Total nitrogen content of deep sea basalts

Geochrmica et Cosmochimico Actn Vol. 46, pp. 371 to 379 0 Pcrgamon Press Ltd. 1982. Printed in U.S.A.

Total nitrogen content of deep sea basal& THOMAS L. NORRIS’ and OLIVER A. SCHAEFFER Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794 (Received

June

16, 1980; accepred in revised form October 16, 1981)

Abstract-Various models have been suggested concerning the origin and evolution of the earth’s atmosphere. An estimate of the nitrogen content of the mantle could further constrain atmospheric models. Total nitrogen content was determined by thermal neutron activation analysis via “N(n,p)W. The ‘“C was converted to carbon dioxide and counted in miniature low level proportional counters. The total nitrogen content of U.S.G.S. standards BCR-1 and G-2 as determined by different laboratories is variable, probably due to atmospheric adsorption by the finely ground samples. Total nitrogen content was determined in deep sea basalt glasses from three regions: East Pacific Rise (15 rt 4, 18 t 4, and 7 f ppm 2 N), Mid-Atlantic Rift (FAMOUS Region: 22 +- 5, 18 It 3, and 10 + 2 ppm N) and the Juan de Fuca Ridge (17 + 4 ppm N). Matrix material from the same samples as the glasses was available from the East Pacific Rise (37 rt 6, 26 + 4, and 34 t 6 ppm N) and the Mid-Atlantic Rift (39 k 4 ppm N) which are about 50 to 100% greater than the associated glasses. The increased matrix abundance may be due to incorporation of chemically bound nitrogen from sea water rather than dissolved molecular nitrogen. The nitrogen content of the FAMOUS samples are inconsistent with the model of Langmuir et al. (1977) for petrogenesis based on trace element data. Factors which can affect the observed nitrogen content in the basalts and the interpretation in terms of the mantle nitrogen abundance are discussed (e.g. partial melting and degassing of the basalts). A lower limit of about 2 ppm N in the mantle can be estimated. _ 1. INTRODUCTION THE initial composition of the primitive atmosphere of the earth suggest two possible compositions. Urey (1959) suggested the atmosphere was highly reduced and composed of methane and ammonia. Holland ( 1962) and Miller and Orgel ( 1974) have cited further evidence for a reducing atmosphere, particularly the apparent necessity of a reducing atmosphere for the formation of life. In contrast, a number of studies (Rubey 1955, Abelson 1966, Baur 1978, Fanale 1971, and Walker 1976, 1977) proposed a carbon dioxide and nitrogen dominated atmosphere based on geologic constraints. Hart (1978) suggested a model which contains both methane and carbon dioxide in the primitive atmosphere with the methane eventually oxidized to carbon dioxide. In addition to the different proposed initial compositions there is no agreement as to whether the atmosphere formed by catastrophically degassing (Fanale 197 I) or by slow degassing throughout geologic time (Rubey 1955). Anders and Owen (1977), Clark et al. (1972), Ringwood ( 1979), and Walker ( 1976 and 1977) have proposed a heterogeneous accretion model for the earth with the atmosphere deriving from a thin volatile rich veneer added in the last stages of the earth’s formation. The speculations concerning the formation and initial composition of the earth’s atmosphere are constrained by the isotopic and element abundances of the volatile elements in the atmosphere and the earth’s mantle. The noble gas element abundance IDEAS ON

’ Present address: Smithsonian Astrophysical tory, Harvard University, Cambridge, Mass.

Observa-

patterns in the atmosphere are nearly identical to the planetary component in ordinary chondrites (Fanale and Cannon 1971) with the exception of xenon. Fanale and Cannon (1971) and Canales et al. (1968) suggested that due to the adsorption properties of xenon large quantities can be adsorbed onto shales and other sedimentary rocks. The amount estimated in this reservoir is sufficient to match the earth’s abundance to that of the meteorites. Fisher (1970 and 1974) and Dymond and Hogan (1973) and others have determined the concentration of the noble gases in deep sea basalt quench glass. The glass is rapidly quenched and appears to have interacted only slightly, if at all, with sea water over the short time it has been in contact with it (Fisher 1971). Fisher (1974) found that the noble gas abundance pattern appears identical to the atmosphere in one sample but non-atmospheric for others. However, the abundance per unit mass for al1 samples is significantly less than the ratio between the atmospheric inventory to the total earth mass, suggesting the mantle is depleted in the noble gases. If the chemical composition of the earth is similar to the ordinary chondrites (Larimer 1971), then the total nitrogen inventory in the earth may be similar. The nitrogen abundance in chondrites is similar for all petrologic groups, but there are differences in the average values obtained by different laboratories. Gibson and Moore (1971) obtained the highest values, ranging from 15 to 120 ppm N with an average value of approximately 54 ppm N. Kothari and Goel (1974) found an average of 23 ppm N though they measured fewer meteorites, and Kung and Clayton (1978) obtained an average of 9 ppm N. The atmosphere contains 3.86 X 102’ g N (Veriani 371

