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Natural variations in the nitrogen isotope abundance ratio in igneous rocks
Nabimi viuiations in th8 nitmg8n idog inign8ousroclrs* -xl
aImdane ratio
K. I. Mavrrx L&&ory, oxford
ourveyofthe smount aad bIotopic oompceition of the nitrogm found iu 8 AbBka-Abrief *“es of igneola stat tbcrc in & mlbetant~ wuaiorl (about & f&tar of 5%) in the yield of of rock; end that the ieotapic compoeitiin of the nitrogen varies mgubrly with the ~~~~~of.fsw,~t. INTBODVCTION
Nrr.uoa~w is cosmically one of the more abundant elements; but on the earth it has been depleted (relative to Si) by a factor of app~~rna~ly 745 x 1O-’ (USEY, 19&X2), because of its ability to form volatile compounds which were probably lost during the formative stage of the planet. A study of the terrestrial occurrence of nitrogen and its isotopic composition would contribute towards understanding the conditions at the time of the formation of the earth and the development of its atmosphere. There is little Feference to be found in the literature to work on nitrogen contained in igneous rocks. In some early work, Loaz, RA~LBI~H ( 1939) found that on heating to redness for 4-5 hr a sample of rock which was ground to a fine powder and mixed with an approximately equal amount of CuO, the yield of nitrogen was constant, within about f2O per cent, at 0-04 cm* s.t.p./g of rock for the wide range of igneous rooks which he investigated. This nitrogen he considered to be in the form of an ammonium compound in the rock. Recently, HOEIUX~ (1956) degassed samples of crushed rock at 14OO'C for several hours in an induction furnace, and found the nitrogen content to be about one hundred times less than the value given by RAYLEIOE. The nitrogen from some granites was less than lo/, enriched in IN with respect to atmospheric nitrogen. The low nitrogen content found by Hoxn~~o has been questioned by workers engaged in extracting argon from igneous rocks. Nitrogen has also been reported as occurring in chondritio meteorites to the extent of 0.0008 cma s.t.p./g (NODDACK, 1934), about fifty times less than RAYLEI~H’S value for igneous rocks. No isotopic analysis was made. Large variations in the isotopic composition of nitrogen from radioactive minerals have been reported by WHITE and YACSODA (ISrSO),but not confirmed by Hoxnrxo. The isotopic ratio showed a marked age effect, and it has been suggested that the high level of radioactivity may be the cause. Only rocks of low activity (several p.p.m. of uranium and thorium) are considered here. Biological fractionation of the nitrogen isotopes has been reported by Holu~lrwa (1956) and POWELL et a.& (1957), but is not dealt with here. *Inths~t~ofI951thesuthbrwespriPilegsdtollpsndetsnnin~.H.C.U~’s~ at the IJniversity of Chicago, and et hia suggestioo en inveetigatkm wee begun of the netureUy occurriog verietiine in the isotopic ratio of the nitrogen found in igneous rocks The work wee never completed, but in view of recent publications by HOBBIN~ (1956) and PAU~ZL st d. (1967), the rem&a obt&ed mey be of some intereat.
185
IL I. MAYNE
EXPERIMENTAL DETAILS In the present work the method of degaaaingthe rock sample wan in principle similar to that adopted by RAYIZIOH. Meet of the samplee obtainable were unfortunately not freshly mined; however, in every cam the outaide expceed portion of the sample was broken off and dincarded and only a piece from the inside wea used. Thie waa crushed, and ground to pess 120-mesh.
Tobighvouambw
Fig. 1. Table 1 Ssmple
Location
Rhyolite
Lipari Inland
Hornblende-micaandeeita
Baflalt
Hooeac Mt., Eureka County, Nevada. Mauna Hualalai Hawaii. Disco Island
Baa&
Holyoke, Mess.
Olivine
Braunnhauaen, S.W. Germany. Zehren. nr. Meiaeen, Germany. Mauna Loa, Hawaii Monolake. Calif. Jackson County N. Caroline
Trachyte
Granite
Olivine besalt Obsidian Dunite*
l
se0
Age (approximate)
Yield (cm~ e.t.p./g)
b
Recent, extrusive Tertiary 30 m.y.
