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Lunar nitrogen: indigenous signature and cosmic-ray production rate
Earth and Planetary Science Letters 184 (2001) 659^669 www.elsevier.com/locate/epsl
Lunar nitrogen: indigenous signature and cosmic-ray production rate K.J. Mathew *, K. Marti Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0317, USA Received 26 June 2000; received in revised form 23 October 2000; accepted 25 October 2000
Abstract Indigenous lunar nitrogen composition and abundances have been determined in old ferroan anorthosite 60025 and in anorthositic breccia 67915 from North Ray Crater, as well as in 55 cm deep volcanic glasses of 74001 double-drive core from Shorty Crater. Also included in the set is the well-documented lunar basalt 75075, collected near the Camelot Crater. Indigenous lunar N abundances are low (at ppm level), but there is some variation between glass-rich cores, mare basalts and anorthosites. The uniform indigenous N isotopic signature of N15 N = +13.0 þ 1.2x, is consistent with data reported previously for Shorty Crater samples. The indigenous N15 N cannot account for the light nitrogen component, observed in the lunar regolith samples. We have determined cosmic-ray production rates P(15 N) for the above rocks and the drill core samples. The average production rate estimate (for low shielding) of P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma is V60% higher than published lunar 15 N production rates, but consistent with the meteoritic production rate derived from silicates in the Enon meteorite, when normalized to 2Z-irradiation geometry. From the observed cosmogenic 15 N excesses and the reported cosmogenic 21 Ne abundances in core 74001 we derive a (15 NC /21 NeC ) production rate ratio of 4.0 þ 0.3 for silicates. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: lunar samples; nitrogen; spallation nitrogen; regolith; Moon; volatiles
1. Introduction Nitrogen in ancient lunar rocks that formed at di¡erent times and depths within the Moon could trace the nitrogen isotopic evolution of the Moon as well as constrain its initial signature. The earliest crustal rocks on the Moon presumably
* Corresponding author. Tel.: +1-858-534-0443; Fax: +1-858-534-7441; E-mail: [email protected]
sampled nitrogen of the period of initial lunar di¡erentiation and might also have preserved signatures of an early lunar atmosphere. Mare basalts have crystallization ages similar to eruption times of volcanic glasses at the Apollo 17 site, but the lunar volcanic glasses originated at greater depths than the basalts. Hence the nitrogen isotopic signatures for di¡erent depths within the lunar mantle can be inferred through a comparative study of lunar basalts and volcanic glasses. The lunar indigenous nitrogen isotopic composition may also be useful as a tracer in models of the Earth^Moon system. Lunar igneous rocks are reported to contain
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less than 1 ppm N [1^3] and, therefore, their isotopic signatures are a¡ected by spallation components. Previous attempts to determine the indigenous lunar N abundances and signatures from lunar rocks were largely unsuccessful. Lunar soils, in contrast, contain about two orders of magnitude higher abundances of nitrogen and other volatiles due to accretion of solar wind and possibly also volatiles from other sources. Lunar soils are not suitable for studies of indigenous nitrogen signatures. Barraclough and Marti [4] carried out a search for Moon's indigenous nitrogen based on analyses of vesicular lunar glasses. The nitrogen to noble gas elemental ratios in these samples was much higher than solar values, and it was suggested that these samples contained indigenous N components, but also spallation 15 NC that precluded the identi¢cation of the indigenous signatures. Murty and Goswami [5] reported N in a clast of lunar meteorite MAC88105 and derived a N15 N of 17 þ 3.4x for indigenous lunar N. Another study regarding the signature of the indigenous nitrogen was reported by Kerridge et al. [6] based on 74001/2 double-drive core samples. These authors measured N apparently trapped on the surfaces of fumarolic glass particles, and a N15 N of V14x was derived for the indigenous lunar nitrogen. We selected suitable rocks to calibrate the 15 N production rate P(15 N) by cosmic rays, as this requires rocks with well-determined cosmic-ray exposure histories and shielding conditions. At this time there exists only one determination, obtained from basalt 12021 [2]. 2. Experimental techniques 2.1. Sample selection In the magma ocean model of the early lunar di¡erentiation, ferroan anorthosites are among the ¢rst crustal rocks to form. The Sm^Nd age of 4.44 Ga makes 60025 the oldest dated crustal anorthosite on the Moon [7]. Most ferroan anorthosites show evidence for post-crystallization disturbance, but 60025 is one of the least metamorphosed. Furthermore, all mineral components of
the polymict breccia 60025 appear to be from the same igneous event. The corrections for a cosmogenic component are expected to be very small in the case of 60025 due to its low surface exposure age of only 2.0 Ma (South Ray Crater). Lunar anorthosite 67915 is a fragmental breccia chipped o¡ a 2 m boulder resting on the rim of 50 Ma old North Ray Crater. A 39 Ar^40 Ar age of V4.1 Ga and a Pu^Xe retention age of 4.3 Ga [8] suggest a somewhat younger age than that of 60025, and it was anticipated that this anorthosite may have sampled indigenous lunar nitrogen characteristic of the later stages of the magma ocean phase of lunar evolution. Breccia 67915 contains more ma¢c inclusions than 60025; those visible by optical inspection of a single chip were removed prior to analyses. The black and orange glasses present in 74001/2 represent pyroclastic deposits, formed by the rapid quenching of molten silicate droplets that were ejected into the lunar atmosphere during the explosive volcanic eruption of ultrama¢c magmas onto the lunar surface along the rim of mare Serenitatis [9]. The source regions of these magmas may have been at depths as great as several hundred kilometers. Chemically these glasses are similar to the mare basalts at the Apollo 17 site. The surfaces of the individual glass particles are coated with volatile elements such as Zn, Pb, and halogens [9^11], suggesting that the glasses have sampled volatiles from the lunar interior (mantle) released during the lava fountaining. It appeared reasonable to expect that the lunar interior nitrogen was enriched in these double-drive tube glasses. The 74001/2 cores do not show agglutinates [12], except for the top 5 cm layer (of the drill core), indicating no surface exposure of the deeper layers, consistent with the exposure history model of Eugster et al. [13]. Formation ages of 3.7 Ga have been obtained for the black and orange glasses of 74001/2 by 39 Ar^40 Ar technique [14]. A short recent cosmic-ray exposure age of 17.2 þ 1.4 Ma was obtained for 74001/2 by 81 Kr^Kr [13]. This probably represents the time when Shorty Crater was excavated and when the glass was emplaced on the crater rim. Lunar basalt 75075 is a medium-grained surface rock picked from the top of a boulder at the rim
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Table 1 N concentrations and isotopic signatures in lunar samples 60025, 74001, 67915 and 75075 Sample
Temp. (³C)
N (ppm)
N15 N (x)
Sample
Temp. (³C)
N (ppm)
N15 N (x)
60025-A
350C 700 900 1100 1400 1550 Total 500C 700 800C 900 1000 1100 1250 1550
0.03 0.08 0.11 1.50 0.12 0.03 1.87 0.04 0.06 0.01 0.23 0.04 0.66 0.06 0.09 1.19
6.00 þ 2.3 0.90 þ 1.6 11.63 þ 1.3 12.83 þ 0.9 30.52 þ 1.4 39.40 þ 2.5 13.70 þ 1.0 8.00 þ 2.3 30.80 þ 1.9 10.29 þ 3.2 11.95 þ 0.9 12.61 þ 1.5 12.80 þ 0.9 26.81 þ 0.9 37.10 þ 1.7 14.31 þ 1.1
74001,434
350C 700 1000 1100 1250 1600 Total 350C 600 800 1000 1100 1400 1600 Total
0.06 0.76 0.76 1.65 0.91 0.32 4.46 0.04 0.55 0.38 0.61 0.73 0.80 0.08 3.19
14.00 þ 4.0 8.87 þ 1.0 13.04 þ 1.0 14.10 þ 0.7 54.20 þ 1.0 71.20 þ 2.0 25.31 þ 1.0 17.00 þ 3.0 7.61 þ 1.2 13.24 þ 1.6 13.61 þ 0.8 15.63 þ 1.0 75.16 þ 1.7 58.10 þ 2.7 29.59 þ 1.3
350C 700 1000 1100 1200 1550 1600 Total
0.06 0.09 0.20 0.36 0.15 0.35 0.05 1.26
5.11 þ 1.2 2.83 þ 1.2 13.05 þ 0.8 21.62 þ 0.7 58.84 þ 1.2 107.50 þ 1.1 86.20 þ 3.0 48.98 þ 1.0
400C 700 900 1000 1100 1300 1450 1550 1600 Total
0.021 0.054 0.052 0.031 0.012 0.335 0.087 0.079 0.032 0.703
2.1 þ 2.0 12.9 þ 1.1 13.0 þ 1.1 13.3 þ 0.9 16.5 þ 1.0 320.2 þ 0.9 392.1 þ 1.2 354.5 þ 1.8 321.8 þ 2.1 258.6 þ 1.2
57.08 mg
60025-B 196.83 mg
67915,233 47.44 mg
3.66 mg
74001,2203 4.45 mg
75075 621.9 mg
C denotes a combustion step in O2 . Kerridge et al. [6] reported N data for number of 74001/2 samples. N data (sum of all temperature steps up to 1050³C) reported for samples from the same layer (same cosmic-ray exposure history) as 74001,434 and 74001,2203 are listed here for comparison. N = 0.56 ppm, N15 N = 46 þ 12.8x (32.5 to 33.5 cm); N = 9.11 ppm, N15 N = 12.6 þ 2.6x (55.5 to 56.5 cm); N = 110 ppm, N15 N = 13.2 þ 1.3x (55.5 to 56.5 cm); N = 371 ppm, N15 N = 13.2 þ 1.7x (55.5 to 56.5 cm); N = 2.82 ppm, N15 N = 19 þ 9.6x (56 to 56.5 cm); and N = 2.33 ppm, N15 N = 20 þ 5.1x (56 to 56.5 cm) (depths in the core from which the samples were taken are indicated).
of the Camelot Crater, from which it was presumably ejected. The basalt crystallized 3.70 þ 0.07 Ga ago, and was ejected in a crater forming event 143 þ 5 Ma ago [15]. Most lunar basalts exhibit complex exposure histories, but basalt 75075 was selected on the basis of a single stage exposure near the lunar surface [15]. This history was inferred from the cosmogenic 131 Xe/126 Xe ratios and the thermal neutron £uences, calculated from measured 150 Sm/149 Sm ratios. Thus the cosmic-ray production rate, P(15 N), obtained from 75075 represents an average surface production rate. A direct comparison of the P(15 N) with those derived by Becker et al. [2] from lunar basalt 12021 is possible.