312

T. L. NORRIS AND 0. A. SCHAEFFER

1966), and with a total earth mass of 5.98 X 10” g this corresponds to approximately 1 ppm N which is significantly less than the nitrogen data in meteorites. Including estimates of nitrogen bound up in sedimentary rocks raises the inventory by less than 1 ppm N (Rubey 1955). If the nitrogen is chemically bound in rocks rather than molecular nitrogen, then the solid earth may contain the “missing” nitrogen. Wlotzka ( 1961) drtermined the chemically bound nitrogen in a large number of crustal rocks and minerals. He found a wide variation, from -3 to -40 ppm N, about the same as chondrites. Because Wlotzka determined only crustal abundances, it is important to estimate the nitrogen inventory in the mantle as it comprises about two-thirds of the total mass of the earth. This study has determined the nitrogen content of a number of deep sea basalt glasses and associated matrix by neutron activation. The measurements reported here are the first determinations of nitrogen in deep sea basalts. We used a procedure originally developed by Shukla et ai. (1978) and Kothari and Goel(1974). Determining the abundance of nitrogen and its characteristics (estimate of chemical form, interaction with sea water, effects of partial melting, etc.) leads to a lower limit to N content and a probable value of the content in the mantle. 2. EXPERIMENTAL TECHNIQUE 2. f . Sample prepuration and irradiation Two neutron irradiations were made at the Brookhaven National Laboratory Medical Reactor in the A-3 facility for about five hours, The thermal neutron fluence was about 8 X 10” n/cm*. For the first irradiation, the samples were wrapped in aluminum and placed into quartz vials. Two of the vials were evacuated for about 30 minutes until the pressure remained less than lo-’ torr at room temperature. The other vial was heated to 75°C for 200 minutes to determine if there was any adsorbed atmospheric nitrogen that was not removed by evacuation. All three vials were sealed under a vacuum to exclude atmospheric nitrogen. For the second irradiation, the samples were loaded into numbered quartz ampules, which were capped with quartz wool, placed into a quartz vial, and sealed at room temperature after evacuation. After irradiation the samples were stored for about one month to permit short lived radionuclides to decay. 2.2. Neutron monitor and i~ter~err~ngreactions For the first irradiation, NBS Steel Standard #lo96 with a nitrogen content of 40.4 + .9 ppm N was milled into small fragments. The “C activity for each sample is listed in Table 1 and gives an average calibration of 14.5 + 1.5 dpm ?Z/ppm N. As it proved difficult to extract the “C from the steel, a synthetic glass containing 2.06 wt. % N (Muller, 1972) was used as the monitor in the second irradiation. The activity for irradiation 2 was 19.8 t- 1.2 dpm ‘%/nom N for the glass and 20.2 f 0.8 dpm “C/ppm N for- the NBS Steel. The difference between the steel activities for the two irradiations may be due to the differenee in neutron fluence caused by variation in the placement of the samples within the core. 14C is also produced by the “0 (n, a)“C reaction from oxygen in silicates. This could theoretically provide a large amount of “C, equivalent to an apparent nitrogen content

of 18 ppm N for a silicate with 40 wt. YOoxygen. Shukla ef al. (1978) studied the interferring reaction with an I70 enriched AIrOr and found the 14C activity was three times less than that predicted by theory. They indicate there is apparently an unsolved problem with the theoretical calculations, e.g., the neutron cross section, or excitation energy, etc. Based on the emperical results of Shukla et al. (1978) correction of 6 ppm N to the observed abundance is obtained. The rock nitrogen contents have been corrected for this effect. The correction is not serious so that it is not necessary to make the correction based on the measured oxygen content of each sample because an increase to 50 wt. % oxygen only increases the correction to 7 ppm N, No correction was needed for the NBS Steel samples nor for the syntheticgiass standard, which has such a large nitrogen content, 2.06 wt. %, that the correction is insignificant.

2.3. ‘“C extraction procedure The sample is heated in a CO2 atmosphere and the 14C extracted by oxidation to CO by the CO,. The procedure was slightly modified from that described by Fireman et al. (1976). Each samole is loaded into a new alumina crucible which is placed inside a platinum crucible and heated by an RF induction heater. Platinum was used because it is one of the few metals that does not react with CO? at high temperatures to form a carbide. A small amount of 14C free carrier gas (0.5 ccSTP, prepared from coal) is placed in the furnace to provide the oxidizing CO2 atmosphere. Copper oxide was heated to 4OO’C to convert CO or other unoxidized carbon to CO:. To be sure all the 14C was extracted, several runs were made for each sample, each time using new carrier gas to determine the amount of 14Cremaining in the sample. The “C extraction was considered complete when less than 5% of the total 14C was removed in the final fraction. Usually about 50% of the total 14Cwas contained in the first high temperature fraction, and it required from 3 to 5 additional extractions to remove all the ‘V. The time required to heat each sample varied depending on the particular experiment. For samples where the 14C was first extracted at a low temperature, the extraction was three hours. For those samples where only the total carbon was extracted, the time was about six hours to maximize the amount of “C removed. The high temperature extraction was between 1300 and 1400°C. Because there was no thermocouple available, the temperature for the low temperature fraction was estimated at between 400 to 600°C by reducing the power level of the RF generator to the appropriate temperature based on a power versus temperature curve extropolated from higher temperatures which could be measured with an optical pyrometer. Prior to each sampie the interior of the furnace was cteaned with HF, rinsed with distilted water and dried. The crucibles were occasionally soaked in HNOl to remove some of the outside layers of sample which had deposited on the surfaces. After a rinse in distilled water and followed by drying, the furnace along with a new or refurbished sample crucible was preassembled and degassed at 1,300”C. Because a small amount of the 14Cfrom the previous run dissolves in the alumina, the furnace is decontaminated by placing approximately 2 ccSTP of carbon dioxide carrier gas into the system at 1,300”C for a few hours, and then the system was degas& for another hour or two. After denomination a blank was taken using the carrier gas. Blanks were run for three hours, and only when the activity of the blank was sufficiently low (less than 0.5 dpm/ccSTP) CO1 was the sample loaded into the furnace. If the blank was not low then additional carrier gas was used to continue to decontaminate the furnace until the blank activity was low enough.

313

N, IN BASALTS Table 1.

Monitor Results* Measured Content

Sample Number Irradiation 1 NBS 1096 Steel 41 42 43 Average USGS Standard Rocks BCR-1 #15 BCR-1 1116 Average G-2 1121 G-2 1122 G-2 ii23b Average Irradiation 2 Synthetic Glass 101 115 Average NBS 1096 11125

Mass (mg)

Nitrogen in Low Temp. Fraction in I

Standard Nitrogen content 40.4kO.9

..

DDE

dpm 14C/g

Total N ppm (Corrected for 6 pm from &

dpm 14C/N ppm

40.4i 0.9c

14.6il.O 13.0i0.8 15.8il.O 14.5i1.4

N 590+29 525+22 640?23 585?58

(38.6) (98.6) (60.7)

1043?54 1238?88

(39.0) (108.1)

See Table 2

30.3 2.4

(39.7) (67.6) (57.1)

See Table 2

19.4 0.04 29.9

607r46 7?3?36 579?27

65 79 71 36 49 34 38

file -t14e ? 9e f 7e ?8e -t6e ? 4e

34.8k5.0 41.3i6.8 10.8iO.8 14.2i2.1 10.3i1.6

2.06 wt.% (3.6 i.35)x105 (4.59+.37)x105

(9.9) (9.4) (95.7)

815?32

40.4 0.9

2.06' wt.% 41.2? 4.ld

17.6i1.7 22.221.8 19.8i1.2 20.2tO.8

(a) Data from Gibson & Moore (1970). (b) Sample degassed at 75°C for 200 minutes under vacuum prior to irradiation. (c) Nitrogen content of standard assumed correct. (d) Based on synthetic glass calibration. (e) Average calibration NBS standard steel from IRRl. *The errors listed in the table are one sigma and are based on known sources that could be estimated (e.g. counting statistics, volumes, etc.). The total error for the samples prior to calibration with the standards was usually better than 10%. The calibration of each sample with the standards, particularly the NBS steel, increases the error of the absolute nitrogen concentration to about 15%.