0448
- 16*0
O-0187
-11.8
Recent, highly eiliceo~. Tertiary 40 m.y. Triaeaic 160 m.y. Permian 200 m.y. Cerboniferoue 260 m.y.
0.016
Lava from 1942eruption Recent Pre-Cambrian 500 m.y.
text. 186
0.015
-1.2 -1.0 -0.2
0.016 0.018 0.017
+4*r) +6*2 +&Q
0.0074 0@069 0@070 0.0025
+8.1 +8.0
0.0022\ 0.003 1 O&O18
+Q.Q + 30.9 + 5.5
Natural varidionm
in the nitrogenisotope8bund8nw ratioin @mowm&m
dried in an oven, snd ebout SOg weighed out. A somewhet greater 8mount of CuO w8a weighed out (the CuO wee prepared by debasing Merck CuO and then re-oxidizing with nitrogen-f+ee oxygen). Ma& of this wea mixed with the powdered rock and loaded into 8 aili08 tube, oloeedoff et one end, whioh formed 8 lining for 8 etainleae-&eel combuetion tube: the rem8inder of the CuO wlyl edded on top of the mixture. The lining wee inserted in the combuetion tube, which w8e joined on to the gaeh8ndling eyetem ehown in Fig. 1 by meens of 8 gre8wd oone and no&et joint. The tube wee pumped out very slowly, vecuum-teetad, and degemed 8t 8bOUt 109Y.!for several hours while being pumped, to remove 8dsorbed ti. A fimd tat of the vacuum wae mede by ieol8ting the system overnight. In order to degas the rook, the temper8ture of the oombuetion tube, while cloeed off from the vacuum system, wee rsiwd elowly to 960°C snd nxsintainedthere for et leest 16hr. It ~88 found by eXp&mmt th8t 8 few houre Only were 8pp8Rintly tioient to ertXeCtall Of the nitrogen; 8t lee& oontinuing the tratment for periods up to 30 hr geve no further yield. The yielde from three eemplee of the Beme piece of granite subjected to widely v8rying beet freetment ere given in Table 1; they indicate the coneietencyto be expected in the experiment& The gaeeoue producta of the combuetion were then cleaned, to leeve only nitrogen end the r8re g8m0; the 8mount of theeelletter repreeented Only 8 nm8ll pert of the t.ot8lgas abmple. Toeppler Firstly, the gee wee pumped into the CuO furnece, by me8ne.of an eleotricallyOpereted pump, end then, while the CuO fuxaoe wea heated to 890°C end allowed to 0001,the gea wee continu8lly oirou&ed over the CuO, condeneibleebeing removed by the liquid-nitrogen-cooled trap. In this wey, q. CO, CO,, SO,, end hydraxrbonm 8re effeotively removed. The gee was then Groul8ted over hot copper meintained 8t 4OO“C,in order to remove 0,. Fimdly, the NI end thereregaeeremeining were pumped into 8 oonstant-volumem8nometer, end the volume of gee me8euredby obeerving the premure. For ieotopic8n8lyei8,the gee w8e compreeaedinto 8 Bample tube and t&erred to the g8sh8ndling eyetem of 8 double-collectionM-epectrometar of the Nier type, previouely deearibed by ALLEN d d. (1960). Thin wan deeigned for the meeaurement of em811differenceein ieotopic r8tioe between eemplea and 8 etand8rd.
