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Search for3He in deep-sea basalts
EARTH AND PLANETARY SCIENCE LE1TERS 8 (1970) 77-78 . NORTH-HOLLAND PUSLISHIN; COMPANY
SEARCH FOR 3 He IN DEEP-SEA BASALTS David EXISHER fMiverdly of Miami, Rosenstiel School ofMarine and A tmospheric Sciences, 10 Rickenbacker Causeway, Miami, Florida 33149, USA
Received 22 January 1970 Johnson and Axford [ I 1 have suggested that the dominant form of 3 He production in the earth's atiinosphere is from aurora] precipitation of solar wind plasma. Clarke et al . [21 have suggested that the original 3 He content of the earth might have been about the same as the average chondritic primordial abundance, that most of this 3 He might be retained still in the deep earth, and that a significant portion of the atmospheric 3 He may result from leakage through the ocean floor . The present atmospheric 3 He content is 4 .4 X 10 -13 cc STP/g ; the gas-rich meteorites contain up to 5 X 10 -6 cc STP/g primordial 3 He ; the ordinary chondrite contains about 10 -10 cc STP/g ; an "average c .hon-
dritic" (107., gas-rich, 90% ordinary) abundance is 8 about 10_ cc STP/g . We have found [3, 41 that certain deep sea ûasalts contain excess 4 He and 40 Ar, with 4 He/ 4~o Ar ratios of up to about 15 . This indicates a rare gas component present in the magma before eruption and trapped in the rock ; i .e ., not released during eruption of the rock onto the surface of the earth . Such an inhibition of the normal (sub-aerial) degassing process is reasonable in light of the nearzero temperatures and high hydrostatic pressures of the ocean water environment into which the magma erupts. Since the highest observed 4 He/ 40 Ar ratios are approximately the maximum values that one would expect from Th, U and K decay in a closed system, with terrestrial Th, U and K values, it appears that these rocks have not lost significänt ;.mounts of 4 He
Contribution from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami, Florida 33149 .
by post-eruption diffusion. If there is a significant source of 3 He in the deep-earth regions sampled by these magmas, the 3 He should be trapjped in these rucks along with the excess 4 He and Ar . In this study I present results of a search for this 3 He component in rocks with high 4 He/ 40 Ar ratios. The measurements were made on a glass Reynoldstype 4 .5 inch, 60° rare gas mass spectromet,,r . Approximately 1-2 g samples were boiled at 2000° C for 20 min by RF-heating, and the evolved gases were purified over hot Ti, which was allowed to cool to absorb H 2 . The samples were not previously degassed at elevated temperatures, to remove the possibility of 3 He loss during such degassing ;1ater runs with samples degassed at -200° for 24 hr show substantially identical 4 He contents . The data are presented in table 1 . We had noted previously that glass crusts from tire basalts generally showed greater amounts of the trapped component ; such glass crusts are denoted by the symbol G following the sample number . The He values are total concentrations ; the 40Ar is total 40 Ar minus 296 X 36 Ar (atmospheric component) . Samples 32, 44 and 50 are part of a basaltic basement which outcrops along the crest of the East Pacific Rise . The rare gas component trapped in these rocks may be indicative of the mantle-ambient atmosphere, if current theories regarding upwelling of mantle material along the crests of active oceanic rises are valid . Sample 50 is from a seamount which did not arise on the crest of the Rise . _ The limit of 10 - 10 cc STP/g 3 He found here is significantly less than the amount expected if the source material has an average or gas-rich chondritic primordial 3 He abundance . It would be interesting
D.E.FISHER
78 Table 1 He and Az values in selected deep-sea basalts Basalt [31
4He/4 0er
3He (cc STP/g)
32:G 44G 40G 40G 50 SOG
6 15 20 10 18 12
< 1.1 X lo-10 < 1 X 19-10 < 1 X 10 10 <1 .5 xlo"10 < 1 X lo-1 0 < 1.5 X 1o-10
to tower the limits of detection by another order of magnitude, in order to compare the results more significantly with the "ordinary" chondrites, but 3 the limiting factor in my machine is the (HD. H) peak which is not resolvable fre,m 3 He . A machine with greater mass resolution should be capable of extending this work. These data do not argue against the conclusion of Clarke et al. [2] that there is a leak source of 3H-. beneath the ocean floor. To argue meaningfully against their conclusion it would be necessary to set a limit of < 10 -13 cc STP/g for the earth's interior. Such a limit may be obtainzble with sufficiently sophisticated apparatus . The pre sent data
do spec,'fy that if the earth at one time had an average or gas-rich chondritic 3 He abundance it has long since degassed thoroughly. Since the estimated residence time of 3 He in the atmosphere is roughly 146 years, this primordial He is no longer with us. Acknowledgements I am grateful to T.Middleton for laboratory assistance . This work was supported in part by the National Science Foundation Grant GA-1370 . Refer, ves [11 H.h .aohnson and W.I .Axford, Production and loss of He3 in the earth's atmosphere, J. Geophys. Res., Space Phys. 74 (1969) 2433-2438. [2[ W.B .Clarke, M.A .Beg, and H.Craig, Excess 3He in the sea: evidence for terrestrial primordial helium, Earth Planer. Sci. Letters 6 (1969) 213-220. 131 J.G .Funkhouser, D.E.Fisher and E.Bonatti, Excess argon in deep-sea rocks, Earth Planet Sci. Letters 5 (1968) 95100 . [41 D LJ isher, Fission track ages of deep sea glasses. Nature 221 ('1969) 549-550.