2.4. Purification of the carbon dioxide Carbon dioxide is trapped onto activated charcoal with a dry ice acetone bath. In most cases all the gas transferred from the furnace was adsorbed onto the trap indicating the samples contained very little gases other than carbon dioxide and water vapor. For those samples, particularly the standard glass which released large amounts of ammonia, some of which was trapped in the sample bottle, the carbon dioxide was adsorbed for I to 11/2 hours to be sure all the carbon dioxide had been adsorbed onto the charcoal. After adsorption of the CO, the charcoal was evacuated to a pressure of less than 10e4 torr. The carbon dioxide is released by placing the charcoal trap in a water ice bath at 0°C and collecting the carbon dioxide with a liquid nitrogen cold finger. The carbon dioxide was collected for two hours with =95% recovery of the gas. The charcoal acts as a gas chromatograph and separates CO2 from contaminant gases (Bruns 1976). For each sample new activated charcoal is used to prevent cross contamination between samples. For each CO2 sample of a given rock, the charcoal was degassed at 300°C under vacuum for a minimum of five hours before using the charcoal to clean the next gas fraction. The advantage of the charcoal procedure, in addition to its simplicity, is that it removes any radon from the sample gas (Bruns 1976). As a result we did not have to wait for the radon decay before counting. 2.5. Counting procedure The carbon dioxide was counted in small 0.5 cm3 gas proportional counters of the type designed by R. Davis (Kummer et al. 1972). Two counters were used which had a background of 20 to 30 counts per day and counting efficiencies of 60.4 f 0.8 and 56.9 + 0.9% respectively. The counting procedure is described in detail by Stoenner et al. (1970). All the counting was conducted at Brookhaven National Laboratory with the kind cooperation of R. Stoenner.

The gas counters were filled with carbon dioxide to a pressure of about one atmosphere. An Fe” source (5.6 kev x-rays) is used to calibrate the counter by increasing the voltage until the peak is centered at the center of the energy spectrum on a multichannel analyzer. Typical operating voltages are between 1800 and 2000 volts. To verify the purity of the gas, the iron spectrum is collected. For the pure sample, the resolution is -30% at half width for the Fe” peak. Water vapor and other contaminants tend to broaden the peak, and if this occurs the sample gas is removed and dried or recleaned. The multichannel system is designed to detect and count all pulses from the counter which occur above the lower discriminator level. This gives the total carbon- 14 activity of the sample. The advantage of the multichannel system over a scaler counter is that it is possible to determine if there is any noise component in the spectrum. As the counter is filled by use of mercury, sometimes the mercury inadvertently gets into the cathode which results in a large noise peak in the spectrum. The mercury can be removed by removing the sample gas temporarily and baking the counter at 300°C under vacuum. Noise from the electronics, particularly caused by high humidity, or any other noise source, will result in a spectrum that differs from the normal spectrum (i.e., by giving low energy breakdown count) in addition to giving unstable counting rates. Only counting rates without noise are used in the analysis of the samples. The samples were counted long enough to reduce the error due to counter statistics to 3%. Typical counting times fractions were 5 to 10 hours with the lower fractions taking proportionally longer. 3. NITROGEN 3.1.

USGS

atmospheric

ABUNDANCES

standard rocks and the affects contamination

of

USGS Standard rocks are commonly used as interlaboratory standards for a variety of elements but

T. L. NORRIS AND 0. A. SCHAEFFER

374 Table

2.

Nitrogen

Analysis

-

U.S.G.S.

Semdard

..____I_

Sample

Rocks

-.___. 1 Total N (2 5%)

W-l

52

C-l G-2 GSP-1 AGV-I DTS-1 PCC-1 BCR-1

59 56 48 44 27 43 30

2 3 4 Total N* Total N Chemically (C 10%) -___ __----.- Bound N 29

IO.8 2

16,

. I.

_.... .

N* --.

K.R.

1:s.r

‘3 i?

I

:! 2H 3.‘ 12 il i :?

5 Total

18 *

3 I’

1

i.

/j

0

-1. 3. 5.

Gibson and Becker and This work.

Moore (1970). Clavton (1977). *Correited

2.

Shukla et al. (lY78) __4. WlIer (1972. 1976). fr>r “n.

have not been used as a geologic standard for nitrogen. There is significant variation in the literature on the total concentration of nitrogen in these standards. Numerous techniques, mass spectrometry, neutron activation, and Kjeldahl extraction, have been used and the reproducibility is poor. Table 2 lists the nitrogen concentrations from various laboratories. Notice that the range is from a few ppm N up to approximately 59 ppm N for the single standard G-l. Goel and Kothari ( 1972) have attributed this to the effects of absorbing atmospheric nitrogen onto the powdered samples. They suggested that their standards were degassed of surface nitrogen and hence are more accurate values. When compared to Gibson and Moore (1970), whom Goel and Kothari (1972) and Shukla el al. (1978) claim have more adsorbed nitrogen, the pattern is not as simple as suggested. For samples measured by both laboratories, Goel and Kothari and Shukla et al. give lower values for W- 1, G-2, GSP- 1, AGV- 1, DTS- 1, while PCC- 1 and BCR- 1 are similar to Gibson and Moore. If all these variations were caused by atmospheric nitrogen all the values from Goel and Kothari should be lower than Gibson and Moore-but this is not the case. Even with one sample (BCR-1) Shukta et al. found a large variation with repetitive determinations (see their Fig. 2). Some of the variation may be due to sample preparation; however, all the samples are very fine powders and should have absorbed similar amounts of nitrogen from the atmosphere. Becker and Clayton (1977) measured nitrogen concentrations which are an order of magnitude smaller than either of the above groups and have suggested that both groups have samples which contain large amounts of adsorbed nitrogen. In comparison, Muller (1972) and Muller et al. (1976) determined by the Kjeldahl procedure which gives abundances nearly identical to that of Becker and Clayton. The Kjeldahl procedure measures chemically bound nitrogen only and hence is unaffected by atmospheric molecular nitrogen. Any indigeneous molecular nitrogen would not be measured. MuIler’s values are much less than those of Gael and Kothari, Shukla et al., and Gibson and Moore, suggesting that there is some molecular nitrogen present in the samples which may or may not be of atmospheric origin.