RESULTS The yield and isotopic composition of the nitrogen obtained from a range of igneous rocks are given in Table 1. The isotopic variation d shown in the last column is expressed in the usual way as 615Ns
=
(‘5W4NLm~e - (‘5N/‘4Wt.mmx (‘5V”%w,,~
1000
The standard for comparison in all this work was atmospheric nitrogen, obtained by cleaning a sample of air in the gas-handling system. Thus, positive values for b indicate an excess of lbN in the sample, and negative values an excess of l*N, with respect to atmospheric nitrogen. The uncertainty of the measurements of the isotopic composition of the standard was less than O-27,. Samples of nitrogen from the same rock prepared at different times were about as good as this, when the amount of gas sample was large. However, where the yield of nitrogen is very small, the precision is adversely affected by background in the mass spectrometer and any CO contamination which may remain in the gas sample. Furthermore, with low yield the effect of incomplete desorption of atmospheric nitrogen, or of a small leak in the vacuum system, is most pronounued. Thus, in the case of the Jaokson County Dunite, HOERIN~found a nitrogen yield of less than O-0004 cm8 s.t.p./g of rock. If the higher yield reported here is due to contamination by atmospheric nitrogen, the true value of d for the nitrogen of the rock is greater than 30YW. 187
K. I. MAYNE
From the data in Table 1 it ie evident that there is a substantial variation in the yield of nitrogen per gram of rock, and in its isotopio composition. The extent of the isotopic fractionation is, however, less than that reported for hydrogen, carbon, and sulphur, and about the same as the measured fractionation of the oxygen isotopes in nature. The high values of nitrogen content reported here correspond closely to LORD RAYLEIOE'S abundance figures, the lowest ones are in reeeonable agreement with HOEEIN~‘S determinations; and there is a continuous variation from one to the other. Contrary to RAYLEIOH’Sfindings, the nitrogen Although the data are insufficient to content of igneous rocks is not constant. draw any detailed conclusions, there is a parallelism between the nitrogen content and its isotopic composition, in that the more depleted the rock is in nitrogen, the more enriched this is in lsN. The logarithm of the nitrogen yield is an approximately linear function of 6. There appears to be no well-defined correlation of the age of the rock sample with the isotopic composition or yield of its nitrogen; poesibly any age-effect which may exist is obscured in some cases by a m-melting process; considerably more data from well-selected rocks are required on this point. The measured effect can hardly be produced by an incomplete extraction of nitrogen from the samplea; if this were the case, one would expect that the nitrogen which is extracted, if showing any fractionation at all, would be enriohed in “N. It could be produced by preferential loss of l’N during the preliminary desorbing process; but the fact that the German granite samples gave consistent results under widely varying desorbing treatment makee this appear unlikely. One might suppose that the total nitrogen content of the rocks ia constant, aa RAYLEZOH fin&, but occurs in various chemical compounda, from Borne of which it can be extracted by the present method, and some of which are exceedingly stable, even in the presence of water, at 960°C; and that 14N ia enriched in the more stable form. Since it is most unlikely that all of the rocks of the earth’s crust would be leached of nitrogen to exactly the same extent, the constant nitrogen yield found by RAYLEIOHwould suggest that the earth.8 atmosphere is not derived from the crust, but is in large part primordial. On this basis also, together with the present isotopic analyaia, atmospheric nitrogen would be “heavy” with respect to the average of crustal nitrogen. It seems more likely that the effects governing the distribution of nitrogen and its isotopes, as measured here, represent a proceee of loss of nitrogen from the cruet to the atmoaphere, and in such a way that the nitrogen remaining in the rocks is concentrated in 15N. The evidence from the present work, and from that of HOERINO, and of the NODDACKS, would suggest that very old rocke contain only small amounta of nitrogen. If the Pre-Cambrian rocks of the earth originally held as much nitrogen aa some of the recent ones now contain (i.e. O-05 cmS N/g), then from an estimated 2.5 x 1O*sg of rock in the earth’s cruet (of which about 90 per cent is &-Cambrian) approximately one-half of the 4 x lO**g of nitrogen in the atmosphere would be derived by leaching and weathering, quite apart from any contribution from the mantle. A substantial nitrogen-containing primitive atmosphere would not be 188
Net~~~~~t~~
-tiin~molp
