Nitrogen isotopic signatures in Cape York: Implications for formation of Group III A irons

Geochimica et Cosmochimica Acta. Vol. 58. No. 7. DD. 1841-1848. 1994 Copyright 0 1996ksevier Scienk Ltd Printed in the USA. Allrights reserved

L22izh

Pergamon

0016-7037/94 $6.00 + .OO

Nitrogen isotopic signatures in Cape York: Implications

for formation of Group III A irons

S. V. S. MURTY * and K. MARTI Department of Chemistry, 03 17, University of California, San Diego, La Jolla, CA 92093-03 17, USA (Received Mu!: 10, 1993; accepted in revised,form November 28, 1993)

Abstract-Nitrogen isotopic abundances and concentrations of cosmic-ray-produced 3He,, “Net, and “Ar, are reported in metal and troilite separates of the Cape York III A iron meteorite. High resolution (20 step release) nitrogen isotopic data are obtained in a 0.2 g metal chip which contains 33.1 ppm N of 6”N( %o) = -94.8 + 1.1. Two troilite samples each contain 1.13 ppm N with signatures 6”N( %o) = -10.7 and -3.8, respectively. Spallogenic *‘Ne is nineteen times more abundant in troilite than in metal due to cosmic ray reactions on sulphur. HowLver, the distinct nitrogen isotopic signatures of troilite cannot be accounted for by the additional presence of spallogenic nitrogen. The nitrogen isotopic signatures of the metal for the temperature steps 2 1200” are within a narrow range of 4%0 and reveal a light N reservoir of 6 15N( %OO) = -94.8%0 in Cape York. The nitrogen isotopic signatures of the metal and troilite phases appear to conflict with a proposed magmatic origin, unless the observed signature in troilite is due to a secondary alteration process. We discuss possible genetic links between iron meteorite groups based on nitrogen isotopic signatures in metal and oxygen isotopic signatures in silicate inclusions. Our nitrogen isotopic studies reveal artifacts due to adsorption/desorption of N2 by reactive metal vapors deoosited on the surfaces of the auartz extraction system, and we discuss an extraction protocol which reduces nitrogen loss and exchange effects. INTRODUCTION

a heavy nitrogen reservoir. Identification of such a nitrogen will help in understanding 6 15N variations among various meteorite groups. Replicate analyses of some meteorites ( FRANCHI et al., 1993) produced much larger variations than those obtained from standards, suggesting that nitrogen is distributed heterogeneously. Nitrogen abundance variations may be due to contributions from specific phases such as nitrides ( BUCHWALD, 1975).Detailed high resolution studies of nitrogen systematics in separated phases of meteorites appear to offer a suitable approach to achieve this documentation. Compared to stone meteorites, the mineralogy of iron meteorites is obviously simpler. When considering the metal phase in particular, the cosmic-ray production rate of nitrogen is small and does not alter the isotopic composition of indigenous nitrogen. If there are no late thermal events, the metal can preserve the original nitrogen signature intact. This characteristic makes nitrogen a good tool to investigate if the meteorite parent body formed in a magmatic or nonmagmatic process. In a magmatic process one would expect uniform 6 “N signatures in the metal, while heterogeneity might still be preserved in nonmagmatic processes. PROMBO and CLAYTON ( 1993) found that the two major iron meteorite groups I AB and III AB which comprise 68% of all irons and which are believed to be of nonmagmatic and magmatic origin, respectively ( WASSON et al., 1980), have very light nitrogen. The single step melt analyses of Cape York, a group III A member, revealed very light nitrogen, but of varying isotopic signatures (e.g., FRANCHI et al., 1993, and Table 5). We undertook a detailed study of nitrogen isotopic systematics using a stepped heating pyrolysis of Cape York metal and troilite to better understand the nature of its nitrogen reservoir. We also measured noble gases in these samples to monitor fractionation effects on nitrogen, as well as to decipher the spallation nitrogen content. reservoir

NITROGEN ISONE OF the more abundant elements in the solar nebula, but it is incorporated into solid bodies only at a trace level. This is partly due to the thermodynamic instability of the nitrides (FEGLEY, 1983)in solids and partly due to the inert nature of the dominant molecular form in which it exists under solar nebular conditions (NORRIS, 1980). Therefore, very sensitive static mass spectrometer techniques were developed (FRICK and PEPIN, 1981; FREDRIKSSON et al., 1985; WRIGHT et al., 1988) for precise isotopic analysis of small samples. Earlier studies of nitrogen in meteorites have revealed a wide range of isotope ratios “N / 14N both in stone as well as iron meteorites ( KUNG and CLAYTON, 1978; PROMBO and CLAYTON, 1993; FRANCHI et al., 1987; NEAL et al., 1989). Even though most of these differences appear to be primary, their origin is still elusive. The broad groupings of 6 15N values among various classes of stone and iron meteorites ( KUNG and CLAYTON, 1978; PROMBO and CLAYTON, 1993; FRANCHI et al., 1987, 1993; NEAL et al., 1989) are clearly suggestive of a large scale heterogeneity of nitrogen isotopes in the solar nebula. But to what extent the heterogeneity reflects distinct nuclear signatures and/or fractionation effects remains to be explored. Extreme isotopic variations observed in interstellar diamond and SIC isolated from carbonaceous chondrites (LEWIS et al., 1987; ZINNER et al., 1987) indicate a nucleosynthetic origin, while the large 615N excesses observed in the meteorites Bencubbin and Weatherford and in polymict ureilites ( PROMBO and CLAYTON, 1985; FRANCHI et al., 1986; GRADY and PILLINGER, 1988) also indicate the existence of

* Presenf address: Earth Sciences and Solar System Physical Research Laboratory. Navrangpura, Ahmedabad India.

