ar/36Ar of 40,000–44,000 for the convecting upper mantle inferred from midocean ridge basalts (Marty and Humbert, 1997; Marty and Zimmermann, 1999), and the δ15N value of the sub-continental lithosphere mantle (CLM) of −5‰ to −8‰ recorded by pristine diamond samples, there is little doubt that nitrogen in the Earth's upper mantle is depleted in 15N relative to atmospheric nitrogen, by a globally uniform δ15N value of −5 ± 2‰. There are two independent estimates of upper mantle N concentration. Mantle N content is estimated at 1 to 2 ppm inferred from covariations of N2/36Ar, 40Ar/36Ar, and 3He/4He in MORB (here computed with an upper mantle 40Ar/36Ar ratio of 44,000; Moreira et al., 1998), and assuming that nitrogen, like He and CO2, behaves as an incompatible element during melting, and that the mantle carbon content is about 400 ppm (Marty, 1995). Alternatively, mantle N content is estimated at 40 ppm based on nitrogen being regarded as not a totally incompatible element from diamond δ13C–N systematics (Cartigny et al., 2001b). According to Mohapatra and Murty (2000a, 2004), the N content of continental lithospheric mantle is 0.1 to 1.5 ppm.

Peridotitic Diamonds

Eclogitic Diamonds

Fibrous Diamonds

3. N-isotopic composition of mantle plumes

Mid-Ocean Ridge Basalts

Mantle plume-related lavas

Deep mantle-related fluids

-30

-20

-10

δ15N

0

10

(‰)

Fig. 2. δ15N histogram of mantle derived diamonds including perodotitic and eclogitic diamonds (Cartigny et al., 1997, 1998) and fibrous diamonds (Boyd et al., 1987, 1992; Javoy et al., 1984); mid-ocean ridge basalts with 40Ar/36Ar N1000 (Marty and Humbert, 1997; Marty and Zimmermann, 1999); mantle plume-related lavas and carbonatites with 40 Ar/36Ar N1000 (Marty and Dauphas, 2003); and deep mantle-related fluids (Marty et al., 1991).

In their papers on MORB, Marty and Zimmermann (1999) discount fractionation of nitrogen isotopes during degassing. However, degassing fractionation cannot be neglected because it would otherwise lead to N-isotopic variability being assigned to source heterogeneity (Cartigny and Ader, 2003; Exley et al., 1987). In MORB vesicles, δ15N values are strongly correlated with other indices of degassing,

The nitrogen isotopic character of the deep mantle from mantle plumes, as sampled by ocean island basalts (OIB), has been studied by Dauphas and Marty (1999) and Marty and Dauphas (2003). Their results show generally positive δ15N values of −0.2 to 8.0‰ (mean of 2.8‰). Carbonatites from the Kola region, Russia, span 0.7 to 6.0‰ with a mean of 3.2‰ (Fig. 2). They interpreted the range of δ15N data in terms of an oxidized environment after the Archean, leading to recycling of 15N-enriched organic N stemming from kinetic isotope effects associated with denitrification (~ 7‰) into the deep mantle, and reflecting the deep mantle character. Noble gas isotopic compositions of oceanic island basalts, such as from Iceland and Hawaii, have been used to infer the existence of an undegassed terrestrial mantle reservoir, most likely from the deep mantle (Allègre et al., 1983, 1986; Breddam et al., 2000; Honda et al., 1991; Moreira et al., 2001). Most basalts undergo some degassing at or near the Earth's surface, loosing some mantle gases to the atmosphere with commensurate isotopic fractionation in basalts relative to the mantle source (Moreira et al. (2001) and references therein). Recent studies on the evolution of δ15N during degassing in OIB magmas show that a shift of up to + 3‰ can be produced from a source having δ15N of −5‰ (see Cartigny and Ader (2003) for a review). Positive correlations of δ15N with 3He/4He ratios are present in both studies of Marty and coworkers. For example, seventeen analyses of δ15N and R/Ra (where R is the 3He/4He ratio and Ra is the atmospheric ratio of 1.38 × 10− 6; Allègre et al., 1995b) from mantle plume lavas by Marty and Dauphas (2003) are plotted in Fig. 3A, which show a correlation coefficient (R) of 0.71. Seven samples from Kola carbonatites have a correlation coefficient between δ15N and R/Ra of 0.88 (Fig. 3B; Dauphas and Marty, 1999). This feature from both suites may indicate that the more positive nitrogen isotopic compositions of OIB reflect degassing fractionation trends. Mohapatra and Murty (2000b), and Mohapatra et al. (2002) report noble gas and nitrogen data in oceanic basalts collected from Hawaii and Reunion. Using stepped pyrolysis (temperature) techniques, they obtained 15N-depleted nitrogen of 0 to −15‰ from low temperature steps (600°–800°C), but 15N-enriched components of 0 to 13‰ at high temperature (900°–1500°C), accompanied with 40Ar/36Ar ratios from 300 at low temperatures to 12,638 at high temperatures. These characteristics of OIB were interpreted as a three-component mix of a deep