It appears there are problems in using the USGS standards for interlaboratory comparisons. These standards have been used in this study to attempt to understand the basis for the discrepancies. Two techniques were used: one was to heat the standards under vacuum prior to irradiation in an attempt to remove any remaining absorbed nitrogen and the other was to measure the 14C released at 500°C. Table 3 lists the 14C activity for BCR-1, #15 for four sequential low temperature fractions. The decline rn activity from 29% to 1.3%, a decrease of a factor of 20, indicates that only the surface is being sampled. The low temperature fraction apparently samples the nitrogen bound at or near the surface and in particular may represent the nitrogen adsorbed from the atmosphere. Table 1 gives the percentage of nitrogen in the low temperature fractions for a number of USGS samples. The values are variable but many show a high surface nitrogen content even after previous evacuation of the sample. Because some of the samples show very little surface nitrogen but similar total abundances, apparently the low temperature release was not always effective in removing the surface “C. The high values are consistent with the suggestion of Goel and Kothari (1972) and Becker and Clayton (1977) that these powdered samples have a large proportion of atmospheric nitrogen. This may account for a minimum of 30% of the totai nitrogen observed in these standards and thus is a major source of contamination. The differences between laboratories in the nitrogen content of the USGS standards may be due to different efficiencies in removing the adsorbed nitrogen. The total nitrogen abundances from this study are summarized below. The USGS nitrogen values listed in Table 2 have not been corrected for atmospheric nitrogen because it is not clear whether the 30% low temperature fraction represents the average or minimum adsorbed nitrogen remaining on the samples. (1) Sample G-2 shows fair agreement with the values of Shukla et al. (1978) though the results of our work are slightly higher. The values of Gibson and Moore (1970) are higher than both of these rcsults. (2) BCR-I from this study is the highest value yer obtained at 71 ppm N whereas the other vaIues are 32, 30, and, 1.3 ppm N. The variation in this one sample is quite large. If one assumes that the sample Table

3.

Temperature

Release

for

BCR-L”

i?l5

-.._._~-__ 14C

Fraction 1

*C

% of

Carbon

500

2

500

;5

500 900

1.3 69.7

is

Combined

Total

Extraction Tine

extracted

26.1 2.9

ca) BCR-1 (b)

Temp.

basalt

powder

6 high

temperature

I f

(min. 60

0.1 0.2

h0

’ 0.1 f 7.7 -..“_--_l

standard

5‘1 ,lf;3 . .._-_. _ from

iractit,ns

I’.“.:;.S.

1.

N, IN BASALTS measured in this experiment contains 30% adsorbed nitrogen, this reduces the corrected value to about 43 ppm N, which is still higher than the other works. Shukla et al. (1978) have found that BCR-1 may have problems with sample heterogeneity (see their Fig. 2). There is significant variation between laboratories on the USGS standards which can probably be attributed to adsorbed nitrogen and sample heterogeneity and thus more work is necessary to clarify the situation before these can be used as interlaboratory standards for nitrogen. 3.2. Deep sea basalts The nitrogen determinations are given in Table 4 for basalts from the Atlantic and Pacific spreading centers. The low temperature releases are all small, most less than a few percent. The consistency of the data in the low temperature fractions indicates that atmospheric nitrogen is only a small contribution to the total nitrogen in these samples. This is probably due to the small surface area of the samples because most were single chips. As most of the matrix material consisted of coarsely broken pieces (-0.1 to 1.O mm) the amount of adsorbed nitrogen remaining on the samples should be small. This is supported by

Table 4. .__

Total Nitrogen Content in Deep Sea Basalts Mass, mg

Sample

TYPO

% N in Low Temp. Fraction

N (PP~)"

East Pacific Rise DSB

35

N1l.P

(25'5'5, 64.6

#llJ

39.3

1112" P13"

98.4 75.6

109"3O'W,

% 10% o. 90% % 10%

1951

glass

meters) 25 i 5

1.9

matrix glass

Cl

x 90% matrix matrix matrix

10 + 3 2.7 2.1c

25 ? 4 21 * 4

DSB 46 (23'31'5, 115"33'W, 2575 meters) /!1" 6 2" ii3.' ii4"

54.5 74.9 94.2 82.0

glass glass matrix matrix

5.7 1.7= 0.9 1ostC

15 18 37 26

2 ? t ?

L 4 h 4

DSB 157 (6'10'5, 106"39'W, 2258 meters) 116" I'7"

58.8 111.3

matrix glass

5.7 5.7c

34 ? 6 7 t 2

Mid Atlantic Rift (FAMOUS: 33'15'W, 36'=47'N,2700 meters) 527-l-l ii62a 527-l-l /is-2b 527-l-l iilz7b 527-l-l /illrob 523-l ill43b

20.8 34.0 86.4 54.1 14.5

glass glass matrix matrix glass

6.7 _e 5.0 _e _e

22 18 39 27 10

i i ? ? ?

5 3 4 4 2

Juan de Fuca Ridged (44'40'N, 130"2O'W, 2195-2220 meters) (172" 17 k 4 55.2 glass 0.6 -__ (a) Samples in irradiation 1 were calibrated by the NBS steel 111096standard. (b) Samples in irradiation 2 were calibrated by a synthetic nitrogen glass standard. (c) These samples were degassed at 75OC for 200 minutes under vacuum before the irradiation. (d) Sample was U. S. National Museum Rlll-240-82, Dredge 10. (e) Only high temperature extraction. *Corrected for l'0.