2.2. Mass spectrometry Chips of the anorthosites 60025-A and 60025-B and basalt 75075 were loaded into the gas extraction and analyses system without wrapping, whereas the 74001,434 and 74001,2203 glasses and the anorthosite 67915 were wrapped in predegassed Au foils. Details of the experimental techniques are described elsewhere [16]. The samples were step heated by an external resistance heater up to 1000³C in a double walled quartz system with a separately pumped vacuum jacket, and then transferred in vacuo into a Mo crucible mounted in a double walled quartz system for radio frequency (RF) heating and melting. In all
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cases low temperature combustion steps in pure O2 , released from puri¢ed CuO, were carried out to remove adsorbed terrestrial N and other surface contamination. N was analyzed in the static mode with a magnetic sector mass spectrometer equipped with a Baur^Signer source. Blank measurements were carried out between the runs, and increased with extraction temperature. N2 blanks were 6 0.3 ng in the 9 1400³C steps and s 2 ng in the 1550³C and 1600³C extractions. Sensitivity and instrumental mass discrimination were monitored by pipettes of air standards and were found to be constant (within 5%) during the set of runs. For N2 , the reliability of the CO corrections at mass 30, based on 12 C18 O signal, was tested on di¡erent sized gas splits from the same extraction. Data presented are corrected for system blanks, N2 recovery (typically V80% for Mo crucible; loss of the sample N to the Mo crucible was monitored by exposing N standard pipettes to the crucible), spectrometer background, and instrumental mass discrimination. Listed errors include the statistical uncertainties in the measurement and variation in the blank and background correction. Uncertainties of isotopic ratios represent 95% con¢dence levels. 3. Indigenous lunar nitrogen : abundances and signatures As mentioned previously, the ferroan anorthosites formed as part of the initial lunar di¡erentiation, presumably as £oating crust above a magma ocean. Thus the indigenous nitrogen of ferroan anorthosite 60025, with a crystallization age of 4.44 Ga [7], may represent the N15 N of indigenous lunar nitrogen at the time of the initial lunar di¡erentiation. Nitrogen data of the two 60025 samples are consistent with each other. The 60025-A (57 mg) was the pilot sample to study the release, permitting optimization of temperature steps used for pyrolysis of 60025-B (197 mg). The release systematics of both the samples are strikingly similar (Fig. 1). The ¢rst combustion step released 6 4% N with N15 N V8x. The 700³C release is isotopi-
Fig. 1. Release systematics of N from lunar anorthosite 60025. 60025-A (57.1 mg) was used as a pilot sample to determine the temperature steps for the larger 60025-B (196.8 mg). The numbers close to data bars indicate the extraction temperature in 100³C. The combustion steps at 350³C for 60025-A, and at 500³C and 800³C for 60025-B (at 5 Torr O2 ) was carried out to remove adsorbed terrestrial N and other contaminants. The plateau release between 700³C and 1100³C represents the indigenous N signatures and the shifts in the s 1100³C releases are due to a spallation 15 N component.
cally lighter and close to terrestrial. The s 700³C and 9 1100³C release de¢ne a well-de¢ned plateau with isotopic composition of +13x and accounts for V80% of the total N in both the samples. This uniform N release presumably represents the signature of indigenous lunar N in 60025. The s 1100³C steps have higher N15 N, due to the presence of a spallation N component. In 74001 core samples, the 350³C combustion step released 6 2% of the total N with a signature of V17x (Fig. 2). The s 700³C and 9 1100³C pyrolysis steps release s 50% of the N in these
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to 1050³C, and that the extraction of spallationfree indigenous N is almost complete at extraction temperatures 6 1100³C. Note that the 74001/2 double-drive core samples analyzed by Kerridge et al. [6] were separated in heavy liquids and washed in acetone and methanol and the magnitude of a possible contamination during the chemical processing was di¤cult to assess. In the Kerridge et al. [6] experiments also, as observed here, the nitrogen released at low temperatures had heavy N15 N values. Fig. 3 shows the N release systematics of anorthosite 67915 and of lunar basalt 75075. The s 1100³C steps of both 67915 and 75075 show shifts due to spallation derived 15 N excesses. In
Fig. 2. Release systematics of N from volcanic glasses 74001,2203 (4.45 mg) and 74001,434 (3.66 mg). The numbers close to data bars indicate the extraction temperature in 100³C. An initial combustion step at 350³C and at 5 Torr O2 was carried out to remove contamination. Both samples are fresh untreated samples from a depth of 55 cm in the double-drive core 74001/2 and received a combined exposure of 33 Ma in the two-stage exposure model of Eugster et al. [13]. N in the s 1100³C releases is heavy, indicating a spallation N component.
samples with a uniform signature of 13x. The 700³C extraction of 74001,434 and the 600³C extraction of 74001,2203 released slightly lighter N with N15 N V8x. N15 N data of the s 1100³C steps are shifted to heavier N, signaling the release of spallation 15 N. The N release systematics and isotopic signatures observed here are in excellent agreement with data reported by Kerridge et al. [6]. Table 1 shows that for 74001 volcanic glasses the measured N abundances are within the range reported by Kerridge et al. [6]. Note that N data reported by Kerridge et al. [6] represent extractions only up
Fig. 3. N release systematics of lunar anorthosite 67915 (47.4 mg) and basalt 75075 (621.9 mg). The numbers close to data bars indicate the extraction temperature in 100³C. For 75075, four pyrolysis steps (700³C, 900³C, 1000³C and 1100³C) de¢ne a uniform signature of +13x, that contrasts the s 1100³C steps with N15 N s 320x, and represent the indigenous N signature.
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75075, N released in the temperature range of 400³C to 1000³C is apparently not a¡ected by a spallation component. The 1100³C is only marginally a¡ected by spallation component. The N isotopic compositions in these steps are consistent with the N15 N = +13x identical to that observed in volcanic glass and in anorthosites. Unfortunately in anorthosite 67915, the 700^1000³C temperature steps do not permit an identi¢cation of an intermediate plateau. However, the 1000³C step is consistent with a N15 N = 13x value. The 1100³C step is already a¡ected by the spallation component. The release systematics of N in all lunar samples, with the exception of basalt 75075, show a characteristic pattern that is interesting. The lowest combustion step, intended as a cleaning step, shows heavy N15 N. This heavy N release is followed by release of N that is somewhat lighter, close to terrestrial in the case of anorthosite 60025 and breccia 67915 and V8x for the volcanic glass 74001. We should also point out that there is excellent reproducibility in the release patterns as well as the N15 N in replicate runs of both 60025 and 74001. The N15 N = +13x signature appears ¢rmly established for the selected samples and this indicates the existence of a uniform indigenous nitrogen component on the Moon. Kim et al. [17] studied the nitrogen isotopic signature of the recent solar wind in surface samples of rock 68815 and soil 67601. We note that in their study the temperature steps that were not a¡ected by either solar wind or spallation components, released nitrogen with N15 N = V+13x, consistent with the indigenous lunar nitrogen signatures obtained here. We note that the isotopic composition of indigenous N is in agreement with approximate values suggested in the literature [2,5,6]. An average N15 N = 13.0 þ 1.2x is obtained from nitrogen released in the steps s 700³C to 6 1100³C (plateau release). We consider this signature to represent the indigenous lunar nitrogen. Further, we make the assumption that in the absence of a spallation N component, the s 1100³C steps reveal the same lunar N15 N = 13x component. Thus, when considering indigenous N abundances, all temperature steps s 700³C need to be
taken into consideration. We obtain the following indigenous N abundances: 1.4 þ 0.4 ppm in anorthosites, 3.1 þ 0.6 ppm in volcanic glasses, and 0.7 ppm in basalt 75075. These N abundances in anorthosites and in the basalt are comparable to previously reported values [1^3,5], but the volcanic glasses are enriched in the indigenous N. Considering that solubility experiments predicts only V0.08 ppm N in the silicate melt [18], the high N abundances in 74001 glasses ( s 100 ppm, in two samples reported by Kerridge et al. [6], see footnote to Table 1) may appear surprising. However, the indigenous N is surface-correlated in the fumarolic volcanic glasses. The abundances of the volume-correlated nitrogen component may be much lower, e.g. the lowest reported N = 0.18 ppm (Kerridge et al. [6]) may represent an upper limit for the volume-correlated indigenous N. 4. Indigenous N signatures : implications 4.1. Lunar vs. terrestrial signatures Fig. 4 shows N15 N values (total N for each sample) plotted vs. e¡ective 2Z-exposure time (discussed later) on the lunar surface. The data de¢ne
Fig. 4. N15 N vs. the e¡ective irradiation time (Myr/N (ppm)) on the lunar surface. Replicate analyses of 60025 are in excellent agreement with each other. Data de¢ne a linear correlation and the intercept on the N15 N axis yields a signature of +13x for the indigenous N in bulk samples.
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an approximately linear array. The intercept on the N15 N axis de¢nes the indigenous lunar nitrogen signature and the slope the 15 N spallation production rate, discussed later. This correlation is consistent with an indigenous lunar nitrogen signature N15 N = +13x for bulk samples. As discussed earlier, identical indigenous N signatures are derived from the oldest (4.44 Ga) studied ferroan anorthosite 60025, from anorthositic breccia 67915 with a formation age of V4.1 Ga, from volcanic glasses formed by lava fountaining 3.7 Ga ago, and from a basaltic rock with a crystallization age of V3.7 Ga. The fact that the indigenous N15 N values derived from lunar samples with di¡erent formation histories, mineralogy, and sources within the Moon leads to the conclusion that there was a uniform indigenous lunar nitrogen component. These signatures are not compromised by solar wind components. The Sm^Nd age of anorthosite 60025 dates back to the time of the early evolution of the Earth^Moon system and naturally leads to another planetology issue. If the lunar materials are derived from a proto-earth, by way of a giant impact, one has to expect approximate consistencies in the N isotopic signatures. However, since the primitive nitrogen isotopic signature of the Earth is poorly constrained, and since fractionation e¡ects at temperatures implied for the separation and formation of the Moon are di¤cult to assess, no simple answer can be expected. Nevertheless, it appears appropriate to make some comparisons. Terrestrial diamonds provide a N15 N range of 311x and +6x [19]. This range has been interpreted as being due to isotopic fractionation of a primitive mantle composition with initial N15 N 6 311x [19]. In this case, light N of terrestrial diamonds is not inconsistent with the indigenous N signature of the Moon, if lunar nitrogen represents a residue of a larger reservoir, and if in the formation process nitrogen was fractionated by 20^40x, favoring the heavy isotope. On the other hand, Kurz et al. [20] and Rison and Craig [21] noted that deep-seated mantle material carried high 3 He/4 He ratios, and these samples show signi¢cantly heavier N, with N15 N values up to +14 þ 1.2x (Exley et al. [22]). We note that indigenous lunar nitrogen signatures do
665
match signatures in samples from the Lau Basin [22] associated with high 3 He/4 He ratios. Marty and Humbert [23] report analyses of oceanic basalts with N15 N in the range of 37x and +7x which has been interpreted to represent a mantle signature of N15 N = 37x and contamination with crustal material of N15 N = +7x. Similarly Sano et al. [24] and Mohapatra and Murty [25] have shown that the primary signatures of terrestrial mantle have been modi¢ed by variable addition of recycled components from sur¢cial reservoirs. 4.2. Can indigenous N explain the variations in the regolith?