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Fig. 3. Relationships between δ15N and R/Ra (where R is the 3He/4He ratio and Ra is the atmospheric ratio of 1.38 × 10−6) of plume-related lavas (A) (all data from Marty and Dauphas (2003)) and carbonatites (B) from the Kolar region, Russia, which are also called mantle plume related rocks by Marty et al. (1998). Nitrogen isotope data from Dauphas and Marty (1999) and helium isotope data from Marty et al. (1998).

mantle component, with recycled sediments, and atmospheric contamination. Sano et al. (2001) also modelled the range of δ15N in OIB and the other arc basalts as some combination of mantle N, recycled sedimentary nitrogen having δ15N of 2 to 10‰ with a mean of 7‰, and atmospheric N defined to be 0‰ (Bebout, 1995; Bebout and Fogel, 1992; Peters et al., 1978; Rau et al., 1987; for a compilation of crustal δ15N see Jia and Kerrich (2004)). The δ15N values of plume-related lavas (OIB) reported by Marty and Dauphas (2003) range from close to atmospheric value of 0‰ to a sedimentary value of ~7‰. These results, in conjunction with both N2/36Ar and 40Ar/36Ar showing higher values than air, but much lower than the mantle, demonstrate that nitrogen in OIB is some combination of deep mantle N, recycled sediment, and atmospheric contamination either during subduction and/or from groundwaters during emplacement, with superimposed degassing fractionation in shallow lavas (see Table 1). Continental lithospheric mantle (CLM), as sampled by xenoliths, also records three N components. Deep mantle of ~−9‰ and recycled sediments of ~15‰, both of which possess MORB-like Ar and Xe-isotopes, as well as atmospheric N either from subducted N2 and/or air saturated water (ASW) groundwaters during emplacement (Mohapatra and Murty, 2000b). The proportion of nitrogen derived from the three endmembers for the OIB samples of Marty and Dauphas (2003) can be estimated utilizing mass balance calculations following equations of Sano et al. (1998, 2001):         15 15 15 15 δ N ¼ fM δ N þ fA δ N þ fS δ N o

M

A

ð1Þ

S

        36 36 36 36 þ 1= N2 = Ar þ 1= N2 = Ar ð2Þ 1= N2 = Ar ¼ 1= N2 = Ar o

fM þ fA þ fS ¼ 1

M

A

S

ð3Þ

where subscripts o = observed, M = mantle-derived, A = air-derived, S = sediment-derived nitrogen; fM, fA, and fS are the respective fractional contributions of mantle-, air-, and sediment-derived nitrogen. In the above scheme, endmember compositions are generally well constrained: The mantle and sedimentary endmembers both have N2/36Ar ratios of 6 × 106 (air is 1.8 × 104) but their δ15N values are distinct. The mantle has a δ15N of − 5‰ (Cartigny et al., 1997, 1998; Marty and Humbert, 1997; Sano et al., 1998) whereas sedimentary nitrogen is assumed to be 7‰ (Bebout, 1995; Peters et al., 1978) and air has δ15N = 0‰ (see Table 1). The wide difference in δ15N between the potential end-members makes this approach a sensitive tracer of N2 provenance. In this calculation, the elemental fractionation of N2/36Ar, which may occur during magma degassing, is not taken into account, given that the N2/36Ar ratio is expected to remain constant during gas phase separation (Marty, 1995; Sano et al., 2001). Based on these assumptions, contributions of atmospheric and sedimentary nitrogen to plume-related lava samples range from 2.4% to 72%, and 0.6% to 45%, respectively. These results are also compatible with earlier studies of Sano et al. (2001) (see Table 1). Consequently, like arc-related basalts, variations in δ15N of mantle plumes are a common feature (c.f., Farley and Neroda, 1998; Fischer et al. 2002). In an early study on the gas geochemistry of geothermal fluids in the Hengill area (Iceland), Marty et al. (1991) observed uniform helium isotope ratios (R/Ra) of 14.4, confirming a deep mantle origin of the gases (Moreira et al., 2001). δ15N values vary from −0.2‰ to −10.5‰, and do not show any covariations with 3He/4He (Figs. 2 and 4). These may be rare samples where degassing fractionation is minimal. 15 N-enriched values of 1.5 to 6.2‰ in ~100 Ma ultramafic lamproites, eastern India, have also been found by Jia et al. (2003). They interpreted the lamproites as derived from subduction–erosion of continental crust or subducted sediments, hybridisation of their low degree partial melts with mantle lithosphere, and incubation prior to decompressional melting. Consequently, the enrichment is considered a crustal, not mantle, signature. In summary, mantle derived MORB and diamonds may all have had a similar and uniform primary nitrogen isotope composition of about − 5‰ since the Archean eon. Basalts from MOR show variations of ~ 3‰ due to shallow degassing fractionation, whereas diamonds that form at N 150 km do not. Large variations in δ15N are an intrinsic feature of mantle-plumes, in common with variability observed in arc-related basalts and volcanic gases in island arcs. Variations stem from some combination of three components: deep mantle N, recycled sedimentary N, and atmospheric contamination, as well as degassing fractionation. 4. N-isotope characteristics of the primitive mantle The primitive bulk mantle δ15N can be constrained via chondritic meteorites. Recent Cr-isotope studies on various terrestrial minerals and rocks, and chondritic meteorites, are consistent with the solid Earth being a mixture of enstatite with carbonaceous chondrites. The bulk silicate earth has a 53Cr/52Cr ratio in ε units (1ε unit = one part in 104) defined to be 0ε, intermediate between mean enstatite chondrites (εEC) of +0.17ε and carbonaceous chondrites (εC1) at − 0.43ε (Lugmair and Shukolyukov, 1998; Shukoolyukov and Lugmair, 1998). Consequently, the bulk earth (δ15Nearth) may be independently constrained as follows: 15