SEARCH FOR 3 He IN DEEP-SEA BASALTS David EXISHER fMiverdly of Miami, Rosenstiel School ofMarine and A tmospheric Sciences, 10 Rickenbacker Causeway, Miami, Florida 33149, USA
Received 22 January 1970 Johnson and Axford [ I 1 have suggested that the dominant form of 3 He production in the earth's atiinosphere is from aurora] precipitation of solar wind plasma. Clarke et al . [21 have suggested that the original 3 He content of the earth might have been about the same as the average chondritic primordial abundance, that most of this 3 He might be retained still in the deep earth, and that a significant portion of the atmospheric 3 He may result from leakage through the ocean floor . The present atmospheric 3 He content is 4 .4 X 10 -13 cc STP/g ; the gas-rich meteorites contain up to 5 X 10 -6 cc STP/g primordial 3 He ; the ordinary chondrite contains about 10 -10 cc STP/g ; an "average c .hon-
dritic" (107., gas-rich, 90% ordinary) abundance is 8 about 10_ cc STP/g . We have found [3, 41 that certain deep sea ûasalts contain excess 4 He and 40 Ar, with 4 He/ 4~o Ar ratios of up to about 15 . This indicates a rare gas component present in the magma before eruption and trapped in the rock ; i .e ., not released during eruption of the rock onto the surface of the earth . Such an inhibition of the normal (sub-aerial) degassing process is reasonable in light of the nearzero temperatures and high hydrostatic pressures of the ocean water environment into which the magma erupts. Since the highest observed 4 He/ 40 Ar ratios are approximately the maximum values that one would expect from Th, U and K decay in a closed system, with terrestrial Th, U and K values, it appears that these rocks have not lost significänt ;.mounts of 4 He
Contribution from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami, Florida 33149 .
by post-eruption diffusion. If there is a significant source of 3 He in the deep-earth regions sampled by these magmas, the 3 He should be trapjped in these rucks along with the excess 4 He and Ar . In this study I present results of a search for this 3 He component in rocks with high 4 He/ 40 Ar ratios. The measurements were made on a glass Reynoldstype 4 .5 inch, 60° rare gas mass spectromet,,r . Approximately 1-2 g samples were boiled at 2000° C for 20 min by RF-heating, and the evolved gases were purified over hot Ti, which was allowed to cool to absorb H 2 . The samples were not previously degassed at elevated temperatures, to remove the possibility of 3 He loss during such degassing ;1ater runs with samples degassed at -200° for 24 hr show substantially identical 4 He contents . The data are presented in table 1 . We had noted previously that glass crusts from tire basalts generally showed greater amounts of the trapped component ; such glass crusts are denoted by the symbol G following the sample number . The He values are total concentrations ; the 40Ar is total 40 Ar minus 296 X 36 Ar (atmospheric component) . Samples 32, 44 and 50 are part of a basaltic basement which outcrops along the crest of the East Pacific Rise . The rare gas component trapped in these rocks may be indicative of the mantle-ambient atmosphere, if current theories regarding upwelling of mantle material along the crests of active oceanic rises are valid . Sample 50 is from a seamount which did not arise on the crest of the Rise . _ The limit of 10 - 10 cc STP/g 3 He found here is significantly less than the amount expected if the source material has an average or gas-rich chondritic primordial 3 He abundance . It would be interesting
D.E.FISHER
78 Table 1 He and Az values in selected deep-sea basalts Basalt [31
4He/4 0er
3He (cc STP/g)
32:G 44G 40G 40G 50 SOG
6 15 20 10 18 12
< 1.1 X lo-10 < 1 X 19-10 < 1 X 10 10 <1 .5 xlo"10 < 1 X lo-1 0 < 1.5 X 1o-10
to tower the limits of detection by another order of magnitude, in order to compare the results more significantly with the "ordinary" chondrites, but 3 the limiting factor in my machine is the (HD. H) peak which is not resolvable fre,m 3 He . A machine with greater mass resolution should be capable of extending this work. These data do not argue against the conclusion of Clarke et al. [2] that there is a leak source of 3H-. beneath the ocean floor. To argue meaningfully against their conclusion it would be necessary to set a limit of < 10 -13 cc STP/g for the earth's interior. Such a limit may be obtainzble with sufficiently sophisticated apparatus . The pre sent data
do spec,'fy that if the earth at one time had an average or gas-rich chondritic 3 He abundance it has long since degassed thoroughly. Since the estimated residence time of 3 He in the atmosphere is roughly 146 years, this primordial He is no longer with us. Acknowledgements I am grateful to T.Middleton for laboratory assistance . This work was supported in part by the National Science Foundation Grant GA-1370 . Refer, ves [11 H.h .aohnson and W.I .Axford, Production and loss of He3 in the earth's atmosphere, J. Geophys. Res., Space Phys. 74 (1969) 2433-2438. [2[ W.B .Clarke, M.A .Beg, and H.Craig, Excess 3He in the sea: evidence for terrestrial primordial helium, Earth Planer. Sci. Letters 6 (1969) 213-220. 131 J.G .Funkhouser, D.E.Fisher and E.Bonatti, Excess argon in deep-sea rocks, Earth Planet Sci. Letters 5 (1968) 95100 . [41 D LJ isher, Fission track ages of deep sea glasses. Nature 221 ('1969) 549-550.