375

two observations. First, samples DSB-35 fl2 and #13 which consist of broken fragments give the same total nitrogen abundance as #10 which consists of three large fragments. The low value for #l 1, consisting of 2 large fragments and some smaller pieces, is probably due to sample heterogeneity. Sample FAMOUS 527-l-l #140 was a single fragment of matrix material, which though it gave a lower total nitrogen than #127 (coarse fragments) still gave an abundance greater than the quench glass pieces. Secondly the NBS steels were milled into small fragments (- 1 mm), yet sample #125 had 41.2 + 4.1 ppm N when calibrated against the synthetic glass, compared to the certified content of 40.4 ppm N. The above, in addition to the lack of any significant nitrogen in the low temperature release fractions, suggest the total nitrogen values for the basalt samples may represent the actual content. The total nitrogen abundances in basalts, given in Table 4, indicate that the samples are relatively homogeneous over a wide geographic range including the East Pacific Rise, the Mid-Atlantic Rift, and the Juan de Fuca Ridge. A number of quench glasses were examined at each site and also some of the associated matrix material. 3.2.1. Ea.st paci’c rise. Sample DSB 46, numbers 1 and 2, consisted of two single glass chips with very little weathering alteration present. The chips were individually measured and agree within experimental error with a mean of about 16 a 4 ppm N (see Table 4). In addition, two samples of the matrix (coarsely broken chips), numbers 3 and 4, gave a mean value of 31 f 5 ppm N. It is clear the matrix nitrogen abundance, though variable, is greater than the glass by about a factor of 2. DSB 35 which is from the same locality, consisted mainly of matrix material with only a very thin layer of quench glass present on some samples. Both samples 10 and 11 contained a thin layer of glass, probably less than 10 volume percent. It appears the values for all samples of DSB 35 are identical with the exception of number 11 (see Table 4). Sample 11 may be low due to the sample containing more glass than sample 10 or to sample heterogeneity. As is suggested by sample DSB 46, the glass has a much lower nitrogen abundance so that if there was more glass in number 11 than estimated this would result in a lower value. Samples 10, 12, and 13 indicate that the matrix can be fairly homogeneous because samples 12 and 13 were taken from 2 to 5 cm below the surface sample number 10. DSB 157 shows the same nitrogen pattern between the glass and matrix as seen for DSB 46 (Table 4). DSB 157 6 is from the matrix with a value of 34 ppm N which is five times that for sample 7, a glass chip from the same sample (7 ppm N). 3.2.2. Mid-Atlantic rift. The FAMOUS samples were identical to those described by Langmuir et al. (1977) and all were collected within 3 km. of one another. Langmuir et al. describes the petrography

376

T. L. NORRIS

AND 0. A. SCHAEFFER

and phase chemistry of the samples in detail. The samples consist of slightly vesicular glasses that are pale brown and matrix that contains some vesicles and cracks within the interior portion containing very little altered material. The lack of alteration suggest a young age of 10,000 years (Langmuir et al., 1977) and indicates these glass samples have been little altered by reaction with sea water, and hence the nitrogen content probably reflects the concentration in the melt. Sample 527-l-l (Table 4) shows the same pattern observed for the glass and matrix samples from the East Pacific Rise. The glass abundance is identical within experimental error between the individual chips but is about one-half the matrix content. Sample 523-1, for which there was only one sample, is lower than 527-l-l but similar to DSB 157 from the East Pacific Rise. Langmuir et ai. (1977) have modeled the petrogensis of these particular samples with 523-1 being from the first stage of a continuous melt sequence and 527-l-1, a sample from the end of the same melt sequence. The nitrogen abundances, however, are opposite to that expected (527- I- ‘1should be less than 523-l). We will discuss this point in section 5. 3.2.3. Juan de Fuca Ridge. There was only one glass sample from the Juan de Fuca Ridge and matrix material was not available. The glass had a nitrogen content of 18 2 4 ppm N (Table 4) and is consistent with that of the other glasses. All the glass nitrogen contents are simiiar with an abundance of 7 to 22 ppm N. 4.

DfSCUSSION OF GLASS VS. MATRIX ~ON~NTRATrONS

It is interesting to compare this study with the work of Dalrymple and Moore (1968) and Fisher ( 1971) on the argon isotopes because a similar type of pattern between the matrix and glass appears. Dalrymple and Moore (1968) found that excess A?” decreased from the surface glass to the interior of historic Hawaiian lava flows. Fisher ( I97 1) measured A?’ and A?’ in deep sea basalts and found that the A?’ excess was greatest in the glassy rims while Arx6 concentration was greatest in the matrix. This is interpreted to indicate that the rims were able to retain their initial inventory of argon because the samples were rapidly cooled on contact with sea water. However, because the matrix cools more slowly, it has more time to interact and exchange Ar with the seawater; and may result in overall partial desorption of the Arm and adsorption of Ar36 to partially equilibrate with the 40Ar/36Ar in seawater. Arm is not adsorbed in the matrix because the 40/36 ratio in the basalt is initially greater than seawater. Samples DSB 46 and 157 and FAMOUS 527-l1 show the same type of pattern as the argon data, which suggests that the matrix may have incorporated nitrogen from seawater as it cooled. Though most of the matrix samples were coarsely broken

chips, even single chip samples (e.g. FAMOUS 5271-l #140 and DSB 35 #lo) show increased nitrogen content relative to the glass. Some of the variation in the matrix may be due to adsorbed nitrogen due to the larger surface area of the matrix fragments. However, some of the increase is probably indigenous to the sample as all matrix material, whether single chips or coarsely broken fragments, have increased nitrogen content. The source of this nitrogen cannot be dissolved molecular nitrogen because the increase in the matrix is too large. If one assumes all the Ar3” in the matrix was from adsorption from seawater, then it is possible to estimate the maximum nitrogen that would be adsorbed because both are similarly inert gases. Norris (1979) calculates the amount of nitrogen that would be adsorbed in the matrix with this comparison. The observed increase in the matrix, about 5- 10 ppm N, is 500 to 1000 times greater than that predicted from the Ar” increase. A more probable source for the excess may be the incorporation or organically bound nitrogen. The tota amount of organic nitrogen in seawater at depth is 0.6 mgN/l (Hill 1963) which is not much less than the dissolved molecular nitrogen, 16 mgN/l {Craig rf al. 1967) for this discussion, but the chemical reactivity is significantly greater fOF the organic nitrogen 5. DISCUSSION OF NITROGEN ABUNDANCES A number of factors can affect the concentration of the noble gases, nitrogen, and other volatiles in the melt relative to that of the parent material. Dymond and Hogan (1978) have discussed the effects of solubility, partial melting, and fractional crystallization on the noble gas distribution between the melt and the source material. Of particular interest to the interpretation of nitrogen in deep sea basalts is the effect of partial melting on the nitrogen concentration in the melt. The distribution of trace elements in batch melting (removal of a single melt) is described by Schilling (1966) as: C, =. ci‘_z c,