15

15

δ Nmantle ¼ δ NEC  MEC þ δ NCI  MCI

ð4Þ

εEC  MEC þ eCI  MCI ¼ 0

ð5Þ

MEH þ MCI ¼ 1

ð6Þ

where δ15NEC and δ15NC1 represent the median values for the 15Ndepleted E-chondrites of −25‰ and 15N-enriched C1-chondrites of +39‰, respectively (Fig. 1; Kung and Clayton, 1978; Javoy and Pineau,

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135

Table 1 Calculated contributions of atmospheric and sedimentary N on mantle plume-related lavas (data are from Marty and Dauphas (2003)). Location

δ15Ν

3

Sample

(‰)

R/Ra

Teahitia, Society SO47-5DS SO47-9DS TH09-02 TH09-05 TH14-02 TH14-03-4 TH14-05

3.89 0.58 1.38 1.35 −0.23 1.46 4.05

Mehetia, Society TH10-04 Cyana, Society TH25-03-4 Seamount no.1, Society DTH02-01-1 DTH02-02-1 Seamount no.2, Society DTH03-02 MacDonald, Cook-Austral TH30-03 Loihi, Hawaii (Sano et al., 2001) KH85-4D17-DE51 KH85-4D17-DE55-1 KH85-4D17-DE55-3 Component ⁎ Air & air-saturated water Mantle Sediments

He/4He

6.92 6.37 7.69 7.52 6.73 6.27 6.5

N2/36Ar

40

Ar/36Ar

(×104) 12 2.7 2.5 2.9 4.6 86 23

905 329 348 299 490 9995 2597

N component Air

Mantle

Sediment

0.15 0.67 0.72 0.59 0.39 0.02 0.08

0.68 0.15 0.232 0.283 0.235 0.528 0.723

0.174 0.188 0.049 0.126 0.376 0.448 0.202

0.45

9.24

5.2

660

0.33

0.315

0.351

3.58

1.56

32

5175

0.05

0.693

0.254

6.71 3.38

3.44 1.26

53 144

4786 7202

0.03 0.01

0.963 0.695

0.006 0.296

1.33

8.31

4.1

360

0.44

0.345

0.218

1.82

5.62

2.5

291

0.64

0.301

0.057

0.721 0.330 0.602

0.196 0.358 0.200

0.08 0.31 0.2

1 0 0

0 1 0

0 0 1

-0.4 0.4 0.4

0 −5 7

23 24.5 24.5

1 8.2/N14.3 0.02

2.49 5.42 2.98

1.8 600 600

360 669 369

296 44,000 N300

⁎ Data source: Mantle δ15N of −5‰ recorded by diamonds and MORB and our estimate in this study, A mean δ15N of 7‰ for sediment is from Peters et al. (1978), Rau et al. (1987), Bebout and Fogel (1992) and Bebout (1995); R/Ra of 8.2 (MORB) from Allègre et al. (1995a); and R/Ra N14.3 (mantle plumes) from Moreira et al. (1998). Other values are adopted from Sano et al. (2001).