15
N/14 N
Lunar regolith samples have been extensively used for studies of the implanted `solar' component (e.g. [2,26^28]). Nitrogen studies of the lunar regolith samples have shown that (i) N isotopic ratios show at least 30% variation, (ii) N/36 Ar ratios are an order of magnitude larger than `solar' values (N/132 Xe about 3 times higher than `solar') [26,28], (iii) Ar/Kr/Xe ratios show no evidence of fractionation due to losses [29], and (iv) the C/N ratios in regolith material are well correlated [30]. Secular changes in the solar wind source [27,28] and a non-solar source for majority of the N are evoked [31,32] to explain this lunar regolith nitrogen `excesses'. Geiss and Bochsler [33] and Signer et al. [34] suggested that the lunar indigenous component released from the Moon's interior and subsequently re-implanted in the regolith (e.g. [35]) could explain the very light N released at intermediate temperatures of some lunar soils and breccias. The required indigenous nitrogen signature is N15 N 6 3200x, close to the lightest measured N in lunar soils. If, according to the model, the relative abundances of the indigenous light nitrogen component decreased as a function of time, this would account for the observed variation in the 15 N/14 N ratio in regolith samples. However, the lunar indigenous N signature obtained in this work is clearly inconsistent with the composition required by the model. In order to understand the lunar regolith nitrogen data, both the nitrogen `excesses' and the var-
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iation in the isotopic composition need to be addressed [2,27,28,36]. The present study eliminates one of the suggested non-solar sources, invoked to explain lunar nitrogen excesses. 5. Cosmic-ray production rates of Moon
15
N on the
In contrast to lunar soils, which have abundant solar wind derived nitrogen and noble gases, igneous lunar rocks have low N abundances. Hence, these rocks are in principle good candidates for deriving cosmic-ray induced production rates of nitrogen. One of the objectives of the present study is to quantify the cosmic-ray induced production rate of nitrogen in lunar igneous rocks with documented surface exposure history. Oxygen is the major target for the production of nitrogen, and since the O abundance is rather constant in lunar rocks, no signi¢cant compositional dependence is expected. The proton induced production of 15 N is through the direct reaction 16 O(p,2p)15 N and also through the 16 O(p,pn)15 O reaction. Reedy [37] estimated the 15 N production rates (P(15 N)) in the lunar regolith from the 16 O(p,2p)15 N reaction. The contribution of the 16 O(p,pn)15 O reaction was estimated to be 55% of the direct production, based on a comparison of the predicted 16 O(p,2p)15 N production rate with the empirical estimates of Becker et al. [2]. Mathew and Murty [38] utilized the 16 O(p,2p)15 N and 16 O(p,pn)15 O cross-section measurements that became available after the Reedy [37] calculations and estimated a P(15 N) 30% higher than derived by Becker et al. [2]. Important are also neutron induced production reactions 16 O(n,2n)15 O and 16 O(n,np)15 N. These reaction cross-sections, however, are not available and theoretical estimates mostly rely on the proton induced reaction cross-sections for estimates of the production rates. As indicated earlier, lunar basalt 75075 had a single stage exposure history close to the lunar surface for 143 þ 5 Ma [15], and average lunar production rates P(15 N) can be derived for nearsurface locations. The 700³C to 1000³C extraction steps of 75075 released a uniform indigenous N
signature (N15 N = +13x), which contrasts with the higher temperature steps (N15 N s 320x), shifted due to the release of spallation-produced 15 NC (Fig. 3). The entire excess is taken as the 15 NC spallation component, which accounts for V30% of the measured 15 N in lunar basalt 75075. Therefore, the production rate is more precise than those derived from other samples. From spallation 15 NC excesses in 75075 we calculate a production rate P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma, and this P(15 N) is not compromised by assumptions about the indigenous N signature. The uncertainties in the production rates listed include a 5% systematic uncertainty in the N concentrations. A sub-surface production rate applies for the two 60025 anorthosite measurements, but the spallation component is much smaller. This anorthosite has the short cosmic-ray exposure age of South Ray Crater (2.0 Ma). Nevertheless, the s 1100³C extractions of 60025 (both -A and -B), representing V10% of the total N, show clear shifts to heavier signatures, relative to the plateau release of N15 N = +13x in the 700³C to 9 1100³C steps. An average production rate of 5.6 þ 0.9 pg 15 N/g/Ma is obtained from 60025-A and 60025-B, consistent with the derived production rates from lunar basalt 75075. Anorthositic breccia 67915 had a partially shielded exposure to cosmic rays on the lunar surface and needs a shielding correction and the volcanic glasses from a depth of 55.5 cm in the drill core determine the 15 N production rates at an inferred effective depth. First, consider breccia 67915, hammered o¡ the `Outhouse' boulder, a 2 m rock resting on the rim of the North Ray Crater. This rock was partially shielded by the V10 m `House' boulder and its present e¡ective cosmic-ray exposure geometry of 1.33Z can be estimated from the photo-documentation on the lunar surface (shielding angle 45 þ 15³). The exposure geometry requires an adjustment in the calculated P(15 N) = 3.6 pg 15 N/g/Ma (to normalize to the exposure geometry 2Z) and corresponds to a production rate P(15 N) = 5.4 þ 1.2 pg 15 N/g/Ma corrected for the shielded exposure. The 15 N excesses in the two 74001 samples, coupled to an exposure age of 33 Ma (Eugster
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et al. [13]), yield an average P(15 N) = 6.5 þ 1.0 pg 15 N/g/Ma, slightly higher than but consistent with the rate determined from 75075. We note that the location of these 74001 samples corresponds to a shielding depth of V80 g/cm2 beneath the lunar surface, and a somewhat enhanced production in our 74001 samples is consistent with an expected increase in the P(15 N) due to shielding. In addition to the recent 17 Ma exposure at shielding depth of V80 g/cm2 , 74001,434 and 74001,2203 apparently received pre-exposure irradiation for V16 Ma at a shielding depth in the range of 85^120 g/cm2 (Eugster et al. [13]). Eugster et al. [13] also noted that the spallation 21 Ne, 38 Ar, 83 Kr and 126 Xe abundances are constant for the layer with a shielding depth range of 20 to 100 g/cm2 (from which the two present 74001 samples and the three 74001 samples of Becker and Clayton [39], discussed below, were taken). The di¡erent production rates discussed above are consistent and permit the calculation of an average production rate (Table 2) of P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma, about 60% larger than that estimated by Becker et al. [2]. Becker and Clayton [39] reported V0.22 ng15 N/g spallation excesses in 74001/2 assuming a trapped signature of N15 N = 10x. When recalculating their data using a N15 N = 13x, determined here, for the indigenous component, spalTable 2 Measured 75075
15
N abundances and cosmic-ray induced
15
667
lation excesses in agreement with the values measured by us are obtained. Spallation excesses in 74001/2 were not reported by Kerridge et al. [6], since the samples were heated only up to 1050³C, below the temperature where most of the spallation nitrogen appeared. Thus the cosmogenic 15 N excesses in ¢ve samples of 74001 are uniform. The £at 15 N depth pro¢le is similar to the cosmogenic noble gas depth pro¢les reported by Eugster et al. [13]. The observation that the cosmogenic 21 Ne and 15 N concentrations are constant for a shielding depth range of 20 to 100 g/cm2 in 74001 allows us to derive a production rate ratio P(15 NC )/ P(21 NeC ) in silicates. The calculated P(15 NC )/ P(21 NeC ) = 4.0 þ 0.3, using the 15 NC measured in this work and the 21 NeC reported by Eugster et al. [13], in agreement with model estimates by Mathew and Murty [38], also reporting a ratio of 4.0 þ 0.5. Fig. 4 shows a plot of the 15 N excesses as a function of e¡ective (adjusted) 2Z-irradiation time. In this correction, partially shielded sample (67915) is adjusted, using photo-documentation on the lunar surface, to the same 2Z surface irradiation conditions. The ¢gure shows that the 60025 and 74001 aliquots, the adjusted 67915, and basalt 75075 are on the same trend line and also shows that the 15 N excesses increase linearly
N production rates derived from lunar samples 60025, 67915, 74001, and
Sample
Shielding depth (cm)
15
Na (ng/g)
N15 N (x)
Exposure agec (Ma)
15
NdC (ng/g)
Production rate (pg/g/Ma)
60025-A 60025-B 67915b 74001,434 74001,2203 74001,1072 74001,1082 74001,1085 75075
sub-surface sub-surface sub-surface 55.5 cm 55.5 cm 38.5 cm 47.5 cm 59.0 cm sub-surface
0.57 0.57 3.55 10.9 6.2 n.r. n.r. n.r. 2.8
32.3 þ 1.6 33.0 þ 1.4 64.3 þ 1.0 33.1 þ 1.0 47.3 þ 1.4 n.r. n.r. n.r. 313 þ 1.1
2.0 þ 0.1 2.0 þ 0.1 50 þ 1.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 143 þ 5.0
0.0110 0.0114 0.18 0.22 0.21 0.21, BC 0.22, BC 0.17, BC 0.83
5.5 þ 1.0 5.7 þ 1.0 5.4 þ 1.2 6.7 þ 0.9 6.4 þ 1.1 6.4 6.7 5.2 5.8 þ 0.6
Data reported in literature by Becker and Clayton [39] for three 74001 drill core samples are included for comparison. n.r., not reported; BC, Becker and Clayton [39]. a Only the s 1000³C extractions with cosmogenic 15 N are included. b The production rate derived from anorthosite 67915 requires a correction for shielding (see text). c See sample selection for references to the cosmic-ray exposure ages of the samples. d15 NC are evaluated using 15 NC = 15 N{(N15 N313.0)/1000}.