required for the earth. On thie bseia 8180the nitrogen re&ling in the cnmt is only a few per cent of that in the atmosphere. Unyy, (1962, p. 123, eta.)ha8 coneidemd in Bornedetail the thermodynamic stability of nitrogen compound8 in the presence of cosmic proportion8 of the element+ over a range of totalpreaanre(in effect the hydrogen pre88ure)audtemperature, In particular, he ha8 shown (in Rig. 13, p. 129) the region8 of temperature and pre88m-ein which nitrogen can be retained a8 nonvolatile N&Cl; and in which more than half of the nitrogen will be pre8ent a8 ammonia. If the earth accretcd under thege condition8 of (low) temperature and pre88ure, then the nitrogen of the rock8 would be largely in ammoniacal form, ae RA~LEIC+Ebelieved it was. Nitride8 of some of the element8 may al80 be present. (At the higher temperatures, -1200°C, which exiet in the earth’s cru8t today, thiS eituation may not hold; and unless the pressure of hydrogen i8 great enough, the nitrogen may exiet largely a8 ga8, di88olvedin the melt. A8 the melt rices and coofB,the unction favour ammonium-acrltformation.) A8 theett8alt8are leached from the rock8, %ome fractionation of the nitrogen ieotope8 can be produced (hm~ara3?r and RIoELEISEN, 1960). It would 8eem de&able to examine the ohemical form in which the nitrogen is present in the rocks. It would aleo be interesting to have data on the nitrogen of etone meteoritea. AcR author ie extremely grateful t0 Prof. H. C. U~EY for making possible his visit to Chicago in 1QSl. Prof. WBEY ie alway a 8ouree of inspiration, and ha8 shown intereet in the pre8ent work; he ha8 been good enough to read thie manuscript. Thank8 are al8o due to Prof. L. IL W~cnntand Dr. H. D. HOLLAND for reading the manwcript. REFEI~~NCES hIEDMAN
H. A. d 4. (1960) Rev. Se& Inshutn. $3,724. L. and B~ommsm J. (1960)J. t%un. P&u. ~s,lS~S.
EOEZEXIW T. peaf3) Nuclear -
in GeoIogit?settm NSrnC, proasedingrrof Ek%xui chfkf9n-,p. 39. NODDAOX I. end NODDACK W, (1934)S-k. Ksm. Tidskr.4& 173. Pmwn~A.,RnuwcR., and WIF. E. (1967)Ueochim. SLCYosmocil&n. Ada 11,166. RALPH LOBD (1939)Pwc.Roy.Soc.Al'?0,451.
Uauv H. C. (1962)The Pkm&. oxford UniversityPrem. WurrwW, and YA~oDAH.(~QUO)&~TWX ill,307.
189
aImdane ratio
K. I. Mavrrx L&&ory, oxford
ourveyofthe smount aad bIotopic oompceition of the nitrogm found iu 8 AbBka-Abrief *“es of igneola stat tbcrc in & mlbetant~ wuaiorl (about & f&tar of 5%) in the yield of of rock; end that the ieotapic compoeitiin of the nitrogen varies mgubrly with the ~~~~~of.fsw,~t. INTBODVCTION
Nrr.uoa~w is cosmically one of the more abundant elements; but on the earth it has been depleted (relative to Si) by a factor of app~~rna~ly 745 x 1O-’ (USEY, 19&X2), because of its ability to form volatile compounds which were probably lost during the formative stage of the planet. A study of the terrestrial occurrence of nitrogen and its isotopic composition would contribute towards understanding the conditions at the time of the formation of the earth and the development of its atmosphere. There is little Feference to be found in the literature to work on nitrogen contained in igneous rocks. In some early work, Loaz, RA~LBI~H ( 1939) found that on heating to redness for 4-5 hr a sample of rock which was ground to a fine powder and mixed with an approximately equal amount of CuO, the yield of nitrogen was constant, within about f2O per cent, at 0-04 cm* s.t.p./g of rock for the wide range of igneous rooks which he investigated. This nitrogen he considered to be in the form of an ammonium compound in the rock. Recently, HOEIUX~ (1956) degassed samples of crushed rock at 14OO'C for several hours in an induction furnace, and found the nitrogen content to be about one hundred times less than the value given by RAYLEIOE. The nitrogen from some granites was less than lo/, enriched in IN with respect to atmospheric nitrogen. The low nitrogen content found by Hoxn~~o has been questioned by workers engaged in extracting argon from igneous rocks. Nitrogen has also been reported as occurring in chondritio meteorites to the extent of 0.0008 cma s.t.p./g (NODDACK, 1934), about fifty times less than RAYLEI~H’S value for igneous rocks. No isotopic analysis was made. Large variations in the isotopic composition of nitrogen from radioactive minerals have been reported by WHITE and YACSODA (ISrSO),but not confirmed by Hoxnrxo. The isotopic ratio showed a marked age effect, and it has been suggested that the high level of radioactivity may be the cause. Only rocks of low activity (several p.p.m. of uranium and thorium) are considered here. Biological fractionation of the nitrogen isotopes has been reported by Holu~lrwa (1956) and POWELL et a.& (1957), but is not dealt with here. *Inths~t~ofI951thesuthbrwespriPilegsdtollpsndetsnnin~.H.C.U~’s~ at the IJniversity of Chicago, and et hia suggestioo en inveetigatkm wee begun of the netureUy occurriog verietiine in the isotopic ratio of the nitrogen found in igneous rocks The work wee never completed, but in view of recent publications by HOBBIN~ (1956) and PAU~ZL st d. (1967), the rem&a obt&ed mey be of some intereat.