1983; Kerridge, 1985); MEC and MC1 represent the mass of E-chondrites (72%) and C-chondrites (28%) calculated from their respective Crisotope compositions. A simplifying assumption is that after accretion, impact, and degassing C- and E-chondrites had comparable N contents. The Si/Mg and Al/Mg ratios, and initial Os-isotope ratio of the silicate Earth are greater than for E- and C1-chondrites (Drake and Righter, 2002). However, major elements and isotopic systems may become decoupled. Drake and Righter (2002) state “…prima facie evidence for the existence of Earth-building materials sharing some properties in common with various extant meteorites, although no extant meteorites share all of the properties of Earth material”. Nitrogen isotope data on ordinary chondritis scatter between −10‰ and 20‰ and so cannot be used in a mass balance (Kung and clayton, 1978).

Under these assumptions, the calculated bulk earth had a primitive nitrogen isotopic composition of − 7 ± 3‰. This value overlaps with, but is at the lower bound of δ15N values recorded in mid-ocean ridge basalts and most diamonds of − 5 ± 2‰ from a global compilation of diamonds and MORB (Boyd et al., 1987, 1992; Cartigny et al., 1997, 1998; Javoy et al., 1984; Marty and Humbert, 1997; Marty and Zimmermann, 1999; Van-Heeerden et al., 1995). Some enstatite chondrite-like δ15N values down to −24‰ measured in very rare diamonds (Cartigny et al., 1997) could be interpreted to be a result of a heterogeneous mantle, in keeping with the mantle “reservoir jumping” model of Albarade (2001). Consequently, a 15N-depleted nitrogen isotopic composition is an intrinsic attribute of the Earth's mantle. An independent estimate of primitive mantle δ15N can be made from Mesoarchean cherts. Isley and Abbott (1999) and Condie et al. (2001) have shown that komatiite–basalt sequences, thought to be erupted from plumes originating in the deep mantle, have common time series with chert-BIF. The latter are hydrothermal sediments generated from the large volumes of magma erupted into ocean basins. The most depleted values for kerogen in 3.8 Ga 3.4 Ga cherts are − 7‰ and − 6‰ respectively (Beaumont and Robert, 1999; D.L. Pinti et al., 2001). A mantle signature in the 3.4 Ga cherts from Marble Bar, Pilbara craton, is confirmed by Xe-isotopes (D.Z. Pinti et al., 2001).

5. Secular evolution of the mantle δ15N

Fig. 4. Correlations of δ15N versus R/Ra of mid-oceanic ridge basalts with 40Ar/36Ar N1000 (A) and geothermal fluids derived from deep mantle (B). Data for mid-oceanic ridge basalts are from Marty and Zimmermann (1999) and data of deep mantle fluids from Marty et al. (1991).

If the primitive mantle had a δ15N of −7‰ there has been a shift of ~2‰ to the value of −5‰ for MORB and most diamonds. This possible shift, within the limitation of errors in estimating − 7‰, would have occurred between ~4.5 and 3.2 Ga, because the latter age is preserved in diamonds (Richardson et al., 1984; Richardson et al., 2001). The shift can be evaluated on the basis of recent studies of nitrogen flux

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rates, both of input from recycled subducted oceanic sediments and crust and outputs from arcs (island arcs and back arcs) and the midocean ridge system. We use the following input parameters and assumptions: (1) In the Archean (4.5 to 3.0 Ga) the oceanic lithosphere-recycling rate would be ten times the current value. The present nitrogen input fluxes of subducted sediment and oceanic crust, and outputs from upper mantle through arcs, MOR, and from lower mantle through hot spots are presented in Table 2. The flux from plumes is 3 to 4 order of magnitude lower than arcs and MOR (see Table 2), so this is a negligible contribution in these calculations. (2) Oceanic sediments are composed of 0.1 wt.% chemical/biochemical sediments (Archean cherts) with a mean δ15N of −0.1‰ and 28 ppm N content inferred from the data of Beaumont and Robert (1999), and 99.9% continentally derived clastic sediments from upper continental crust represented by Archean S-type granite averaging 3.2‰ and bulk rock N content of 4 ppm. The latter estimates are based on an average N content of 20 ppm in analysed K-feldspar and muscovites, coupled with 20% modal K-silicates (Jia and Kerrich, 2000). Consequently, Archean oceanic sediments would have been ~3‰. (3) Archean (N3.0 Ga) oceanic crust may have had a similar δ15N to the primitive mantle δ15N of − 7‰ as estimated in this study. Sparse data for 2.7 Ga tonalitic rocks in the Uchi subprovince, Superior Province, span −5.3 to +5.2‰, averaging −0.9‰ (Jia and Kerrich, 1999, 2000). The Archean tonalite–trondhjemite– granodiorite (TTG) suite is considered to form by melting of basaltic oceanic crust, either underplated or on a subducting slab (Drummond and Defant, 1990; Martin, 1999; Smithies, 2000). Archean (N3.0 Ga) oceanic crust may have had a similar δ15N to contemporaneous basalts of − 7‰ as estimated in this study. (4) From studies of compositional and isotopic gradients in the Catalina subduction complex, metasedimentary rocks in the European Variscan Belt, and the Cooma metasedimentary complex, we assume a + 3‰ shift of oceanic sediments and crust due to metamorphic devolatilization during subduction, and decrease of N content of ~ 17% (Bebout and Fogel, 1992; Jia, 2004; Mingram and Bräuer, 2001). The lower mantle is also assumed to remain unchanged (Moreira et al., 2001).