Division, 380 009,

1841

S. V. S. Muriy and K. Marti

1842 EXPERIMENTAL

PR~EDURE

The studied piece of Cape York was obtained from the main mass Agpalilik and was located >40 cm below the surface. The sample heating and gas extraction procedures were discussed earlier (MURTY and MARTI, I987), but some relevant details will be elaborated here. Recovery of Nitrogen in the Extraction Process In the early studies of nitrogen in metal, we encountered problems of nitrogen loss in the extraction system. We have investigated the problem and worked out a protocol that minimizes nitrogen loss and avoids fractionation effects. In the following we discuss our loss Calibrations and document the exchange effects. The first Cape York metal analyzed yielded only 7 ppm N, with 6”N = -80%~. The value reported by F’ROMBO and CLAYTON ( 1993) for Cape York metal is 35 ppm N, with 615N = -84%0. Our data indicated incomptete/complete recovery, though the 615N was only slightly different. We investigated the problem of nitrogen loss in the extraction system (confined to the MO crucible and the surrounding, water cooled, quartz walls) under varying conditions using known splits of nitrogen from our air standard. When using a new MO crucible which had been thoroughly degassed, a significant loss of nitrogen was observed. This may indicate the existence ofactive sites that can chemisorb nitrogen. This nitrogen loss almost vanished after vaporizing piecesofquartz in the crucible. Since we used aluminum foil to wrap our samples and nickel fortemperature calibration of the crucible, we tested both aluminum and nickel vapor deposits on the walls of the extractionbottle. The aluminum foil did not cause an appreciable loss of nitrogen, but nickel vapor increased the nitrogen loss rate substantially. Subsequently silicates were vaporized to cover the nickel deposits. For a given set of conditions of the crucible, we checked the effect of crucible temperature and the residence time of nitrogen in contact with the crucible. In all cases the loss of nitrogen increased with increasing crucible temperature and also with increasing contact time of nitrogen with the crucible; these data are summarized in Table 1. We further observed that nitrogen which was lost in a previous exposure is partially exchanged with successive splits of nitrogen in the extraction bottle. This resulted in shifts in the isotopic composition towards those of the previous nitrogen sample. If the nitrogen released from a sample at successive extraction temperatures was isotopically uniform, then the exchange process did not affect the measured isotopic composition. This was studied by exposing atmosp~e~c nitrogen to the crucible. Under all conditions the observed S lSNwas the same, even though the recovery was not. This is further demonstrated by the Cape York metal-3 results, where the > I2OO”Cextraction steps yield the same 6 “N value, These tests show that if proper procedures are not adopted, both the nitrogen contents and isotopic composition in metals can be affected by loss and exchange reactions. It appears that some of the discrepancies reported in the literature (Table 5) may be due to similar Table 1. Nitrogen recovery tests using different surface conditions of the quartz extraction system and the MOcrucible. t System Condition

vaporized

Al foil vaporized

Crucible Exposure Recovery Temp. Tnne of Nittogen (min.) (“c, f%> 1 >99 1400 100: 1700 1000 :Z

1 2: :o”

:; 90 a2 24 66

22

1

>99

Ni 22 1 7 vaporized &own splits of atmospheric N2 in standard pipettes were used for these tests.