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with e¡ective exposure time. Slope of the correlation line in Fig. 4 implies a P(15 N) = 5.8 þ 0.7 pg 15 N/g/Ma, consistent with the average P(15 N) obtained from individual samples. R. Wieler and B. Marty brought to our attention that Hashizume et al. [40] reported a 15 N/ 38 Ar production rate ratio of 13.1 þ 1.6 atoms/ atom in an ilmenite separate, about a factor of two higher than estimates based on the then existing 15 N and 38 Ar production rates. Kim et al. [41] deduced a silicate 4Z production rate of 11.6 pg 15 N/g/Ma in the meteorite Enon. The new lunar production rates derived here are consistent with the rate determined in Enon, when normalized to the same exposure geometry. P(15 N) rates derived here are also in agreement with the production rates calculated by Mathew and Murty [38]. 6. Conclusions Indigenous lunar nitrogen composition and abundances have been determined in lunar samples with short exposure ages. The studied samples include ferroan anorthosite 60025, one of the oldest crustal rocks on the Moon, volcanic glassrich samples of the double-drive drill tube 74001, lunar basalt 75075 with a well-determined exposure age, and anorthositic breccia 67915. The abundances of the indigenous lunar nitrogen are at ppm level, two orders of magnitude lower than the observed abundances in lunar regolith soils with implanted solar gases. The N isotopic composition is found to be uniform in all lunar rocks and is constrained to be N15 N = +13.0 þ 1.2x. The N15 N = 13.0x derived for the indigenous lunar nitrogen eliminates one of the suggested mechanisms for the origin of nitrogen `excesses' in lunar regolith samples ^ viz. degassing of indigenous lunar nitrogen followed by re-trapping into the regolith. We calculate a near-surface 15 N cosmic-ray production rate of P(15 N) = 5.8 þ 0.6 pg 15 N/g/ Ma for the lunar surface. This rate is consistent with both theoretical estimates based on production rate calculations and with meteoritic production rates determined from Enon silicates [41].
The production rates derived here, however, exceed that given by Becker et al. [2] by 60%. A 15 N/ 21 Ne production rate ratio of 4.0 þ 0.3 is derived for silicates. Acknowledgements We thank NASA, JSC for the lunar samples studied in this work and K. Geddes for assistance in sample preparation. The paper bene¢ted from detailed and constructive reviews by R. Wieler, B. Marty and S.V.S. Murty. Work was supported by NASA grant NAG5-8167.[AH] References [1] O. Mu«ller, Solar wind nitrogen and indigenous nitrogen in Apollo 17 lunar samples, Proc. 5th Lunar Sci. Conf., 1974, pp. 1907^1918. [2] R.H. Becker, R.N. Clayton, T.K. Mayeda, Characterization of lunar nitrogen components, Proc. 7th Lunar Sci. Conf., 1976, pp. 441^458. [3] D.J. DesMarais, Light element geochemistry and spallogenesis in lunar rocks, Geochim. Cosmochim. Acta 47 (1983) 1769^1781. [4] B.L. Barraclough, K. Marti, In search of the Moon's indigenous volatiles: noble gases and nitrogen in vesicular lunar glasses, Lunar Planet. Sci. XVI, 1985, pp. 31^32. [5] S.V.S. Murty, J.N. Goswami, Nitrogen, noble gases and nuclear tracks in lunar meteorites MAC88104/105, Proc. 22nd Lunar Planet. Sci. Conf., 1992, 225^237. [6] J.F. Kerridge, O. Eugster, J.S. Kim, K. Marti, Nitrogen isotopes in the 74001/74002 double-drive tube from Shorty Crater, Proc. 21st Lunar Planet. Sci. Conf., 1991, pp. 291^299. [7] R.W. Carlson, G.W. Lugmair, The age of ferroan anorthosite 60025: oldest crust on a young Moon?, Earth Planet. Sci. Lett. 90 (1988) 119^130. [8] K. Marti, U. Aeschlimann, P. Eberhardt, J. Geiss, N. Gro«gler, D.T. Jost, J.C. Laul, M.-S. Ma, R.A. Schmitt, G.J. Taylor, Pieces of the ancient lunar crust: Ages and composition of clasts in consortium breccia 67915, Proc. 14th Lunar Planet. Sci. Conf., J. Geophys. Res. 88 (1983) B165^B175. [9] C. Meyer, D.S. McKay, D.H. Anderson, P. Butler, The source of sublimates on the Apollo 15 green and Apollo orange glass samples, Proc. 6th Lunar Sci. Conf., 1975, pp. 1673^1699. [10] E.H. Cirlin, R.M. Housley, A £ameless atomic absorption study of the volatile trace metal lead in lunar samples, Proc. 8th Lunar Sci. Conf., 1977, pp. 3931^3940. [11] U. Kra«henbu«hl, Distribution of volatile and non-volatile
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[12]
[13]
[14] [15]
[16]
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[21] [22]
[23] [24] [25] [26]
elements in grain-size fractions of Apollo 17 drive tube 74001/2, Proc. 11th Lunar Planet. Sci. Conf., 1980, pp. 1551^1564. D.S. McKay, G. Heiken, G.A. Waits, Core 74001/2: Grain size and petrology as a key to the rate of in situ reworking and lateral transport on the lunar surface, Proc. 9th Lunar Planet. Sci. Conf., 1978, pp. 1913^1932. O. Eugster, N. Gro«gler, P. Eberhardt, J. Geiss, W. Kiesl, Double drive tube 74001/2: A two-stage exposure model based on noble gases, chemical abundances and predicted production rates, Proc. 12th Lunar Planet. Sci. B, 1981, pp. 541^558. P. Eberhardt, O. Eugster, J. Geiss, N. Gro«gler, M. Jungck, P. Maurer, M. Morgeli, A. Stettler, Shorty crater, noble gases and chronology, Meteoritics 10 (1975) 93^94. G.W. Lugmair, N.B. Scheinin, K. Marti, Sm-Nd age and history of Apollo 17 basalt 75075: Evidence for early di¡erentiation of the lunar exterior, Proc. 6th Lunar Sci. Conf., 1975, pp. 1419^1429. K.J. Mathew, R.L. Palma, K. Marti, B. Lavielle, Isotopic signatures and origin of nitrogen in IIE and IVA iron meteorites, Geochim. Cosmochim. Acta 64 (2000) 545^ 557. J.S. Kim, Y. Kim, K. Marti, J.F. Kerridge, Nitrogen isotope abundances in the recent solar wind, Nature 375 (1995) 383^385. F. Humbert, B. Marty, G. Libourel, Enhanced solubility of nitrogen in basaltic melt under reducing conditions: A way to enrich nitrogen relative to noble gases during planetary Formation, Lunar Planet. Sci. XXX, 1999, Lunar and Planetary Institute, CD-ROM. M. Javoy, F. Pineau, D. Demai¡e, Nitrogen and carbon in the diamonds of Mbuji Mayi (Zaire), Earth Planet. Sci. Lett. 68 (1984) 399^412. M.D. Kurz, W.J. Jenkins, S.R. Hart, D.A. Clague, Helium isotopic variations in volcanic rocks from Loihi Seamount and the island of Hawaii, Earth Planet. Sci. Lett. 66 (1983) 388^406. W. Rison, H. Craig, Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths, Earth Planet. Sci. Lett. 66 (1983) 407^426. R.A. Exley, S.R. Boyd, D.P. Mattey, C.T. Pillinger, Nitrogen isotope geochemistry of basaltic glasses: implications for mantle degassing and structure?, Earth Planet. Sci. Lett. 81 (1987) 163^174. B. Marty, F. Humbert, Nitrogen and argon isotopes in oceanic basalts, Earth Planet. Sci. Lett. 152 (1997) 101^ 112. Y. Sano, N. Takahata, Y. Nishio, B. Marty, Nitrogen recycling in subduction zones, Geophys. Res. Lett. 25 (1998) 2289^2292. R.K. Mohapatra, S.V.S. Murty, Origin of air-like noble gases in oceanic basalts, Geophys. Res. Lett. 27 (2000) 1583^1586. U. Frick, R.H. Becker, R.O. Pepin, Solar wind record in the lunar regolith: Nitrogen and noble gases, Proc. 18th Lunar Planet. Sci. Conf., 1988, pp. 87^120.