185
IL I. MAYNE
EXPERIMENTAL DETAILS In the present work the method of degaaaingthe rock sample wan in principle similar to that adopted by RAYIZIOH. Meet of the samplee obtainable were unfortunately not freshly mined; however, in every cam the outaide expceed portion of the sample was broken off and dincarded and only a piece from the inside wea used. Thie waa crushed, and ground to pess 120-mesh.
Tobighvouambw
Fig. 1. Table 1 Ssmple
Location
Rhyolite
Lipari Inland
Hornblende-micaandeeita
Baflalt
Hooeac Mt., Eureka County, Nevada. Mauna Hualalai Hawaii. Disco Island
Baa&
Holyoke, Mess.
Olivine
Braunnhauaen, S.W. Germany. Zehren. nr. Meiaeen, Germany. Mauna Loa, Hawaii Monolake. Calif. Jackson County N. Caroline
Trachyte
Granite
Olivine besalt Obsidian Dunite*
l
se0
Age (approximate)
Yield (cm~ e.t.p./g)
b
Recent, extrusive Tertiary 30 m.y.
0448
- 16*0
O-0187
-11.8
Recent, highly eiliceo~. Tertiary 40 m.y. Triaeaic 160 m.y. Permian 200 m.y. Cerboniferoue 260 m.y.
0.016
Lava from 1942eruption Recent Pre-Cambrian 500 m.y.
text. 186
0.015
-1.2 -1.0 -0.2
0.016 0.018 0.017
+4*r) +6*2 +&Q
0.0074 0@069 0@070 0.0025
+8.1 +8.0
0.0022\ 0.003 1 O&O18
+Q.Q + 30.9 + 5.5
Natural varidionm
in the nitrogenisotope8bund8nw ratioin @mowm&m
dried in an oven, snd ebout SOg weighed out. A somewhet greater 8mount of CuO w8a weighed out (the CuO wee prepared by debasing Merck CuO and then re-oxidizing with nitrogen-f+ee oxygen). Ma& of this wea mixed with the powdered rock and loaded into 8 aili08 tube, oloeedoff et one end, whioh formed 8 lining for 8 etainleae-&eel combuetion tube: the rem8inder of the CuO wlyl edded on top of the mixture. The lining wee inserted in the combuetion tube, which w8e joined on to the gaeh8ndling eyetem ehown in Fig. 1 by meens of 8 gre8wd oone and no&et joint. The tube wee pumped out very slowly, vecuum-teetad, and degemed 8t 8bOUt 109Y.!for several hours while being pumped, to remove 8dsorbed ti. A fimd tat of the vacuum wae mede by ieol8ting the system overnight. In order to degas the rook, the temper8ture of the oombuetion tube, while cloeed off from the vacuum system, wee rsiwd elowly to 960°C snd nxsintainedthere for et leest 16hr. It ~88 found by eXp&mmt th8t 8 few houre Only were 8pp8Rintly tioient to ertXeCtall Of the nitrogen; 8t lee& oontinuing the tratment for periods up to 30 hr geve no further yield. The yielde from three eemplee of the Beme piece of granite subjected to widely v8rying beet freetment ere given in Table 1; they indicate the coneietencyto be expected in the experiment& The gaeeoue producta of the combuetion were then cleaned, to leeve only nitrogen end the r8re g8m0; the 8mount of theeelletter repreeented Only 8 nm8ll pert of the t.ot8lgas abmple. Toeppler Firstly, the gee wee pumped into the CuO furnece, by me8ne.of an eleotricallyOpereted pump, end then, while the CuO fuxaoe wea heated to 890°C end allowed to 0001,the gea wee continu8lly oirou&ed over the CuO, condeneibleebeing removed by the liquid-nitrogen-cooled trap. In this wey, q. CO, CO,, SO,, end hydraxrbonm 8re effeotively removed. The gee was then Groul8ted over hot copper meintained 8t 4OO“C,in order to remove 0,. Fimdly, the NI end thereregaeeremeining were pumped into 8 oonstant-volumem8nometer, end the volume of gee me8euredby obeerving the premure. For ieotopic8n8lyei8,the gee w8e compreeaedinto 8 Bample tube and t&erred to the g8sh8ndling eyetem of 8 double-collectionM-epectrometar of the Nier type, previouely deearibed by ALLEN d d. (1960). Thin wan deeigned for the meeaurement of em811differenceein ieotopic r8tioe between eemplea and 8 etand8rd.