problems. For our work we adopted procedures which result in minimum losses of nitrogen. During the extraction we continuously adsorb the evolved gases onto a zeofite at liquid Nz temperature until 10 min after the R.F. power is shut off. The volume containing the extraction bottle is isolated before desorbing gases from the zeolite, to prevent contact between nitrogen and the crucible. A volume split of collected gas (usually - 5%) is isolated in the processing line for nitrogen measurements and the rest of the gas is exposed to a cooling titanium sponge, starting at 8OO”C, and the noble gas mixture is adsorbed onto a charcoal finger at liquid Nz temperature. The gas phase now contains only helium and neon. This He-Ne mixture is then exposed to a pair of Ti-Pd getters (400 and 5OO”C,respectively) and a Ti-Zr getterloy at room temperature to further remove hydrocarbons and Hz, respectively. Also, a charcoal fmger at liquid N2 tem~rature is exposed to this He-Ne mixture before its admission to the static mass spectrometer. At the end of the analysis of neon and helium, the background peaks at masses 2, 18, 40, 42, and 44 are measured in order to assess interferences at masses 3( Hi ), 20( H2i80t), 20(Ar2”), 21 (C3Hp). and 22(CO?). The nitrogen fraction is exposed to Cu/CuO at 700°C for IO min to oxidize CO and organic contaminants to CO2 and f-I@. The CuO is then allowed to coal to 400°C for 15 min, during which time the O2 in the gas phase is removed by the Cu/CuO. The H20 and COZ are frozen in a cold finger at liquid NZ temperature and the uncondensed gases are collected onto a stainless steel (S.S.) mesh. The cold finger is now isolated, and gases are desorbed into the gas phase. Condensibles are trapped again and separated from gases occluded in the frozen CO2 and H20. The gas phase is once more transferred to the S.S. mesh, and the CO, and Hz0 are pumped off. The pressures of CO1 and H&J are monitored while pumping, and if considered excessive, the NZ fraction is cleaned a second time on the Cu/CuO. In most cases one cleaning was sufficient, and only a few cases exist where a second cleaning on Cu/CuO was needed. The nitrogen is allowed to desorb from the S.S. mesh at room temperature and then is admitted to the mass spectrometer and measured on the Faraday cup. Peaks and baselines at masses 28, 29. and 30 were measured by the peak jumping method, and generally about 8 cycles were run, with three measurements of each peak being obtained. Data reduction was done off line and measured isotope ratios were extrapolated to time zero (gas admission). Interferences and Bianks Measurement of the mass 30 peak was required to assess the interference by “Ci60. After each analysis we scanned the mass 30 and 3 I peaks using the multiplier to precisely assess the interference at mass 30. From the mass 3 I peak we estimate that 39.5% of mass 30 background is due to CO. In our correction schedule we attribute all excesses over “N2 at mass 30 to 12C’a0. This correction does not alter the d “N by more than I %Oin most cases. Blanks were measured prior to and after each sample under identical conditions and are reproducible within error limits, with a range &15N= 3 i 5%0.Total nitrogen blanks which include contributions by the CuO furnace, the processing line, and the extraction system ranged from (1 to 5 ng and increased with temperature and extraction time. The 1500°C, 30 min blank was 2 ng. Unless indicated, blank corrections were <5W and often ~2%. The extraction line was monitored also for nitrogen losses. When necessary, the extraction system was baked and the crucible conditioned by vaporizing pieces of quartz until no further loss of nitrogen was observed. Mixed He-Ne standards ( 3He/4He mixture and air neon) and air standards (for N2 and argon) were used to determine the mass discriminations and the sensitivities. Typical mass discrimination values (in %/amu) are 12 and 1.9 for helium and neon, respectively, using the multiplier and 0.6 for N2 and argon (Faraday). RESULTS Cosmic-Ray-Produced

Noble Gases in Cape York

The spallogenic 3He, “Ne, and “Ar concentrations for metal-3 and troilite-2 are presented in Table 2. The neon concentrations

in metal and troilite are exceedingly low. Our

Isotope composition of ?i’in the Cape York iron meteorite Table 2. condensations of ~osrni~-by-pmdu~ 3Hee,

ZrNe,, 3%~ and of 4He in metaI-3and troilite-2of Cape York. (Uncertaintiesin concentrationsare 10%). Temp. c Metal-3 (0.21 g) 750 1000 1100 1200 1300 1400 1500 1600 1700 Total Troilite-2 (0.70 g) 400 700 900 1050 1300

Total

3He,

ZtNe, 3%4r, STP/g)

(lo-12 cm3

51 122 248 334 470

521

0.5 2.3 5.8 5.9 8.5

9.2

4He

(10-o cm3 SIP/g)

6.8 17.7 40.0 43.7 53.5

70.7

1,090 345 <5

13.9 0.3 10.1

122 15.0 308

3180

46.4

677

38.2 742 3,040 6,120 628



226:
885

475

335

11,170


objective in measu~ng neon is to estimate spallogenic 2’Ne and to correct for the spallogenic j5Nc in measured nitrogen. In calculating spallation Ne, contents, the measured neon in metal and troilite is assumed to be a mixture of trapped neon and spallation neon as measured in the metal phase (e.g., VOSHAGE et al., 1983) and in troilite (BEGEMANN, 1965 ). Since the composition of trapped neon is uncertain, we used both atmospheric and Ne-A compositions, and the uncertainty in the choice of trapped neon is included in the quoted uncertainties. Since 4He concentrations in metal are close to blank levels, all ‘He is taken to be of spallogenic origin. The 3RArcconcentration of metal-3 is in very good agreement with the metal-1 and metal-2 data. The “Ne, content of troilite is nineteen times higher than in the metal, due to the larger production rate from sulfur. On the other hand, we would expect similar amounts of 3He, in both metal and troilite. The depletion of 3He, in the metal by a factor of 3.5 indicates a loss of 3He from the metal (S~MJLT~, 1967). If the original amount of 3Hec in the metal is assumed to be the same as in troilite (assuming that *‘Ne and 38Arc are quantitatively retained), we calculate the radio ( 3He/3*Ar)c = 16.5 for the metal, a reasonable value for iron meteorites (VOSHAGE et al., 1983; HONDA, 1985). Using the 93 Ma exposure age ( MURTY and MARTI, I987), production rates of 5.0 X 10 -” cm3 STP/g Ma and 7.3 X IO-” cm3 STP/g Ma are obtained for 2’Ne and 38Ar, respectively. These extremely low production rates indicate very heavy shielding conditions. It is interesting to note that the troilite shows amounts of 4He above blank levels in three temperature fractions (700,900, and 1050°C). The total 4He = 335 X lob9 cm3 STP/g is at least a factor of 3 higher than expected from spallation reactions, suggesting a radiogenic contribution. The reported uranium content of 3 ppt in the Cape York troilite (CHEN and WASSERBURG, 1983) can account for only 2% of the excess 4He. However, FISHER ( 198.5) reported the detection of micro-inclusions rich in uranium and heterogeneously distributed in the troihte of Cape York.