669
[27] J.F. Kerridge, Long-term compositional variation in solar corpuscular radiation: evidence from nitrogen isotopes in the lunar regolith, Rev. Geophys. 31 (1993) 423^437. [28] R.N. Clayton, M.H. Thiemens, Lunar Nitrogen: Evidence for secular change in the solar wind, in: R.O. Pepin, J.A. Eddy, R.B. Merrill (Eds.), Proc. Conf. Ancient Sun, 1980, pp. 463^473. [29] R. Wieler, H. Baur, Fractionation of Xe, Kr, and Ar in the solar corpuscular radiation deduced by closed system etching of lunar soils, Astrophys. J. 453 (2 Part 1) (1995) 987^997. [30] J.F. Kerridge, I.R. Kaplan, C. Petrowski, Carbon isotope systematics in the Apollo 16 regolith, Lunar Planet. Sci. IX, 1978, pp. 618^620. [31] R. Wieler, F. Humbert, B. Marty, Evidence for a predominantly non-solar origin of nitrogen in the lunar regolith revealed by single grain analyses, Earth. Planet. Sci. Lett. 167 (1999) 47^60. [32] D.R. Brilliant, I.A. Franchi, C.T. Pillinger, Nitrogen components in lunar soil 12023 - complex grains are not carriers of isotopically light nitrogen, Meteoritics 29 (1994) 718^723. [33] J. Geiss, P. Bochsler, Nitrogen isotopes in the solar system, Geochim. Cosmochim. Acta 46 (1982) 529^548. [34] P. Signer, H. Baur, P. Etique, R. Wieler, Nitrogen and noble gases in the 71501 bulk soil and ilmenite as records of the solar wind exposure: Which is correct?, In: Workshop of past and present solar radiation, Mainz, LPI tech. Report 86^02, Lunar and Planetary Institute, Houston, 1986, pp. 34^35. [35] R.H. Manka, F.C. Michel, Lunar atmosphere as a source of lunar surface elements, Proc. 2nd Lunar Sci. Conf., Lunar and Planetary Institute, Houston, 1971, pp. 1717^ 1728. [36] R.H. Becker, An evaluation of possible explanations for `excess' nitrogen in the lunar regolith, Lunar Planet. Sci. XXVI, Lunar Planet. Institute, Houston, 1995, pp. 89^90. [37] R.C. Reedy, Cosmic-ray-produced stable nuclides: Various production rates and their implications, Proc. 12th Lunar Planet. Sci. B, Lunar and Planetary Institute, Houston, 1981, pp. 1809^1823. [38] K.J. Mathew, S.V.S. Murty, Cosmic ray produced nitrogen in extraterrestrial matter, Proc. Indian Acad. Sci. (Earth Planet. Sci.), 102, 1993, pp. 415^437. [39] R.H. Becker, R.N. Clayton, Nitrogen isotopes in some exceptional Apollo 12 and Apollo 17 soils (abstract), Lunar Planet. Sci. IX, Lunar and Planetary Institute, Houston, 1978, pp. 64^66. [40] K. Hashizume, B. Marty, R. Wieler, Single grain analyses of the nitrogen isotopic composition in the lunar regolith in search of the solar wind component, Lunar Planet. Sci. XXX, 1999, Lunar and Planetary Institute, Houston, CDROM. [41] J.S. Kim, Y. Kim, K. Marti, Isolation of cosmic-ray produced nitrogen in meteoritic silicates and some implications, Lunar Planet. Sci. Conf. XXV, Lunar and Planetary Institute, Houston, 1994, pp. 701^702.
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Lunar nitrogen: indigenous signature and cosmic-ray production rate K.J. Mathew *, K. Marti Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0317, USA Received 26 June 2000; received in revised form 23 October 2000; accepted 25 October 2000
Abstract Indigenous lunar nitrogen composition and abundances have been determined in old ferroan anorthosite 60025 and in anorthositic breccia 67915 from North Ray Crater, as well as in 55 cm deep volcanic glasses of 74001 double-drive core from Shorty Crater. Also included in the set is the well-documented lunar basalt 75075, collected near the Camelot Crater. Indigenous lunar N abundances are low (at ppm level), but there is some variation between glass-rich cores, mare basalts and anorthosites. The uniform indigenous N isotopic signature of N15 N = +13.0 þ 1.2x, is consistent with data reported previously for Shorty Crater samples. The indigenous N15 N cannot account for the light nitrogen component, observed in the lunar regolith samples. We have determined cosmic-ray production rates P(15 N) for the above rocks and the drill core samples. The average production rate estimate (for low shielding) of P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma is V60% higher than published lunar 15 N production rates, but consistent with the meteoritic production rate derived from silicates in the Enon meteorite, when normalized to 2Z-irradiation geometry. From the observed cosmogenic 15 N excesses and the reported cosmogenic 21 Ne abundances in core 74001 we derive a (15 NC /21 NeC ) production rate ratio of 4.0 þ 0.3 for silicates. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: lunar samples; nitrogen; spallation nitrogen; regolith; Moon; volatiles
1. Introduction Nitrogen in ancient lunar rocks that formed at di¡erent times and depths within the Moon could trace the nitrogen isotopic evolution of the Moon as well as constrain its initial signature. The earliest crustal rocks on the Moon presumably
* Corresponding author. Tel.: +1-858-534-0443; Fax: +1-858-534-7441; E-mail: [email protected]
sampled nitrogen of the period of initial lunar di¡erentiation and might also have preserved signatures of an early lunar atmosphere. Mare basalts have crystallization ages similar to eruption times of volcanic glasses at the Apollo 17 site, but the lunar volcanic glasses originated at greater depths than the basalts. Hence the nitrogen isotopic signatures for di¡erent depths within the lunar mantle can be inferred through a comparative study of lunar basalts and volcanic glasses. The lunar indigenous nitrogen isotopic composition may also be useful as a tracer in models of the Earth^Moon system. Lunar igneous rocks are reported to contain
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 0 ) 0 0 3 2 7 - 7
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less than 1 ppm N [1^3] and, therefore, their isotopic signatures are a¡ected by spallation components. Previous attempts to determine the indigenous lunar N abundances and signatures from lunar rocks were largely unsuccessful. Lunar soils, in contrast, contain about two orders of magnitude higher abundances of nitrogen and other volatiles due to accretion of solar wind and possibly also volatiles from other sources. Lunar soils are not suitable for studies of indigenous nitrogen signatures. Barraclough and Marti [4] carried out a search for Moon's indigenous nitrogen based on analyses of vesicular lunar glasses. The nitrogen to noble gas elemental ratios in these samples was much higher than solar values, and it was suggested that these samples contained indigenous N components, but also spallation 15 NC that precluded the identi¢cation of the indigenous signatures. Murty and Goswami [5] reported N in a clast of lunar meteorite MAC88105 and derived a N15 N of 17 þ 3.4x for indigenous lunar N. Another study regarding the signature of the indigenous nitrogen was reported by Kerridge et al. [6] based on 74001/2 double-drive core samples. These authors measured N apparently trapped on the surfaces of fumarolic glass particles, and a N15 N of V14x was derived for the indigenous lunar nitrogen. We selected suitable rocks to calibrate the 15 N production rate P(15 N) by cosmic rays, as this requires rocks with well-determined cosmic-ray exposure histories and shielding conditions. At this time there exists only one determination, obtained from basalt 12021 [2]. 2. Experimental techniques 2.1. Sample selection In the magma ocean model of the early lunar di¡erentiation, ferroan anorthosites are among the ¢rst crustal rocks to form. The Sm^Nd age of 4.44 Ga makes 60025 the oldest dated crustal anorthosite on the Moon [7]. Most ferroan anorthosites show evidence for post-crystallization disturbance, but 60025 is one of the least metamorphosed. Furthermore, all mineral components of
the polymict breccia 60025 appear to be from the same igneous event. The corrections for a cosmogenic component are expected to be very small in the case of 60025 due to its low surface exposure age of only 2.0 Ma (South Ray Crater). Lunar anorthosite 67915 is a fragmental breccia chipped o¡ a 2 m boulder resting on the rim of 50 Ma old North Ray Crater. A 39 Ar^40 Ar age of V4.1 Ga and a Pu^Xe retention age of 4.3 Ga [8] suggest a somewhat younger age than that of 60025, and it was anticipated that this anorthosite may have sampled indigenous lunar nitrogen characteristic of the later stages of the magma ocean phase of lunar evolution. Breccia 67915 contains more ma¢c inclusions than 60025; those visible by optical inspection of a single chip were removed prior to analyses. The black and orange glasses present in 74001/2 represent pyroclastic deposits, formed by the rapid quenching of molten silicate droplets that were ejected into the lunar atmosphere during the explosive volcanic eruption of ultrama¢c magmas onto the lunar surface along the rim of mare Serenitatis [9]. The source regions of these magmas may have been at depths as great as several hundred kilometers. Chemically these glasses are similar to the mare basalts at the Apollo 17 site. The surfaces of the individual glass particles are coated with volatile elements such as Zn, Pb, and halogens [9^11], suggesting that the glasses have sampled volatiles from the lunar interior (mantle) released during the lava fountaining. It appeared reasonable to expect that the lunar interior nitrogen was enriched in these double-drive tube glasses. The 74001/2 cores do not show agglutinates [12], except for the top 5 cm layer (of the drill core), indicating no surface exposure of the deeper layers, consistent with the exposure history model of Eugster et al. [13]. Formation ages of 3.7 Ga have been obtained for the black and orange glasses of 74001/2 by 39 Ar^40 Ar technique [14]. A short recent cosmic-ray exposure age of 17.2 þ 1.4 Ma was obtained for 74001/2 by 81 Kr^Kr [13]. This probably represents the time when Shorty Crater was excavated and when the glass was emplaced on the crater rim. Lunar basalt 75075 is a medium-grained surface rock picked from the top of a boulder at the rim
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Table 1 N concentrations and isotopic signatures in lunar samples 60025, 74001, 67915 and 75075 Sample
Temp. (³C)
N (ppm)
N15 N (x)
Sample
Temp. (³C)
N (ppm)
N15 N (x)
60025-A
350C 700 900 1100 1400 1550 Total 500C 700 800C 900 1000 1100 1250 1550
0.03 0.08 0.11 1.50 0.12 0.03 1.87 0.04 0.06 0.01 0.23 0.04 0.66 0.06 0.09 1.19
6.00 þ 2.3 0.90 þ 1.6 11.63 þ 1.3 12.83 þ 0.9 30.52 þ 1.4 39.40 þ 2.5 13.70 þ 1.0 8.00 þ 2.3 30.80 þ 1.9 10.29 þ 3.2 11.95 þ 0.9 12.61 þ 1.5 12.80 þ 0.9 26.81 þ 0.9 37.10 þ 1.7 14.31 þ 1.1
74001,434
350C 700 1000 1100 1250 1600 Total 350C 600 800 1000 1100 1400 1600 Total
0.06 0.76 0.76 1.65 0.91 0.32 4.46 0.04 0.55 0.38 0.61 0.73 0.80 0.08 3.19
14.00 þ 4.0 8.87 þ 1.0 13.04 þ 1.0 14.10 þ 0.7 54.20 þ 1.0 71.20 þ 2.0 25.31 þ 1.0 17.00 þ 3.0 7.61 þ 1.2 13.24 þ 1.6 13.61 þ 0.8 15.63 þ 1.0 75.16 þ 1.7 58.10 þ 2.7 29.59 þ 1.3
350C 700 1000 1100 1200 1550 1600 Total
0.06 0.09 0.20 0.36 0.15 0.35 0.05 1.26
5.11 þ 1.2 2.83 þ 1.2 13.05 þ 0.8 21.62 þ 0.7 58.84 þ 1.2 107.50 þ 1.1 86.20 þ 3.0 48.98 þ 1.0
400C 700 900 1000 1100 1300 1450 1550 1600 Total
0.021 0.054 0.052 0.031 0.012 0.335 0.087 0.079 0.032 0.703
2.1 þ 2.0 12.9 þ 1.1 13.0 þ 1.1 13.3 þ 0.9 16.5 þ 1.0 320.2 þ 0.9 392.1 þ 1.2 354.5 þ 1.8 321.8 þ 2.1 258.6 þ 1.2
57.08 mg
60025-B 196.83 mg
67915,233 47.44 mg
3.66 mg
74001,2203 4.45 mg
75075 621.9 mg
C denotes a combustion step in O2 . Kerridge et al. [6] reported N data for number of 74001/2 samples. N data (sum of all temperature steps up to 1050³C) reported for samples from the same layer (same cosmic-ray exposure history) as 74001,434 and 74001,2203 are listed here for comparison. N = 0.56 ppm, N15 N = 46 þ 12.8x (32.5 to 33.5 cm); N = 9.11 ppm, N15 N = 12.6 þ 2.6x (55.5 to 56.5 cm); N = 110 ppm, N15 N = 13.2 þ 1.3x (55.5 to 56.5 cm); N = 371 ppm, N15 N = 13.2 þ 1.7x (55.5 to 56.5 cm); N = 2.82 ppm, N15 N = 19 þ 9.6x (56 to 56.5 cm); and N = 2.33 ppm, N15 N = 20 þ 5.1x (56 to 56.5 cm) (depths in the core from which the samples were taken are indicated).