RESULTS The yield and isotopic composition of the nitrogen obtained from a range of igneous rocks are given in Table 1. The isotopic variation d shown in the last column is expressed in the usual way as 615Ns
=
(‘5W4NLm~e - (‘5N/‘4Wt.mmx (‘5V”%w,,~
1000
The standard for comparison in all this work was atmospheric nitrogen, obtained by cleaning a sample of air in the gas-handling system. Thus, positive values for b indicate an excess of lbN in the sample, and negative values an excess of l*N, with respect to atmospheric nitrogen. The uncertainty of the measurements of the isotopic composition of the standard was less than O-27,. Samples of nitrogen from the same rock prepared at different times were about as good as this, when the amount of gas sample was large. However, where the yield of nitrogen is very small, the precision is adversely affected by background in the mass spectrometer and any CO contamination which may remain in the gas sample. Furthermore, with low yield the effect of incomplete desorption of atmospheric nitrogen, or of a small leak in the vacuum system, is most pronounued. Thus, in the case of the Jaokson County Dunite, HOERIN~found a nitrogen yield of less than O-0004 cm8 s.t.p./g of rock. If the higher yield reported here is due to contamination by atmospheric nitrogen, the true value of d for the nitrogen of the rock is greater than 30YW. 187
K. I. MAYNE
From the data in Table 1 it ie evident that there is a substantial variation in the yield of nitrogen per gram of rock, and in its isotopio composition. The extent of the isotopic fractionation is, however, less than that reported for hydrogen, carbon, and sulphur, and about the same as the measured fractionation of the oxygen isotopes in nature. The high values of nitrogen content reported here correspond closely to LORD RAYLEIOE'S abundance figures, the lowest ones are in reeeonable agreement with HOEEIN~‘S determinations; and there is a continuous variation from one to the other. Contrary to RAYLEIOH’Sfindings, the nitrogen Although the data are insufficient to content of igneous rocks is not constant. draw any detailed conclusions, there is a parallelism between the nitrogen content and its isotopic composition, in that the more depleted the rock is in nitrogen, the more enriched this is in lsN. The logarithm of the nitrogen yield is an approximately linear function of 6. There appears to be no well-defined correlation of the age of the rock sample with the isotopic composition or yield of its nitrogen; poesibly any age-effect which may exist is obscured in some cases by a m-melting process; considerably more data from well-selected rocks are required on this point. The measured effect can hardly be produced by an incomplete extraction of nitrogen from the samplea; if this were the case, one would expect that the nitrogen which is extracted, if showing any fractionation at all, would be enriohed in “N. It could be produced by preferential loss of l’N during the preliminary desorbing process; but the fact that the German granite samples gave consistent results under widely varying desorbing treatment makee this appear unlikely. One might suppose that the total nitrogen content of the rocks ia constant, aa RAYLEZOH fin&, but occurs in various chemical compounda, from Borne of which it can be extracted by the present method, and some of which are exceedingly stable, even in the presence of water, at 960°C; and that 14N ia enriched in the more stable form. Since it