1843

~pallation Correction in the Nitrogen Data

The calculated spallation Ne, in the metal is *‘Ne, = 46.4 X lo-‘* cm3/g. For an iron target, 15N, is expected to be <*‘Ne,. This amount is insignificant and does not change the “N abundance, even if all spallation 15N, is released in one step, e.g., the melting step. Since in troilite total nitrogen is 30 times smaller and *‘Ne, is - 19 times larger than in the metal, we need to consider the spallation contribution. Using an empirical relation for the production rate of a nuclide which is AA mass units from the target, P (x (AA)-“, we estimate the production rates for k values ranging between 2.3 and 3.5 and obtain PLs/P21 - l/3. We calculate that the spallogenic i5Nc in troilite (-3 X lO-‘O cm3 STP/g) does not significantly alter the 6 jSN value for the total nitrogen in troilite. If the 15N, is assumed to be released in the melting step only, 6 15Nfor this step would be lowered by only 0.2%. Nitrogen in the Metal of Cape York

The nitrogen data for the three pieces of metal analyzed in this study are presented in Tables 3 and 4. During the analysis of metal- 1 we employed a different protocol for the gas collection. Throughout the heating sequence the evolved gases were in contact with the MO crucible, and the gas collection on zeolite (at liquid nitrogen temperature) started after the R.F. power was shut off. This protocol ensures maximum residence time for nitrogen in the extraction bottle and leads to higher probability of loss and exchange of nitrogen. For the analysis of metals 2 and 3 and troilites 1 and

Table 3. Nitrogen concentrations and isotopic signatures

of metal and troilite in the Cape York meteorite. Uncertainties in SlsN correspond to 95% confidence limits. Temp. Sample Metal-l (1.54 g) :z 1000 1200 1.500 1700 Total

Metal-2 (2.47 g)

600 1000 1200 1400 1700 Re-ext. Total

N

6tsN air

&pm)

(%@)


0.03 0.47 10.1 12.4 06:x 29.9

TroiIite-1 (1.80 g)

400 600 1000 1100 1300 1700 Total


Troilite-2 (0.70 g)

;g

0.011 0.433 0.374 0.283 0.005 0.020

900 1050 1300 1600

16 f -6.1 -77.8 -81.5 -49.1 -24.4 -80.2

12 f 5.2 f. 2.4 f 2.2 f 5.7 + 1.2 * 2.2

5.6 -70.7 -100.0 -82.8 -75.6 -75.9 -86.8

4 5.3 4 1.3 -+ 1.2 rt 0.7 ?I 0.8 * 2.0 f 0.9

2.5 + -0.9 f -0.7 + -3 1.8 f -16.9 f -19.9 f -10.7 + -2.5 -6.6 -2.6 -1.3 -12.2 -2.7 _^

3.7 7.4 4.2 4.5 1.1 1.5 2.9

+ 2.9 + 1.7 * 0.5 f 1.0 f 17.6 It 1.7 ._

S.

I844

V. S. Murty and K. Marti

Table 4. Nitrogen concentrations and isotopic signatures in a high resolution study of Cape York Metal-3 (0.21 g).

step

Nom.

1

Crucible Temp. 750

Heating

f

:

1000 850 1100 1300 1200

;

1500 1400

2

9 10

1600 1700

:: 6

1700

f

:

b

:;: 19 20

29 Signal @W

Contrib. @)

QIztKl 5.5 f 1.8

0:34 Gl

-66.23 0.58 -82.11 -90.11 -92.59

-23.2 ?; -67.5 rt16.3 1.6 -83.3 zt 1.3 -90.4 i: -93.0 f 1.4 1.0

3.78 4.15

0.36 0.33

-94.58 -93.37

-94.9 -93.6 + f 0.8 1.1

1.06 0.96

:::

-89.20 -93.01

-91.3 * 1.3 -95.4 f 1.3

1.67 1.09

2.4

-95.18 -96.37

-96.5 f 0.9 -98.5

:: 5:3 6.5 9.9

-91.63 -96.85 -91.69 -89.45

-101.1 -96.2 fzlzo.9 0.9 9;; i y.;

-83.46 -71.75 -70.49 -78.64

-96:8 f 1:3 -99.1 i 1.9 -97.9 f 2.2 -98.3 1.2

0:002 0.383 0.522 4.00 1.67

130.4 143.3 37.65 95.91 164.6 108.4

:tE :!