Table 2 Compilation of present N fluxes at arcs, mid-oceanic ridges, and subduction zones globally.

N input rates at subduction zones N from subducted sediment N from subducted oceanic crust N output rates at arcs* N from subducted sediment through arcs N from upper mantle through arcs N output rates at mid-oceanic ridges N output from upper mantle through mid-ocean ridges N output rates at hot spots N output from lower mantle through hot spots

Unit (mol/year)

Source(s)

1.37 × 1010 3.11 × 1010

Hilton et al. (2002) Hilton et al. (2002)

1.15 × 1010

Fischer et al. (2002)

0.85 × 1010

Fischer et al. (2002)

2.2 × 109

Marty (1995)

4.1 × 106

Sano et al. (2001)

Note: The total N output rate is 2.0 × 1010 at arcs where it includes 57.5% from subducted sediment and 42.5% from upper mantle. The proportions were from Sano et al. (2001).

may be a consequence of nitrogen isotopic fractionation during degassing (i.e., Cartigny and Ader, 2003). The 15N-depleted character of the deep mantle is a primitive feature and has been directly inferred by hot spot geothermal fluids, showing negative δ15N values (Marty et al., 1991). The primitive bulk mantle having a δ15N of about −7‰ estimated in this study rules out a pure enstatite chondrite model for the primordial earth's composition, consistent with early studies by Allègre et al. (1995a) and Drake and Righter (2002). Secular evolution of the upper mantle δ15N from an initial value of −7‰ to −5‰ by ~3.2 Ga was controlled by a combination of recycling of 15N enriched (~ 7‰) oceanic crust and sediments through subduction, and degassing of magmas from the upper mantle. Acknowledgements

Under these assumptions, a −7‰ initial upper mantle would shift to −4.2‰ by 3.0 Ga, consistent with the observed range for diamonds and mid-oceanic ridge basalts.

This research was supported by a CSIRO Postdoctoral fellowship to Y. Jia, and an NSERC research grant to R. Kerrich. Y. Jia also acknowledges honorary position at Monash University and R. Kerrich acknowledges the George McLeod endowment to the Department of Geological Sciences, University of Saskatchewan. The manuscript benefited from the reviews and discussions on an earlier version by M. Hughes and Z. Gao and they are gratefully acknowledged. We thank P. Cawood for the editorial handling as well as B. Dhuime and an anonymous reviewer for their detailed and extremely useful comments on the manuscript. The authors are also grateful to Lingsa Jia for drafting some of the diagrams.

6. Conclusions and implications

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The Earth's upper mantle is depleted with a mean δ N of −5‰, as identified from Archean and recent diamonds and recent mid-oceanic ridge basalts. The slight enrichment in 15N for MORB compared with diamonds is attributed to N-isotopic fractionation during magma degassing, as indicated by strong correlations between both δ15N and δ13C, and δ15N and N2/Ar ratios in MORB vesicles (Cartigny et al., 2001a, 2001b). Accordingly, the negative δ15N value of −5‰ is a longterm feature of the Earth's upper mantle. The δ15N values of about 3‰ inferred from most mantle plume samples by Marty and co-workers do not represent the primary nitrogen isotope character of lower mantle (Cartigny and Ader, 2003; Mohapatra and Murty, 2000a; Mohapatra et al., 2002). Rather, these values may reflect a mixing of mantle-derived 15N-depleted nitrogen and recycled sediment derived nitrogen, with some atmospheric contamination, as evidenced by correlated δ15N values and 3He/4He ratios, and mixing mass balance calculations. In addition, more positive values

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