;,

_; F

Co is the initiat concentration, C, the concentration in the melt, F the fraction of melt formed, and L) the bulk distribution coefficient given by il = C&Y, where C, is the concentration in solid. Figure 2 of Hanson ( 1977) shows the effect of F and D on C’, and C,. For small F the fractionation between the melt and source is very large for D 5 1. For volatites (particularly the noble gases and molecular nitrogen) which are expected to behave as incompatible elements, the distribution coefficient will be nearly zero (Dymond and Hogan 1978). That is, they will be contained entirely within the meit resulting in a significant depletion of the volatiles in the source material. For the FAMOUS samples there is ~nde~ndent evidence on the extent of melting for each sample,

N2 IN BASALTS

Langmuir et al. (1977) used trace elements in the basalts to model the petrogenesis. They suggest that while both 523-l and 527-l-l appear to be from similar material, sample 523-l is a result of 5 to 8% batch melting while 527-l-l represents a melt derived from a similar mantle which has undergone continuous melting and partial loss of melt. The model itself is relatively insensitive to the actual values of the extent of melting, but the concentrations observed in 527-l - 1 can be produced after about 18% total melting. If D = 0 as expected for the volatiles, then all the volatiles should have been concentrated in the earlier melt and subsequent melts should be severely depleted. However, this is not observed and 523-l has a volatile content lower than does 527-11 (10 and 18 ppm N respectively). It may be that secondary processes have affected the observed concentration in the basalts. First, nitrogen may have been lost to vesicules or to the surrounding water during emplacement of the samples. Craig et al. (1967) have detected excess ‘He in ocean waters on top of mid-oceanic ridges and interpreted this as evidence for mantle degassing of primordial 3He. By analogy, nitrogen may also be degassing from the ridges; however, due to the large concentration of N2 in sea water, it would not be observable. If both samples had lost proportional amounts of nitrogen then, even though the absolute contents would be lower, the initial concentration ratio would have remained the same. Thus if degassing were present, it probably affected the samples unequally. The solubility of N2 in a glass melt is 3.5 X 10e4cm3 N2/cm3 melt at one atmosphere (Mulfinger and Meyer 1963) which is about 100 times lower than the observed nitrogen concentration in the basalts (3 X lo-* cm’/g). Taking into account the added pressure at the ocean floor (about 250 atm.) the solubility of N2 may be similar to the total nitrogen observed in the basalts. The solubility of a gas such as N2 or Ar is proportional to the partial pressure of the gas (Kirsten 1968). The solubility of ammonia in glass is about 104 times that of N2 at one atmosphere (Mulfinger and Meyer 1963). Thus the amount of nitrogen in the basalts is consistent with the solubility range of either N2 or ammonia, and hence neither species can be ruled out as a constituent based on solubility data. Assuming the partial melt values are correct and because 523-l and 527-l-l have nearly identical nitrogen concentration, suggests that the bulk distribution coefficient is nc.-. - to or greater than one. Hence C, = CdD, i.e. “‘, is independent of F and depends only on D (see Figure 2 of Hanson, 1977). However, this is contrary to the expected chemical behavior of nitrogen. In particular the ionic radius of NH: is similar to K+ (1.44 and 1.33 A respectively), they differ by only 7%. Though ammonia is most probably present in the basalts (Norris 1979) it may remain a dissolved species and not be protonNH: because of the similarity in ated to NH:.

371

radius and charge should correlate with K+. No correlation is observed since 523-l has a higher K content (.22 wt.%) than 527-l-l (.07 wt.%) (Langmuir et al. 1977). Whether nitrogen is as NH4, HN3, or N2 it should behave as an incompatible element with D 5 1. The above discussion is based on the assumption that there is no free gas phase present in the mantle and the volatiles are dissolved in the melt. Dymond and Hogan (1978) discuss some of the evidence concerning the existence of a separate CO2 gas phase in the mantle. Yoder (1976) succinctly stated the present beliefs regarding a mantle gas phase. “Some investigators believe there is always a gas phase present in the mantle because of the small amount of CO2 required for saturation of the magma. For example, only 4.8 f 1.0% CO* is required to saturate a diopside melt at 30 kbars and 1625°C whereas 21.5 f 1.0% Hz0 is required at the same pressure and 1265°C. Most workers, however, have the opinion that there is not a free gas phase present and the volatiles are stored in various hydrous and carbonate minerals. Most of the hydrous minerals break down at temperatures below melting and the volatiles if retained are consumed in the melt.” This view is supported by experimental studies on COz-silicate phase diagrams. It has been found that CO2 is not stable as a gas phase under mantle conditions (Newton and Sharp 1975; Irving and Wyllie 1973). Rather it may be present in the form of a carbonatite, e.g., MgCOj or CaC03, though a gas phase may be present in the melt during the emplacement of the melt on the sea floor due to the decreasing pressure. If the gas phase is present in the mantle, then the partitioning of nitrogen will be strongly favored in the gas phase if nitrogen behaves similarly to other volatiles and trace elements (Wendlandt and Harrison 1979). Presence of a vapor phase will control the distribution of nitrogen in the melt and not the solid/melt interface. It is not clear at this point whether the vapor phase is present, but it could be one explanation for the similarity of nitrogen abundances in all deep sea basalts, if it represents the residual nitrogen in the melt based on the distribution of nitrogen between the melt and gas phase and if all gas escaped during emplacement. It is clear from the above discussion that interpretation of the nitrogen concentration of the mantle based on the concentration in deep sea basalts is very difficult. What can best be done is to put limits on the possible range in the mantle. The lower limit is based on sample 523-l with an assumed D = 0 and a 10% partial melt, hence C, = .I C,, and then C,, = 1.6 ppm N. This is assuming the concentration is completely controlled by batch melting with no secondary affects, which is probably most valid for 5231 because it represents the first melt. It is not possible to put an upper limit on the abun-