87.75 99.28 50.60 65.95

2.77 2.42 2.44 1.94

33 6 :!

52.55 30.85 111.4 120.6

1.83 0.224 0.98 1.13

:72.1

15 ;; 30

2, we adopted the procedures described in the experimental section. The metal- 1 data are useful only to demonstrate the problems of loss and exchange of nitrogen in the process. The nitrogen data for metals 1 and 2 are plotted in Fig. 1. In both metals the first temperature fractions are probably due mainly to surface contamination, while the release of indigenous nitrogen starts in the 1000°C step. The lightest nitrogen in metal-l released at the 12OO’C step is heavier than averages found in metal-2 and metal-3, indicating that exchange reactions with a heavier nitrogen reservoir took place. A comparison of the concentration data of metals 1 and 2 reveals that the loss of nitrogen is larger at higher temperatures, as observed in the control experiments. At the same time, exchange processes of nitrogen with the extraction bottle are indicated. Tests before the metal-2 analysis indicated little loss of nitrogen to the extraction bottle. In this run nitrogen was released above 1000°C. A comparison of the release data

20 CAPE

0

s

YORK

815N

(*/WI

4

htstoi -I

6.92

- 80.2

f 2.2

*

Metal -2

29.9

- 66.6i

0.9

N (ppm)

-20

-co L

I P

-40

L 2

-60,

‘0, *o

-80

1i ___-_________--_-----____c______ i

I

20

40 %

60

N

6ISN~it-

Mezed 5.74

$L

13.87 0:9: 18.54 49.20 57.92

:

::. 13 14 15 16

Blank

N2Analyzed3

Ma&

‘Iii (min.) 5

80

Released

FIG. 1. Nitrogen isotopic data in the stepwise release from Cape York metal-l and metal-a.

from metals 1 and 2 reveals that losses of nitrogen to the system are visible only above 1200°C, as the amounts released at 1200°C are comparable. The 6 15Nvalues show a progressive increase from the 1400°C to the 1700°C steps, compared to the minimum value obtained in the 12OO’C step. This indicates either that there was isotopic fractionation in the release or exchange of nitrogen or, alternatively, it may suggest that inclusions are indicated in metal-2, such as phosphides ( FRANCHIet al., 1993). Troilite has a different 6 “N signature and could explain the shift in principle. However, since troilite has thirty times less nitrogen than the metal, the troilite option appears unlikely. Regarding the nitrogen exchange option, we note that the metal melts partially around 14OO’C. A metal vapor deposit on the walls of the extraction bottle can liberate wall nitrogen and at the same time create active surfaces for sorption of nitrogen. The wall effect is more pronounced at higher temperatures, also consistent with the control experiments. In order to further explore these nitrogen experimental artifacts, we studied metal-3 in more detail using many temperature steps to clearly resolve components. A much smaller metal3 sample was used in order to reduce the metal vapor deposition. The results of metal-3 are given in Table 4 and plotted in Fig. 2. Since there was no appreciable release of nitrogen below 800°C in metals 1 and 2, the first pyrolysis step was at 75O”C, wherein only residual adsorbed nitrogen was released, as indicated by a slightly positive d15N value. In the subsequent temperature steps, the release of indigenous nitrogen is clearly documented. A modified protocol with heating times of only 6 min (instead of 30 min) at nominal R.F. settings, coupled with a different sample position in the crucible, account for a large temperature differential between sample and crucible. At 12OO’C the 615N data reached a value of -93%0 and essentially remained constant. Because of the inferred temperature differential, only 50% of nitrogen was released at the nominal 1700°C crucible temperature, A

Isotope composition of N in the Cape York iron meteorite 20

CAPE

YORK

Metal-3

OC Fraction -20,r

7 S -

-4o-

2 2

-60

N (ppm)

8 15N W.)

I

(O-3)%

0.96

-

II

(3-501%

15.6

-93.0fl.I

69.6

t

I.5

III

(50~IOO)%

16.5

TOIOI

33.1

-96.1

i

I.1

-94.6

f

I.

I

-

-aa

-h 3%

,

47%

,

20 %

FIG. 2.

I

40

60 N

50%,

1

80

Released

High resolution (20 steps) study of nitrogen in metal-3 of

Cape York. Ranges I, II, and III as marked in the figure correspond to the level ofterrestrial blank corrections (I-major, II-minor, and

III-negligible); see Table 4.

second extraction step at 1700°C yielded the same amount of nitrogen as in the first 1700°C step with the same 6 15N value. We continued the extractions until the Nz reservoir was almost depleted, based on the spallation 38Ar,data, which were used as monitors. The last step yielded < I % of the total 38Arcconcentration, which is consistent with concentrations observed in metal- 1 and metal-2. However, although the nitrogen yields were clearly decreasing, the recovery may have been less than 100%. The concentration given in Table 3 is considered to be a lower limit. Interestingly, all ten additional extraction steps gave the same (within error limits) 6 15Nvalue of -98%a. The spallation (Ar,) release patterns support the inferred temperature differential, since in metal-2, 81% of Ar, was released in the steps I 1400°C ( MURTY and MARTI, 1987), whereas in metal-3, only 70% of 38Arcwas released at ~17OO”C, indicating an effective temperature between 1200 and 14OO’C (from metal-2 Ar data) for the nominal 1700°C step. The large number of extraction steps in metal3 are useful to test for a possible isotopic fractionation in the process of releasing nitrogen. Since nitrogen has only two isotopes, the fractionation effects are generally difficult to assess.