is most unlikely that all of the rocks of the earth’s crust would be leached of nitrogen to exactly the same extent, the constant nitrogen yield found by RAYLEIOHwould suggest that the earth.8 atmosphere is not derived from the crust, but is in large part primordial. On this basis also, together with the present isotopic analyaia, atmospheric nitrogen would be “heavy” with respect to the average of crustal nitrogen. It seems more likely that the effects governing the distribution of nitrogen and its isotopes, as measured here, represent a proceee of loss of nitrogen from the cruet to the atmoaphere, and in such a way that the nitrogen remaining in the rocks is concentrated in 15N. The evidence from the present work, and from that of HOERINO, and of the NODDACKS, would suggest that very old rocke contain only small amounta of nitrogen. If the Pre-Cambrian rocks of the earth originally held as much nitrogen aa some of the recent ones now contain (i.e. O-05 cmS N/g), then from an estimated 2.5 x 1O*sg of rock in the earth’s cruet (of which about 90 per cent is &-Cambrian) approximately one-half of the 4 x lO**g of nitrogen in the atmosphere would be derived by leaching and weathering, quite apart from any contribution from the mantle. A substantial nitrogen-containing primitive atmosphere would not be 188
Net~~~~~t~~
-tiin~molp
required for the earth. On thie bseia 8180the nitrogen re&ling in the cnmt is only a few per cent of that in the atmosphere. Unyy, (1962, p. 123, eta.)ha8 coneidemd in Bornedetail the thermodynamic stability of nitrogen compound8 in the presence of cosmic proportion8 of the element+ over a range of totalpreaanre(in effect the hydrogen pre88ure)audtemperature, In particular, he ha8 shown (in Rig. 13, p. 129) the region8 of temperature and pre88m-ein which nitrogen can be retained a8 nonvolatile N&Cl; and in which more than half of the nitrogen will be pre8ent a8 ammonia. If the earth accretcd under thege condition8 of (low) temperature and pre88ure, then the nitrogen of the rock8 would be largely in ammoniacal form, ae RA~LEIC+Ebelieved it was. Nitride8 of some of the element8 may al80 be present. (At the higher temperatures, -1200°C, which exiet in the earth’s cru8t today, thiS eituation may not hold; and unless the pressure of hydrogen i8 great enough, the nitrogen may exiet largely a8 ga8, di88olvedin the melt. A8 the melt rices and coofB,the unction favour ammonium-acrltformation.) A8 theett8alt8are leached from the rock8, %ome fractionation of the nitrogen ieotope8 can be produced (hm~ara3?r and RIoELEISEN, 1960). It would 8eem de&able to examine the ohemical form in which the nitrogen is present in the rocks. It would aleo be interesting to have data on the nitrogen of etone meteoritea. AcR author ie extremely grateful t0 Prof. H. C. U~EY for making possible his visit to Chicago in 1QSl. Prof. WBEY ie alway a 8ouree of inspiration, and ha8 shown intereet in the pre8ent work; he ha8 been good enough to read thie manuscript. Thank8 are al8o due to Prof. L. IL W~cnntand Dr. H. D. HOLLAND for reading the manwcript. REFEI~~NCES hIEDMAN
H. A. d 4. (1960) Rev. Se& Inshutn. $3,724. L. and B~ommsm J. (1960)J. t%un. P&u. ~s,lS~S.
EOEZEXIW T. peaf3) Nuclear -
in GeoIogit?settm NSrnC, proasedingrrof Ek%xui chfkf9n-,p. 39. NODDAOX I. end NODDACK W, (1934)S-k. Ksm. Tidskr.4& 173. Pmwn~A.,RnuwcR., and WIF. E. (1967)Ueochim. SLCYosmocil&n. Ada 11,166. RALPH LOBD (1939)Pwc.Roy.Soc.Al'?0,451.
Uauv H. C. (1962)The Pkm&. oxford UniversityPrem. WurrwW, and YA~oDAH.(~QUO)&~TWX ill,307.
189