of the Camelot Crater, from which it was presumably ejected. The basalt crystallized 3.70 þ 0.07 Ga ago, and was ejected in a crater forming event 143 þ 5 Ma ago [15]. Most lunar basalts exhibit complex exposure histories, but basalt 75075 was selected on the basis of a single stage exposure near the lunar surface [15]. This history was inferred from the cosmogenic 131 Xe/126 Xe ratios and the thermal neutron £uences, calculated from measured 150 Sm/149 Sm ratios. Thus the cosmic-ray production rate, P(15 N), obtained from 75075 represents an average surface production rate. A direct comparison of the P(15 N) with those derived by Becker et al. [2] from lunar basalt 12021 is possible.
2.2. Mass spectrometry Chips of the anorthosites 60025-A and 60025-B and basalt 75075 were loaded into the gas extraction and analyses system without wrapping, whereas the 74001,434 and 74001,2203 glasses and the anorthosite 67915 were wrapped in predegassed Au foils. Details of the experimental techniques are described elsewhere [16]. The samples were step heated by an external resistance heater up to 1000³C in a double walled quartz system with a separately pumped vacuum jacket, and then transferred in vacuo into a Mo crucible mounted in a double walled quartz system for radio frequency (RF) heating and melting. In all
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cases low temperature combustion steps in pure O2 , released from puri¢ed CuO, were carried out to remove adsorbed terrestrial N and other surface contamination. N was analyzed in the static mode with a magnetic sector mass spectrometer equipped with a Baur^Signer source. Blank measurements were carried out between the runs, and increased with extraction temperature. N2 blanks were 6 0.3 ng in the 9 1400³C steps and s 2 ng in the 1550³C and 1600³C extractions. Sensitivity and instrumental mass discrimination were monitored by pipettes of air standards and were found to be constant (within 5%) during the set of runs. For N2 , the reliability of the CO corrections at mass 30, based on 12 C18 O signal, was tested on di¡erent sized gas splits from the same extraction. Data presented are corrected for system blanks, N2 recovery (typically V80% for Mo crucible; loss of the sample N to the Mo crucible was monitored by exposing N standard pipettes to the crucible), spectrometer background, and instrumental mass discrimination. Listed errors include the statistical uncertainties in the measurement and variation in the blank and background correction. Uncertainties of isotopic ratios represent 95% con¢dence levels. 3. Indigenous lunar nitrogen : abundances and signatures As mentioned previously, the ferroan anorthosites formed as part of the initial lunar di¡erentiation, presumably as £oating crust above a magma ocean. Thus the indigenous nitrogen of ferroan anorthosite 60025, with a crystallization age of 4.44 Ga [7], may represent the N15 N of indigenous lunar nitrogen at the time of the initial lunar di¡erentiation. Nitrogen data of the two 60025 samples are consistent with each other. The 60025-A (57 mg) was the pilot sample to study the release, permitting optimization of temperature steps used for pyrolysis of 60025-B (197 mg). The release systematics of both the samples are strikingly similar (Fig. 1). The ¢rst combustion step released 6 4% N with N15 N V8x. The 700³C release is isotopi-
Fig. 1. Release systematics of N from lunar anorthosite 60025. 60025-A (57.1 mg) was used as a pilot sample to determine the temperature steps for the larger 60025-B (196.8 mg). The numbers close to data bars indicate the extraction temperature in 100³C. The combustion steps at 350³C for 60025-A, and at 500³C and 800³C for 60025-B (at 5 Torr O2 ) was carried out to remove adsorbed terrestrial N and other contaminants. The plateau release between 700³C and 1100³C represents the indigenous N signatures and the shifts in the s 1100³C releases are due to a spallation 15 N component.
cally lighter and close to terrestrial. The s 700³C and 9 1100³C release de¢ne a well-de¢ned plateau with isotopic composition of +13x and accounts for V80% of the total N in both the samples. This uniform N release presumably represents the signature of indigenous lunar N in 60025. The s 1100³C steps have higher N15 N, due to the presence of a spallation N component. In 74001 core samples, the 350³C combustion step released 6 2% of the total N with a signature of V17x (Fig. 2). The s 700³C and 9 1100³C pyrolysis steps release s 50% of the N in these
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to 1050³C, and that the extraction of spallationfree indigenous N is almost complete at extraction temperatures 6 1100³C. Note that the 74001/2 double-drive core samples analyzed by Kerridge et al. [6] were separated in heavy liquids and washed in acetone and methanol and the magnitude of a possible contamination during the chemical processing was di¤cult to assess. In the Kerridge et al. [6] experiments also, as observed here, the nitrogen released at low temperatures had heavy N15 N values. Fig. 3 shows the N release systematics of anorthosite 67915 and of lunar basalt 75075. The s 1100³C steps of both 67915 and 75075 show shifts due to spallation derived 15 N excesses. In
Fig. 2. Release systematics of N from volcanic glasses 74001,2203 (4.45 mg) and 74001,434 (3.66 mg). The numbers close to data bars indicate the extraction temperature in 100³C. An initial combustion step at 350³C and at 5 Torr O2 was carried out to remove contamination. Both samples are fresh untreated samples from a depth of 55 cm in the double-drive core 74001/2 and received a combined exposure of 33 Ma in the two-stage exposure model of Eugster et al. [13]. N in the s 1100³C releases is heavy, indicating a spallation N component.
samples with a uniform signature of 13x. The 700³C extraction of 74001,434 and the 600³C extraction of 74001,2203 released slightly lighter N with N15 N V8x. N15 N data of the s 1100³C steps are shifted to heavier N, signaling the release of spallation 15 N. The N release systematics and isotopic signatures observed here are in excellent agreement with data reported by Kerridge et al. [6]. Table 1 shows that for 74001 volcanic glasses the measured N abundances are within the range reported by Kerridge et al. [6]. Note that N data reported by Kerridge et al. [6] represent extractions only up
Fig. 3. N release systematics of lunar anorthosite 67915 (47.4 mg) and basalt 75075 (621.9 mg). The numbers close to data bars indicate the extraction temperature in 100³C. For 75075, four pyrolysis steps (700³C, 900³C, 1000³C and 1100³C) de¢ne a uniform signature of +13x, that contrasts the s 1100³C steps with N15 N s 320x, and represent the indigenous N signature.
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75075, N released in the temperature range of 400³C to 1000³C is apparently not a¡ected by a spallation component. The 1100³C is only marginally a¡ected by spallation component. The N isotopic compositions in these steps are consistent with the N15 N = +13x identical to that observed in volcanic glass and in anorthosites. Unfortunately in anorthosite 67915, the 700^1000³C temperature steps do not permit an identi¢cation of an intermediate plateau. However, the 1000³C step is consistent with a N15 N = 13x value. The 1100³C step is already a¡ected by the spallation component. The release systematics of N in all lunar samples, with the exception of basalt 75075, show a characteristic pattern that is interesting. The lowest combustion step, intended as a cleaning step, shows heavy N15 N. This heavy N release is followed by release of N that is somewhat lighter, close to terrestrial in the case of anorthosite 60025 and breccia 67915 and V8x for the volcanic glass 74001. We should also point out that there is excellent reproducibility in the release patterns as well as the N15 N in replicate runs of both 60025 and 74001. The N15 N = +13x signature appears ¢rmly established for the selected samples and this indicates the existence of a uniform indigenous nitrogen component on the Moon. Kim et al. [17] studied the nitrogen isotopic signature of the recent solar wind in surface samples of rock 68815 and soil 67601. We note that in their study the temperature steps that were not a¡ected by either solar wind or spallation components, released nitrogen with N15 N = V+13x, consistent with the indigenous lunar nitrogen signatures obtained here. We note that the isotopic composition of indigenous N is in agreement with approximate values suggested in the literature [2,5,6]. An average N15 N = 13.0 þ 1.2x is obtained from nitrogen released in the steps s 700³C to 6 1100³C (plateau release). We consider this signature to represent the indigenous lunar nitrogen. Further, we make the assumption that in the absence of a spallation N component, the s 1100³C steps reveal the same lunar N15 N = 13x component. Thus, when considering indigenous N abundances, all temperature steps s 700³C need to be
taken into consideration. We obtain the following indigenous N abundances: 1.4 þ 0.4 ppm in anorthosites, 3.1 þ 0.6 ppm in volcanic glasses, and 0.7 ppm in basalt 75075. These N abundances in anorthosites and in the basalt are comparable to previously reported values [1^3,5], but the volcanic glasses are enriched in the indigenous N. Considering that solubility experiments predicts only V0.08 ppm N in the silicate melt [18], the high N abundances in 74001 glasses ( s 100 ppm, in two samples reported by Kerridge et al. [6], see footnote to Table 1) may appear surprising. However, the indigenous N is surface-correlated in the fumarolic volcanic glasses. The abundances of the volume-correlated nitrogen component may be much lower, e.g. the lowest reported N = 0.18 ppm (Kerridge et al. [6]) may represent an upper limit for the volume-correlated indigenous N. 4. Indigenous N signatures : implications 4.1. Lunar vs. terrestrial signatures Fig. 4 shows N15 N values (total N for each sample) plotted vs. e¡ective 2Z-exposure time (discussed later) on the lunar surface. The data de¢ne
Fig. 4. N15 N vs. the e¡ective irradiation time (Myr/N (ppm)) on the lunar surface. Replicate analyses of 60025 are in excellent agreement with each other. Data de¢ne a linear correlation and the intercept on the N15 N axis yields a signature of +13x for the indigenous N in bulk samples.