1845

Althou~ the 615Nvalues for both samples are the same within errors, the nitrogen contents differ by a factor of two. The high temperature release from troilite-1 may be due to a refractory phase that is present in troilite- 1, but absent in troilite2. Inclusions present in troilite- 1 might be the carriers of light nitrogen. Since metal has light nitrogen and also thirty times more nitrogen content compared to troilite, a small amount of metal could also cause the observed dip. Two independent lines of evidence suggest that the carrier of light nitrogen was chromite: ( 1) the occurrence of chromite grains on the surface of troilite 1 and (2) the 6 15N pattern itself. Since chromite (FeCr,O,) contains oxygen, one would expect to see some spallation 15Nc. The initial decrease of 6 15N to -32%0 at 1100°C and subsequent rise to -20%~ at 1300 and 1700°C can be interpreted as due to release of 15Ncat higher temperatures. Since metal does not produce much “NC, the rise in 6 “N at higher temperatures cannot be attributed to metal only. Although it is possible that some metal grains are present in troilite, the signature of chromite seems to be indicated. Published data on nitrogen in troilites of iron meteorites are confined to concentrations (GIBSON and MOORE, 197 1). GIBSON and MOORE ( I97 1) nitrogen data of troilites in nine out of ten meteorites show enrichment relative to metal. However. none of these ten meteorites belongs to the III A group. In the present work we find that Cape York troilite has -30 times less nitrogen than the metal. DISCUSSION Nitrogen Isotopic Signatures in Cape York Metal Nitrogen in metal-2 was much less affected by loss and exchange of nitrogen than metal-l, whereas the metal-3 nitrogen extraction was better controlled. Therefore, we use metal-3 data to derive the 6”N for the Cape York metal. The sum of all twenty temperature fractions yields a nitrogen concentration of 33.1 ppm N with 6 lSN (‘TOO) = -94.8 I_ I. 1. If we use only the fraction of nitrogen released at ~1 100°C which accounts for 97% of the nitrogen, we obtain 6 “N ( %o) = -95.6 4 1.1. A closer inspection of the metal-3 data in Fig. 2 may indicate the presence of more than one nitrogen isotopic signature. In range II (3-50% released) 6 15N values are rather constant at -93 ? 2%0, while the remaining steps

Nitrogen in Troilite Two different pieces of troiiite from the same inclusion were analyzed. The major differences among the troilite chips were the numerous chromite inclusions on al1 visible surfaces of troilite- 1, whereas in troilite-2 only little chromite was observed on the exterior surface. We have made no attempt to remove the chromite. The nitrogen data for troilites-1 and -2 are given in Table 3 and plotted in Fig. 3. In both samples the nitrogen release starts at 600°C. Troilite-1 shows a bimodal release pattern with peak releases at 1000 and 17OO”C, but in troilite-2 the high temperature peak is missing, Since troilite melts around 105O”C, the nitrogen components associated with troilite are expected to be released at s1050°C. The integral nitrogen release and composition up to 1050°C for troilite-1 and troilite-2 are N = 0.55 ppm, 61sN = -0.7 + 4.2 %o,and N = 1.1 ppm, 615N = -3.8 t 1. I%,, respectively.

CAPE

‘=

4 t

YORK

8 ‘5N(X.l

Nfppm)

0

Troilite

-I

1.13

- 10.7 f 2.9

.

Troilits

-2

1.13

-

3.81

1.2

201

I

I

I

I

t

20

40

60

80

%

N Released

FIG. 3. Nitrogen isotopic systematics in the stepwise release in two troilite samples from Cape York. Note a major release above the melting temperature in troilite-I.

S. V. S. Murty and K. Marti

1846

(range III) yielded a value of -98 + 2%0. Isotopic shifts of this magnitude may be the result of ( I ) artifacts of inappropriate blank corrections, (2) diffusive fractionation during the release, (3) different nitrogen signatures in kamacite, taenite, and/or nitrides, coupled with differential releases from these phases. An inspection of the data before blank correction for range III data (Table 4) reveals 6 lSN values in some steps which are more negative than -93%, suggesting that the unce~ainties in blank amounts may not be the cause of the shift in 6 “N values. Let us examine the effect of various parameters on the &15Nvalue in more detail. Procedural Parameters and 8”N