378

I’. L. NORRIS AND 0. A. SCHAEFFER

dance of nitrogen in the mantle because if the FAMOUS data indicate that C, is independent of F then the relationship between C, and C,, is given by C, = CdD, and hence C, is dependent on D only and not F. Figure 2 of Hanson (1977) graphically demonstrates that for a wide range of D greater than or near one, C, is independent of F. If this is true and since D is unknown, then it is not possible to calculate C,. However, because nitrogen is most likely in the form of ammonia or molecular nitrogen and both are expected to behave as incompatible elements, the value of nitrogen in the mantle may lie closer to the lower limit. The presence of degassing would increase this value above the lower limit. Since it is impossible to be more precise with the mantle concentration and because only for the Mid-Atlantic Rift can a mantle content even be discussed, it is not clear whether this value holds for other parts of the mantle. It has been suggested that the mantle is heterogeneous with regards to major, minor, and trace elements as well as with 3He/4He and 4oAr/‘6Ar isotopic ratios (Kaneoka et al. 1978), so there is no reason to assume the mantle is homogeneous in nitrogen. 6. COMMENTS ON THE TOTAL NITROGEN INVENTORY OF THE EARTH The calculation of the total earth inventory of nitrogen requires a number of assumptions particularly on the mantle and core concentrations of nitrogen. Only for the atmosphere and crust are there good determinations of nitrogen content. Wlotzka (1961) in his study on the chemically bound nitrogen in crustal rocks found an average nitrogen content of 20 ppm N for the magmatic rocks. Though the samples may contain molecular nitrogen, the bound nitrogen represents a lower limit in estimating the inventory of the crust of 5 X 10” g N. This will not have a significant affect on the total nitrogen for the earth because the crust comprises only 0.4% of the total mass of the earth (Ringwood 1979). The nitrogen content of the mantle has already been discussed, and for the purposes of this discussion the lower average nitrogen content of 2 ppm N is assumed which would give a nitrogen inventory of 1O22g N. The atmosphere and sedimentary rocks have been grouped together following the suggestion of Rubey (1955) who considered the sediments as sinks for the total volatiles released throughout geologic history. They contribute 5 X 102’ g of nitrogen. Combining the nitrogen inventories for the crust, mantle and atmosphere gives a lower limit of 1.6 X 10” g N or about 3 ppm N for the total earth mass. There are no samples or methods to determine the nitrogen content of the earth’s core, but a poor analogy may be the iron meteorites even though they have formed under different oressure and temoera-

ture conditions, in addition to having a different differential history. The nitrogen content of iron meteorites is very heterogeneous ranging from 0.8 to 58 ppm N with some samples occasionally higher (Kothari and Goel, 1974; and Goel, 1970). The earth’s core inventory is very uncertain; because it comprises approximately 28% of the total earth mass and because the nitrogen content may be significantly greater than the mantle, it appears to clearly dominate any estimate for the total nitrogen inventory of the earth. If the core content is similar to the iron meteorites then up to 70% (based on 30 ppm N in the core) 01 the total nitrogen could be trapped in the cure. This nitrogen is most probably isolated from the mantle and hence from influencing the evolution of the earth’s atmosphere. Thus, even though .j much smaller proportion of the earth’s nitrogen 13 in the atmosphere, the amount that has degassed from the mantle (1 ppm N in the atmosphere from the 3 ppm N in the mantle) represents a significant proportion but is still much less than the apparently nearly complete degassing of the noble gases. .4cknowledgments_This is LO acknowledge the tnvaluable assistance of Dr. R. W. Stoenner of Brookhaven National Laboratory in obtaining the counting results. We would also like to acknowledge conversations with T. Owen, G Hanson, and C. Langmuir which contributed to the discussion. We would like to thank C. Fein for supplying us with the Pacific basalt samples, A. E. Bence for the F.4MOUS samples, and the late 0. Miiller for the synthettc glass. We also thank B. Kothari. D. Fisher, and D. Burnett for many useful suggestions on improving the manuscript. We would also like to thank Ms. Mildred O’Dowd for preparation of the manuscript for publication. The work was partially supported by NASA Grant NGL33-015.-174.

REFERENCES Abelson P. H. (1966) Chemicai Events on the Primitive Earth. Proc. Nat. Acad. Sci. 55, 1365 - 1371 Anders E. and Owen T. (1977) Mars and Earth. Origin and Abundance of Volatiles. Science 198, 453 465. Baur M. (1978) Thermodynamics of Heterogeneous Ironcarbon Systems: Implications for the Terrestrial Primitive Atmosphere. C/rem. Geol. 22, 189-206. Becker R. and Clayton R. ( 1977) Nitrogen Isotopes in Igneous Rocks. EOS 58. 536. Bruns M. (1976) Gaschromatographie nut Radon 222, Diplomarbeit, Heidelberg University. Buchwald V. and Scott E. (1971) First Nitride (CrNt in Iron Meteorites. Nature Whys. Sci. 233, I 13 114. Canales R., Alexander E. and Manuel 0. (196X) Terrestriai Abundances of Noble Gases. J. Geophys. RBJ 73, 333 I 3334. Clark S., Turekian K. and Grossman L. (1972) Model for the Early Historv of the Earth. In The Nature of the Solid E&h, (ed: E. C. Robertson), Chap. I. pp. !3 IX. McGraw Hill Pub. Craig H., Weiss R. and Clark u’. ( 1967) Dissolved Gases in the Equatorial and South Pacific Ocean. J Geoph~~.r Res. 72, 6165-6181. Dalrymple G. B. and Moore J. B. (1968) Argon 40: Excess in Submarine Pillow Basalts from Kilauea Volcano, Ha. waii. Science 161, 1132-l 135. Dymond J. and Hogan L. (1973) Noble Gas Abundance