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an approximately linear array. The intercept on the N15 N axis de¢nes the indigenous lunar nitrogen signature and the slope the 15 N spallation production rate, discussed later. This correlation is consistent with an indigenous lunar nitrogen signature N15 N = +13x for bulk samples. As discussed earlier, identical indigenous N signatures are derived from the oldest (4.44 Ga) studied ferroan anorthosite 60025, from anorthositic breccia 67915 with a formation age of V4.1 Ga, from volcanic glasses formed by lava fountaining 3.7 Ga ago, and from a basaltic rock with a crystallization age of V3.7 Ga. The fact that the indigenous N15 N values derived from lunar samples with di¡erent formation histories, mineralogy, and sources within the Moon leads to the conclusion that there was a uniform indigenous lunar nitrogen component. These signatures are not compromised by solar wind components. The Sm^Nd age of anorthosite 60025 dates back to the time of the early evolution of the Earth^Moon system and naturally leads to another planetology issue. If the lunar materials are derived from a proto-earth, by way of a giant impact, one has to expect approximate consistencies in the N isotopic signatures. However, since the primitive nitrogen isotopic signature of the Earth is poorly constrained, and since fractionation e¡ects at temperatures implied for the separation and formation of the Moon are di¤cult to assess, no simple answer can be expected. Nevertheless, it appears appropriate to make some comparisons. Terrestrial diamonds provide a N15 N range of 311x and +6x [19]. This range has been interpreted as being due to isotopic fractionation of a primitive mantle composition with initial N15 N 6 311x [19]. In this case, light N of terrestrial diamonds is not inconsistent with the indigenous N signature of the Moon, if lunar nitrogen represents a residue of a larger reservoir, and if in the formation process nitrogen was fractionated by 20^40x, favoring the heavy isotope. On the other hand, Kurz et al. [20] and Rison and Craig [21] noted that deep-seated mantle material carried high 3 He/4 He ratios, and these samples show signi¢cantly heavier N, with N15 N values up to +14 þ 1.2x (Exley et al. [22]). We note that indigenous lunar nitrogen signatures do
665
match signatures in samples from the Lau Basin [22] associated with high 3 He/4 He ratios. Marty and Humbert [23] report analyses of oceanic basalts with N15 N in the range of 37x and +7x which has been interpreted to represent a mantle signature of N15 N = 37x and contamination with crustal material of N15 N = +7x. Similarly Sano et al. [24] and Mohapatra and Murty [25] have shown that the primary signatures of terrestrial mantle have been modi¢ed by variable addition of recycled components from sur¢cial reservoirs. 4.2. Can indigenous N explain the variations in the regolith?
15
N/14 N
Lunar regolith samples have been extensively used for studies of the implanted `solar' component (e.g. [2,26^28]). Nitrogen studies of the lunar regolith samples have shown that (i) N isotopic ratios show at least 30% variation, (ii) N/36 Ar ratios are an order of magnitude larger than `solar' values (N/132 Xe about 3 times higher than `solar') [26,28], (iii) Ar/Kr/Xe ratios show no evidence of fractionation due to losses [29], and (iv) the C/N ratios in regolith material are well correlated [30]. Secular changes in the solar wind source [27,28] and a non-solar source for majority of the N are evoked [31,32] to explain this lunar regolith nitrogen `excesses'. Geiss and Bochsler [33] and Signer et al. [34] suggested that the lunar indigenous component released from the Moon's interior and subsequently re-implanted in the regolith (e.g. [35]) could explain the very light N released at intermediate temperatures of some lunar soils and breccias. The required indigenous nitrogen signature is N15 N 6 3200x, close to the lightest measured N in lunar soils. If, according to the model, the relative abundances of the indigenous light nitrogen component decreased as a function of time, this would account for the observed variation in the 15 N/14 N ratio in regolith samples. However, the lunar indigenous N signature obtained in this work is clearly inconsistent with the composition required by the model. In order to understand the lunar regolith nitrogen data, both the nitrogen `excesses' and the var-
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iation in the isotopic composition need to be addressed [2,27,28,36]. The present study eliminates one of the suggested non-solar sources, invoked to explain lunar nitrogen excesses. 5. Cosmic-ray production rates of Moon
15
N on the
In contrast to lunar soils, which have abundant solar wind derived nitrogen and noble gases, igneous lunar rocks have low N abundances. Hence, these rocks are in principle good candidates for deriving cosmic-ray induced production rates of nitrogen. One of the objectives of the present study is to quantify the cosmic-ray induced production rate of nitrogen in lunar igneous rocks with documented surface exposure history. Oxygen is the major target for the production of nitrogen, and since the O abundance is rather constant in lunar rocks, no signi¢cant compositional dependence is expected. The proton induced production of 15 N is through the direct reaction 16 O(p,2p)15 N and also through the 16 O(p,pn)15 O reaction. Reedy [37] estimated the 15 N production rates (P(15 N)) in the lunar regolith from the 16 O(p,2p)15 N reaction. The contribution of the 16 O(p,pn)15 O reaction was estimated to be 55% of the direct production, based on a comparison of the predicted 16 O(p,2p)15 N production rate with the empirical estimates of Becker et al. [2]. Mathew and Murty [38] utilized the 16 O(p,2p)15 N and 16 O(p,pn)15 O cross-section measurements that became available after the Reedy [37] calculations and estimated a P(15 N) 30% higher than derived by Becker et al. [2]. Important are also neutron induced production reactions 16 O(n,2n)15 O and 16 O(n,np)15 N. These reaction cross-sections, however, are not available and theoretical estimates mostly rely on the proton induced reaction cross-sections for estimates of the production rates. As indicated earlier, lunar basalt 75075 had a single stage exposure history close to the lunar surface for 143 þ 5 Ma [15], and average lunar production rates P(15 N) can be derived for nearsurface locations. The 700³C to 1000³C extraction steps of 75075 released a uniform indigenous N
signature (N15 N = +13x), which contrasts with the higher temperature steps (N15 N s 320x), shifted due to the release of spallation-produced 15 NC (Fig. 3). The entire excess is taken as the 15 NC spallation component, which accounts for V30% of the measured 15 N in lunar basalt 75075. Therefore, the production rate is more precise than those derived from other samples. From spallation 15 NC excesses in 75075 we calculate a production rate P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma, and this P(15 N) is not compromised by assumptions about the indigenous N signature. The uncertainties in the production rates listed include a 5% systematic uncertainty in the N concentrations. A sub-surface production rate applies for the two 60025 anorthosite measurements, but the spallation component is much smaller. This anorthosite has the short cosmic-ray exposure age of South Ray Crater (2.0 Ma). Nevertheless, the s 1100³C extractions of 60025 (both -A and -B), representing V10% of the total N, show clear shifts to heavier signatures, relative to the plateau release of N15 N = +13x in the 700³C to 9 1100³C steps. An average production rate of 5.6 þ 0.9 pg 15 N/g/Ma is obtained from 60025-A and 60025-B, consistent with the derived production rates from lunar basalt 75075. Anorthositic breccia 67915 had a partially shielded exposure to cosmic rays on the lunar surface and needs a shielding correction and the volcanic glasses from a depth of 55.5 cm in the drill core determine the 15 N production rates at an inferred effective depth. First, consider breccia 67915, hammered o¡ the `Outhouse' boulder, a 2 m rock resting on the rim of the North Ray Crater. This rock was partially shielded by the V10 m `House' boulder and its present e¡ective cosmic-ray exposure geometry of 1.33Z can be estimated from the photo-documentation on the lunar surface (shielding angle 45 þ 15³). The exposure geometry requires an adjustment in the calculated P(15 N) = 3.6 pg 15 N/g/Ma (to normalize to the exposure geometry 2Z) and corresponds to a production rate P(15 N) = 5.4 þ 1.2 pg 15 N/g/Ma corrected for the shielded exposure. The 15 N excesses in the two 74001 samples, coupled to an exposure age of 33 Ma (Eugster
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et al. [13]), yield an average P(15 N) = 6.5 þ 1.0 pg 15 N/g/Ma, slightly higher than but consistent with the rate determined from 75075. We note that the location of these 74001 samples corresponds to a shielding depth of V80 g/cm2 beneath the lunar surface, and a somewhat enhanced production in our 74001 samples is consistent with an expected increase in the P(15 N) due to shielding. In addition to the recent 17 Ma exposure at shielding depth of V80 g/cm2 , 74001,434 and 74001,2203 apparently received pre-exposure irradiation for V16 Ma at a shielding depth in the range of 85^120 g/cm2 (Eugster et al. [13]). Eugster et al. [13] also noted that the spallation 21 Ne, 38 Ar, 83 Kr and 126 Xe abundances are constant for the layer with a shielding depth range of 20 to 100 g/cm2 (from which the two present 74001 samples and the three 74001 samples of Becker and Clayton [39], discussed below, were taken). The di¡erent production rates discussed above are consistent and permit the calculation of an average production rate (Table 2) of P(15 N) = 5.8 þ 0.6 pg 15 N/g/Ma, about 60% larger than that estimated by Becker et al. [2]. Becker and Clayton [39] reported V0.22 ng15 N/g spallation excesses in 74001/2 assuming a trapped signature of N15 N = 10x. When recalculating their data using a N15 N = 13x, determined here, for the indigenous component, spalTable 2 Measured 75075
15
N abundances and cosmic-ray induced
15
667
lation excesses in agreement with the values measured by us are obtained. Spallation excesses in 74001/2 were not reported by Kerridge et al. [6], since the samples were heated only up to 1050³C, below the temperature where most of the spallation nitrogen appeared. Thus the cosmogenic 15 N excesses in ¢ve samples of 74001 are uniform. The £at 15 N depth pro¢le is similar to the cosmogenic noble gas depth pro¢les reported by Eugster et al. [13]. The observation that the cosmogenic 21 Ne and 15 N concentrations are constant for a shielding depth range of 20 to 100 g/cm2 in 74001 allows us to derive a production rate ratio P(15 NC )/ P(21 NeC ) in silicates. The calculated P(15 NC )/ P(21 NeC ) = 4.0 þ 0.3, using the 15 NC measured in this work and the 21 NeC reported by Eugster et al. [13], in agreement with model estimates by Mathew and Murty [38], also reporting a ratio of 4.0 þ 0.5. Fig. 4 shows a plot of the 15 N excesses as a function of e¡ective (adjusted) 2Z-irradiation time. In this correction, partially shielded sample (67915) is adjusted, using photo-documentation on the lunar surface, to the same 2Z surface irradiation conditions. The ¢gure shows that the 60025 and 74001 aliquots, the adjusted 67915, and basalt 75075 are on the same trend line and also shows that the 15 N excesses increase linearly
N production rates derived from lunar samples 60025, 67915, 74001, and
Sample
Shielding depth (cm)
15
Na (ng/g)
N15 N (x)
Exposure agec (Ma)
15
NdC (ng/g)
Production rate (pg/g/Ma)
60025-A 60025-B 67915b 74001,434 74001,2203 74001,1072 74001,1082 74001,1085 75075
sub-surface sub-surface sub-surface 55.5 cm 55.5 cm 38.5 cm 47.5 cm 59.0 cm sub-surface
0.57 0.57 3.55 10.9 6.2 n.r. n.r. n.r. 2.8
32.3 þ 1.6 33.0 þ 1.4 64.3 þ 1.0 33.1 þ 1.0 47.3 þ 1.4 n.r. n.r. n.r. 313 þ 1.1
2.0 þ 0.1 2.0 þ 0.1 50 þ 1.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 33 þ 3.0 143 þ 5.0
0.0110 0.0114 0.18 0.22 0.21 0.21, BC 0.22, BC 0.17, BC 0.83
5.5 þ 1.0 5.7 þ 1.0 5.4 þ 1.2 6.7 þ 0.9 6.4 þ 1.1 6.4 6.7 5.2 5.8 þ 0.6
Data reported in literature by Becker and Clayton [39] for three 74001 drill core samples are included for comparison. n.r., not reported; BC, Becker and Clayton [39]. a Only the s 1000³C extractions with cosmogenic 15 N are included. b The production rate derived from anorthosite 67915 requires a correction for shielding (see text). c See sample selection for references to the cosmic-ray exposure ages of the samples. d15 NC are evaluated using 15 NC = 15 N{(N15 N313.0)/1000}.