In Table 4 we present measured 6 15Nvalues (before blank correction) for each temperature step along with percentage blank contribution for metal-3 to better understand the factors influencing the 6 “N. Since blank isotopic signatures are consistent with atmosphe~c nitrogen, larger blank cont~butions will obviously affect the measured 6 “N. A test of the blank calibration procedure is useful here, since in the last four extraction steps, the blank contribution is high. In these steps we obtain corrected 6 “N values which are within the range of previous steps. The other data do not show any systematic dependence of 6 “N on percent blank contribution. Furthermore, our data (Table 4) do not indicate any systematic relation between d “N and the amount of nitrogen used for isotopic analysis. The length of time the sample is heated at each temperature step (longer times increased the blank cont~bution), does not appear to shift d”N data, since corrected values of the last ten extractions yielded nearly constant 6 15N values. Hence, it appears unlikely that shifts in the 6 rsN between ranges II and III of the nitrogen release is an artifact of experimental procedures, In order to further evaluate alternatives, a systematic study of separated taenite and kamacite is required. FRANCHI et al. (1993) studied separated kamacite and plessite phases in Mount Edith, another group III AB iron meteorite, and observed a 3%0difference in 6 “N signatures between these phases, and plessite was found to contain twice as much nitrogen as kamacite. We note that the lowest observed 6r5N (s) = -101.1 t 0.9 of metal-3 is similar to the lowest observed &i5N ( %o) = - 100.0 t 1.2 of metal-2. These values are the lowest measured 6 “N values for iron meteorites. In Table 5 we compare our nitrogen results with Cape York data from other laboratories. 6”N values of -84.6%0 from Chicago (PROMBOand CLAYTON,1993) which is close to our metal-2, -32.3%0 from Minnesota ( PEPINand BECKER, 1982) and -80.0% from FRANCHIet al. ( 1993). We cannot

presently exclude the possibility that isotopic heterogeneities may exist in the Cape York iron. However, our data and the literature values in Table 5 suggest that nitrogen exchange problems exist to varying degrees. In the Chicago work ( PROMBOand CLAYTON, 1993) where sample sizes of a few grams are used, the exchange problem is less severe, and the 615N values are close to our data in metals 1 and 2. The PEPIN and BECKER( 1982) data, obtained by oxidation from a 6.7 mg sample, indicate that less nitrogen was lost than in our metal-l; nevertheless, the 6 “N value indicates that exchange of nitrogen has occurred. The FRANCHIet al. ( 1993) data were also obtained from small metal chips and reveal both lower concentrations and less negative 6 “N values. The latter authors observed slightly lighter nitrogen in an acid residue of Cape York. Nitrogen Components in Metal and Troilite

The nitrogen components in metal and troilite of Cape York are different both in content and isotopic composition. Troilite nitrogen is about thirty times less abundant and considerably heavier than nitrogen in metal. These systematics could in principle be understood if the troilite phase had lost most of the nitrogen, leaving a residue that is heavier. The noble gas spallation data (Table 4) show little loss from the troilite and, therefore, the nitrogen loss did not occur during the exposure interval of the Cape York meteoroid to cosmic rays. However, diffuse loss ofnitrogen at solidus temperature oftroilite (< 1050°C) at an earlier time remains a possibility if such a loss mechanism does not affect the nitrogen content of metal. On a laboratory timescale the nitrogen release from metal is very sluggish below 1200°C. Two indications speak against such a loss except during the earliest history: ( 1) Both troilite pieces 1 and 2 reveal significant amounts of radiogenic 4”Ar which are released at lower temperatures ( MURTY and MARTI, 1987), and an inclusion in troilite shows the presence of radiogenic lZ9Xe due to extinct lJ91( MURTY and MARTI, 1987), speaking against a strong thermal disturbance since the formation of the solid body. (2) The 6 15Nvalue of metal is expected to increase with increasing tem~rature, if any diffusive fmctionation was operative in the past. Contrary to this, the metal-3 data show a remarkably constant 6 rsN. Silver isotopic systematics of Cape York metal and troilite (CHEN and WASSERBURG,1983), as well as the petrographic studies (TESHIMA et al., 1986). have clearly shown that there was no extensive thermal metamorphism in support of our conclusion. At present Cape York is the only iron meteorite for which 6”N of both metal and troilite are separately measured, but

Table 5. Comparisonof nitrogen data reportedfor Cape York Metal.

Sample Wt. @

N @pm) 7

8t5Nai (?Jw) 802 22

0:21 57

3’:

-94.8 i: :86:8 f 0’9 1:l

5.3 to 6.1

37

-84.6

0.0067

21

-32.3 f 1.5

0.0058

20.7

-80.0

Reference this work this work this work Prombo and Clayton (1993) Peuin and

Becker(1982) Fran& et al. (1993)

Isotope composition of N in the Cape York iron meteorite also the Acapulco meteorite was observed to contain two distinct nitrogen components (STURGEONand MARTI, 199 1). The silver isotopic system shows a complex behavior between the metal and troilite in several classes of iron meteorites. While the metal phases of II B, III AB, and IV AB iron meteorites show regular and well-behaved trends on Pd-Ag isochron plots, their troilite phases often exhibit distinct trends ( CHEN and WASSERBLJRG,1990). This distinction may extend to the different 6 15N values in metal and troilite. Implications for the Origin of Iron Meteorites