N2 IN BASALTS Patterns in Deep Sea Basalts-Primordial Gases from the Mantle.Earth Planet. Sci. Lett. 20, 131-139. Dymond J. and Hogan L. (1978) Factors Controlling the Noble Gas Abundance Patterns of Deep Sea Basalts. Earth Planer. Sci. Lert. 38, 117-128. Fanale F. (1971) A Case for Catastrophic Early Degassing of the Earth. Chem. Geol. 8, 79-105. Fanale F. and Cannon W. (1971) Physical Adsorption of Rare Gas on Terrigeneous Sediments. Earth Planet. Sci. Lett. 11, 362-368. Fireman E., Defelice J. and D’amico J. (1976) The Abundance of >He and 14C in the Solar Wind. Earth Planet. Sci. Left. 32, 185-190. Fisher D. (1970) Heavy Rare Gases in a Pacific Seamount. Earth Planet. Sci. Letr. 9, 331-335. Fisher D. ( 197 1) Incorporation of Ar in East Pacific Basalts. Earth Planer. Sci. Lett. 12, 321-324. Fisher D. (1974) Planetary Primordial Component of Rare Gases in the Deep Earth. Geophys. Res. Letr. 1, 161164. Gibson E. and Moore C. (1970) Inert Carrier Gas Fusion Determination of Total Nitrogen in Rocks and Minerals. Anal. Chem. 42, 461. Gibson E. and Moore C. (1971a) Total Nitrogen Content of Ordinary Chondrites. Chemie der Erde 30, 115- 13 1. Goel P. (1970) Determination of Nitrogen in Iron Meteorites. Geochim. Cosmochim. Acta 34, 932-935. Goel P. and Kothari B. (1972) Total Nitrogen Content of Some Apollo 14 Lunar Samples by Neutron Activation Analysis. Proc. 3rd. Lunar Sci. Conf, Vol. 2, pp 20412050. Hart M. (1978) The Evolution of the Atmosphere of the Earth. Icarus 33, 23-39. Hanson G. N. (1977) Geochemical Evolution of the Suboceanic Mantle. J. Geol. Sot. Lond. 134, 235-253. Hill M. (1963) The Composition of Sea Water-Comparative and Descriptive Oceanography. In The Sea, Vol. 2, p. 172, Interscience Publ. Holland H. (1962) Model for the Evolution of the Earth’s Primitive Atmosphere. In Petrologic Studies., G.S.A. Spec. Publ., pp. 447-477. Irving A. aud Wyllie P. (1973) Melting Relationships in CaO-CO2 and MgO-CO2 in the Mantle. Earth Planet. Sri. Letr. 20, 220-225. Kaneoka I., Takaoka N. and Aoki K. (1978) Terrestrial Rare Gases, (editors E. C. Alexander and M. Ozima), pp. 7 I-84. Tokyo: Center for Academic Publications Japan, Japan Scientific Societies Press. Kirsten T. (1968) Incorporation of Rare Gases in Solidifving Enstatite Melts. J. Geoohvs. Res. 73. 2807-2810. Koihah B. and Goel P. (1974) Total Nitrogen in Meteorites. Geochim. Cosmochim. Acta 38, 1493- 1507. Kummer F., Stoenner R. and Davis R. (1972) Construction of Miniature Low Level Proportional Counters. Brookhaven National Laboratory, BNL Report 16972. Kung C. and Clayton R. (1978) Nitrogen Abundances and Isotopic Compositions in Stony Meteorites. Earth Planet. Sci. Lett. 38, 421-435. Langmuir C., Bender J., Bence A. and Hanson G. (1977)

379

Petrogenesis of Besalts from the FAMOUS Area-MidAtlantic Rift. Earth Planer. Sci. Lett. 36, 133-156. Larimer J. (1971) Composition of the Earth: Chondritic or A-chondritic. Geochim. Cosmochim. Acta 35, 769786. Miller S. and Orgel L. (1974) The Origin of Life on the Earth, pp. 33-52. Prentice Hall Publ. Mulfinger H. and Meyer H. (1963) Physical and Chemical Solubility of Nitrogen in Glass Melts. Glasrech. Ber. 36, 481-483. Muller O., (1972) Chemically Bound Nitrogen Abundances in Lunar Samples and Active Gases Released by Heating at Lower Temperatures (250°C to 500°C). Proc. 3rd Lunar Sci. Co$, Vol. 2, pp. 2059-2068. Muller 0.. Grallath E. and Fole G. (1976) Nitrogen in Lunar Igneous Rocks. Proc. 7;h Lunar Sk ConfT, Vol. 2, pp 1615-1622. Newton R. and Sharp W. (1975) Stability of Fosterite and CO, and its Bearing on the Role of CO, in the Mantle. Earth Planet. Sci. Lett. 26, 239-244. Norris T. (1979) Total Nitrogen Content of Deep Sea Basalts with Implications for the Origin of the Earth’s Atmosphere. Ph.D. Thesis, State University of New York at Stony Brook. Ringwood A. ( 1979) On the Origin of the Earth and Moon, pi l-295. Springer-Verlag P;bl. Rubev W. (1955) Develoument of the Hvdrosohere and Atkosphdre, with Spec’ial Reference d the’ Probable Composition of the Early Atmosphere. GSA. Spec. Paper 62, pp. 631-650. Schilling J. (1966) Rare Earth Fractionation in Hawaiian Volcanic Rocks. Ph.D. Thesis, Mass. Inst. Tech. Shukla P. N., Kothari B. K. and Goel P. S. (1978) Simultaneous Determinations of Nitrogen and Lithium by Thermal Neutron Activation Analysis. Anal. Chim. Acta 96, 159-269. Stoenner R., Lyman W. and Davis R. (1970) Cosmic Ray Production of Rare Gas Radioactivities and Tritium in Lunar Material. Science 167, 553-555. Urey H. (1959) The Atmospheres of the Planets. In Handbuch der Physik. Vol. 52, pp. 363-418, Springer-Verlag, Berlin. Verniani F. (1966) The Total Mass of the Earth’s Atmosphere. J. Geophys. Res. 71, 385-391. Walker C. (1976) Implications for Atmospheric Evolution of the Inhomogeneous Accretion Model of the Origin of the Earth. In The Early History of rhe Earth, (ed. B. F. Windley), pp. 537-546. Walker C. (1977) Evolution of the Atmosphere, pp. 179208, MacMillan Publ. Wendlandt R. and Harrison W. (1979) Rare Earth Partitioning between Immiscible Carbonate and Silicate Liquids and CO, Vapor: Results and Implications for the Formation of Light Rare Earth Enriched Rocks. Contrib. Mineral. Petrol. 69, 409-4 19. Wlotzka F. (1961) Untersuchungen zur Geochemie des Stickstoffs.Geochim. Cosmochim. Acta 24, 106- 154. Yoder H. (1976) Generation of Basaltic Magma, pp. l265, Nat. Acad. Sci. Publ.