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with e¡ective exposure time. Slope of the correlation line in Fig. 4 implies a P(15 N) = 5.8 þ 0.7 pg 15 N/g/Ma, consistent with the average P(15 N) obtained from individual samples. R. Wieler and B. Marty brought to our attention that Hashizume et al. [40] reported a 15 N/ 38 Ar production rate ratio of 13.1 þ 1.6 atoms/ atom in an ilmenite separate, about a factor of two higher than estimates based on the then existing 15 N and 38 Ar production rates. Kim et al. [41] deduced a silicate 4Z production rate of 11.6 pg 15 N/g/Ma in the meteorite Enon. The new lunar production rates derived here are consistent with the rate determined in Enon, when normalized to the same exposure geometry. P(15 N) rates derived here are also in agreement with the production rates calculated by Mathew and Murty [38]. 6. Conclusions Indigenous lunar nitrogen composition and abundances have been determined in lunar samples with short exposure ages. The studied samples include ferroan anorthosite 60025, one of the oldest crustal rocks on the Moon, volcanic glassrich samples of the double-drive drill tube 74001, lunar basalt 75075 with a well-determined exposure age, and anorthositic breccia 67915. The abundances of the indigenous lunar nitrogen are at ppm level, two orders of magnitude lower than the observed abundances in lunar regolith soils with implanted solar gases. The N isotopic composition is found to be uniform in all lunar rocks and is constrained to be N15 N = +13.0 þ 1.2x. The N15 N = 13.0x derived for the indigenous lunar nitrogen eliminates one of the suggested mechanisms for the origin of nitrogen `excesses' in lunar regolith samples ^ viz. degassing of indigenous lunar nitrogen followed by re-trapping into the regolith. We calculate a near-surface 15 N cosmic-ray production rate of P(15 N) = 5.8 þ 0.6 pg 15 N/g/ Ma for the lunar surface. This rate is consistent with both theoretical estimates based on production rate calculations and with meteoritic production rates determined from Enon silicates [41].
The production rates derived here, however, exceed that given by Becker et al. [2] by 60%. A 15 N/ 21 Ne production rate ratio of 4.0 þ 0.3 is derived for silicates. Acknowledgements We thank NASA, JSC for the lunar samples studied in this work and K. Geddes for assistance in sample preparation. The paper bene¢ted from detailed and constructive reviews by R. Wieler, B. Marty and S.V.S. Murty. Work was supported by NASA grant NAG5-8167.[AH] References [1] O. Mu«ller, Solar wind nitrogen and indigenous nitrogen in Apollo 17 lunar samples, Proc. 5th Lunar Sci. Conf., 1974, pp. 1907^1918. [2] R.H. Becker, R.N. Clayton, T.K. Mayeda, Characterization of lunar nitrogen components, Proc. 7th Lunar Sci. Conf., 1976, pp. 441^458. [3] D.J. DesMarais, Light element geochemistry and spallogenesis in lunar rocks, Geochim. Cosmochim. Acta 47 (1983) 1769^1781. [4] B.L. Barraclough, K. Marti, In search of the Moon's indigenous volatiles: noble gases and nitrogen in vesicular lunar glasses, Lunar Planet. Sci. XVI, 1985, pp. 31^32. [5] S.V.S. Murty, J.N. Goswami, Nitrogen, noble gases and nuclear tracks in lunar meteorites MAC88104/105, Proc. 22nd Lunar Planet. Sci. Conf., 1992, 225^237. [6] J.F. Kerridge, O. Eugster, J.S. Kim, K. Marti, Nitrogen isotopes in the 74001/74002 double-drive tube from Shorty Crater, Proc. 21st Lunar Planet. Sci. Conf., 1991, pp. 291^299. [7] R.W. Carlson, G.W. Lugmair, The age of ferroan anorthosite 60025: oldest crust on a young Moon?, Earth Planet. Sci. Lett. 90 (1988) 119^130. [8] K. Marti, U. Aeschlimann, P. Eberhardt, J. Geiss, N. Gro«gler, D.T. Jost, J.C. Laul, M.-S. Ma, R.A. Schmitt, G.J. Taylor, Pieces of the ancient lunar crust: Ages and composition of clasts in consortium breccia 67915, Proc. 14th Lunar Planet. Sci. Conf., J. Geophys. Res. 88 (1983) B165^B175. [9] C. Meyer, D.S. McKay, D.H. Anderson, P. Butler, The source of sublimates on the Apollo 15 green and Apollo orange glass samples, Proc. 6th Lunar Sci. Conf., 1975, pp. 1673^1699. [10] E.H. Cirlin, R.M. Housley, A £ameless atomic absorption study of the volatile trace metal lead in lunar samples, Proc. 8th Lunar Sci. Conf., 1977, pp. 3931^3940. [11] U. Kra«henbu«hl, Distribution of volatile and non-volatile
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[27] J.F. Kerridge, Long-term compositional variation in solar corpuscular radiation: evidence from nitrogen isotopes in the lunar regolith, Rev. Geophys. 31 (1993) 423^437. [28] R.N. Clayton, M.H. Thiemens, Lunar Nitrogen: Evidence for secular change in the solar wind, in: R.O. Pepin, J.A. Eddy, R.B. Merrill (Eds.), Proc. Conf. Ancient Sun, 1980, pp. 463^473. [29] R. Wieler, H. Baur, Fractionation of Xe, Kr, and Ar in the solar corpuscular radiation deduced by closed system etching of lunar soils, Astrophys. J. 453 (2 Part 1) (1995) 987^997. [30] J.F. Kerridge, I.R. Kaplan, C. Petrowski, Carbon isotope systematics in the Apollo 16 regolith, Lunar Planet. Sci. IX, 1978, pp. 618^620. [31] R. Wieler, F. Humbert, B. Marty, Evidence for a predominantly non-solar origin of nitrogen in the lunar regolith revealed by single grain analyses, Earth. Planet. Sci. Lett. 167 (1999) 47^60. [32] D.R. Brilliant, I.A. Franchi, C.T. Pillinger, Nitrogen components in lunar soil 12023 - complex grains are not carriers of isotopically light nitrogen, Meteoritics 29 (1994) 718^723. [33] J. Geiss, P. Bochsler, Nitrogen isotopes in the solar system, Geochim. Cosmochim. Acta 46 (1982) 529^548. [34] P. Signer, H. Baur, P. Etique, R. Wieler, Nitrogen and noble gases in the 71501 bulk soil and ilmenite as records of the solar wind exposure: Which is correct?, In: Workshop of past and present solar radiation, Mainz, LPI tech. Report 86^02, Lunar and Planetary Institute, Houston, 1986, pp. 34^35. [35] R.H. Manka, F.C. Michel, Lunar atmosphere as a source of lunar surface elements, Proc. 2nd Lunar Sci. Conf., Lunar and Planetary Institute, Houston, 1971, pp. 1717^ 1728. [36] R.H. Becker, An evaluation of possible explanations for `excess' nitrogen in the lunar regolith, Lunar Planet. Sci. XXVI, Lunar Planet. Institute, Houston, 1995, pp. 89^90. [37] R.C. Reedy, Cosmic-ray-produced stable nuclides: Various production rates and their implications, Proc. 12th Lunar Planet. Sci. B, Lunar and Planetary Institute, Houston, 1981, pp. 1809^1823. [38] K.J. Mathew, S.V.S. Murty, Cosmic ray produced nitrogen in extraterrestrial matter, Proc. Indian Acad. Sci. (Earth Planet. Sci.), 102, 1993, pp. 415^437. [39] R.H. Becker, R.N. Clayton, Nitrogen isotopes in some exceptional Apollo 12 and Apollo 17 soils (abstract), Lunar Planet. Sci. IX, Lunar and Planetary Institute, Houston, 1978, pp. 64^66. [40] K. Hashizume, B. Marty, R. Wieler, Single grain analyses of the nitrogen isotopic composition in the lunar regolith in search of the solar wind component, Lunar Planet. Sci. XXX, 1999, Lunar and Planetary Institute, Houston, CDROM. [41] J.S. Kim, Y. Kim, K. Marti, Isolation of cosmic-ray produced nitrogen in meteoritic silicates and some implications, Lunar Planet. Sci. Conf. XXV, Lunar and Planetary Institute, Houston, 1994, pp. 701^702.
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