The bulk of the iron meteorites belongs to groups I AB and III AB. Both these groups have light nitrogen ( PROMBO and CLAYTON, 1993; FRANCHIet al., 1993). Any model for the formation of iron meteorites needs to explain the occurrence of light nitrogen in these groups. If irons formed as cores of asteroid-sized bodies through melting and differentiation, the nitrogen in the undifferentiated presumably asteroidal bodies of 1 AB and III AB group iron meteorites has to be lighter than 615N = -60%0 and -80%0, respectively, because fractionation effects in equilibrium partitioning at higher temperatures are not appreciable. Furthermore, in the case of the Acapulco meteorite the nitrogen component with 6 15N _( - 11O%O(STURGEON and MARTI, 199 1) is observed only in the metal, but not in silicates (KIM and MARTI, 1993). For the more common stone meteorite groups the lightest nitrogen observed is 6 15N= - -33%0 for enstatite chondrites and achondrites ( KUNG and CLAYTON,1978; THIEMENSand CLAYTON, 1983; MURTY and MARTI, 1986). The existence of nitrogen components in the meteorites Bencubbin and Weatherford with 6 15Nvalues > 900%0 indicate the existence of reservoirs of both very heavy and very light nitrogen. So far, positive 6”N have generally been associated with carbonaceous chondrite type material formed in oxidizing conditions, while negative 6 “N are associated with enstatite meteorites formed under reducing conditions. The light nitrogen in 1 AB and III AB irons may represent a characteristic property of such reducing environments. A possible relationship between the oxygen isotopic systematics of the silicate inclusions of I AB irons and of enstatite chondrites might suggest a genetic link (CLAYTONand MAYEDA,1978), but their 6”N signatures differ by some 30%0. Finally, the uniform 6”N values in metal-3 over the entire temperature range are consistent with a magmatic origin of Cape York metal only if the nitrogen signature of troilite is consistent with a fractionated residue from an early loss mechanism. CONCLUSIONS AND OUTLOOK Nitrogen and spallation noble gas studies in metal and troilite phases of the Cape York meteorite showed that ( 1) the metal contains 33.1 ppm N with 6”N = -94.8 + 1.1%0, while pure troilite has -30 times less nitrogen with 61sN = -3.8 + 1.2%0;(2) the spallogenic “Net is nineteen times larger in the troilite due to production from sulfur, but spallogenic 15Ndoes not visibly affect the measured 6 15Nof troilite; (3) loss and exchange of nitrogen can severely affect the nitrogen content as well as the isotopic signatures, but carefully executed experiments yield reliable data; (4) the nearly constant 615N value in all temperature steps of Cape York

1847

metal suggests the occurrence of a nitrogen reservoir with d15N about -loo%, in the early solar nebula; (5) the large difference between the 6 15Nvalues of metal and troilite can, in principle, be explained by a diffusive loss of nitrogen from troilite during the earliest history, and a magmatic origin of Cape York cannot be ruled out based on the nitrogen systematics. Oxygen isotopic compositions from silicate inclusions of I AB, II E, III CD, and IV A and chromite inclusions of III AB have been used to discuss possible genetic links with stone meteorite groups (CLAYTONet al., 1983, 1986). Associations between II E and H chondrites, between IV A and L, LL chondrites, between I AB, III CD and winonaites, and between III AB and pallasites were inferred from such studies. The 6”N signatures of II E and of one IV A meteorites are consistent with such associations ( PROMBOand CLAYTON, 1993). There are at least three iron meteorite groups with large negative 6 “N values. Could a single light nitrogen reservoir be the source for all these groups and could differences in 615N be the result of mixtures with other components? The lightest nitrogen signature observed in the Acapulco meteorite (STURGEONand MARTI, 199 1; KIM and MARTI, 1993) and a possible genetic association between I AB, III CD, and winonaites (CLAYTON et al., 1983) may suggest common light nitrogen reservoirs. However, in such a model ( WASSON et al., 1980), additional nitrogen components were required during the formation of I AB and III CD irons. An association of pallasites and III AB irons based on oxygen isotopes was suggested (CLAYTONet al., 1986). Considering the substantial isotopic differences between metal and silicate phases of pallasites, due to spallation components in the latter ( PROMBO and CLAYTON, 1993), exchange or contamination of the metal might account for some of the heavier nitrogen of pallasite metal, when compared to III AB irons. The distinct grouping of cosmic-ray-exposure ages of iron meteorites (VOSHAGE et al., 1983) is consistent with separate parent bodies for each class of iron meteorites. The oxygen isotopes are also suggestive of distinct parent bodies. In this sense similar nitrogen isotopic signatures in major iron meteorite groups could indicate the general availability of light nitrogen in regions where major iron meteorite groups originated. We expect that nitrogen measurements in pure metal phases of meteorites may lead to a better understanding of the relationship between iron and stone meteorites. Acknowledgments-We

thank V. F. Buchwald for the Cape York (Agpalilik) sample, J. Clarke for essential support, and R. N. Clayton and an anonymous reviewer for helpful suggestions. This research was supported by NASA NAG 9-4 1.

Ediiorial handling: F. A. Podosek

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