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Uranium and thorium in tektites: An additional comment
EARTH AND PLANETARY SCIENCE LETTERS 8 (1970) 141-142 . NORTH-HOLLAND PUBLIF HING COMPANY
URANIUM AND THORIUM IN TEKTITES: AN ADDITIONAL COMMENT J.VV .MORGAN
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA Received 10 February 1970
The relationship between uranium and thorium in tektites and achondrites is reconsidered in the light of the oxygen isotopic composition of lunar rocks . A correlation is found between the uranium and thorium abundances in three high 180 achondrites and those in tektites which is particularly strong in the case of the Australasian group .
Recently [I], selected uranium and thorium abundances in tektites were compared with those of possible source materials. Of the terrestrial rocks and soils considered, granophyres approach the tektites most closely in absolute abundances and Th/U ratio . Even granophyres, however, have too low a Th/U ratio to be tektite parent material, and a fractionation of -20% U and +10% Th would be needed to derive tektite U and Th abundances from these rocks . In support of a possible lunar origin of tektites, it was shown that the uranium and thorium contents of basaltic achondrites were strongly correlated in a manner somewhat analogous to a terrestrial differentiation series [2,31 . If this series developed a granophyre member, from the terrestrial analogy this would be expected to lie on a regression line through the ba%iltic achondrite abundances. Here again it was found that a relative loss of uranium and/or gain of thorium would be necessary to develop the observed tektite abundances from a postulated lunar granophyre . The recent release of results for the Apollo 1 I hinnr rocke hi- nrnritirpll mnnv new nrotimente hnth
for and against a lunar origin, and preliminary results from Apollo 12 indicate that lunar rocks of high silicon abundance do exist . It is not the purpose of this brief communication to consider the significance of the new results, but rather to point out one particular feature of the achondrite uranium and thorium abun-
dances that was omitted in my previous discussion [ 1 ] . Oxygen isotope studies indicate that the Apollo 11 rocks [4-6] have pyroxene 18 0 values similar to terrestrial and cliondritic values, but decidedly different from those of the basaltic and hypersthene achondrites . There are three achondrites, namely Nakhla, Shergotty and Angra dos Reis (stone) for which uranium and thorium data are available but which were not discussed in {1 ] . In their isotopic oxygen abundances the:,c achondrites more closely resemble the chondrites and terrestrial rocks than basaltic achondrites . Angra dos Reis (stone) is of particular interest in view of the apparently widespread Ti enrichment of lunar rocks, in that it is largely composed of titi :naugite [7] . Additionally, it shows rare earth concentrations and a negative Eu anomaly somewhat similar to those of lunar rocks 181 . There is an increase in Th/U ratio in the basaltic and hypersthene achondrites with increasing uranium abundances, and the same is true of the three high 18n ~,Ir nd,,itoc Ti,r lattar °rn~F:n hn ovar hnç mtirh higher Th/U ratios than basaltic achondrites with similar uranium contents 19,101 . For example, Nakhla contains 0.049 ppm 1) and has a Th/U ratio of 3 .9. Frankfort which has a very similar uranium abundance has a ratio of only 2. i . Similarly, Shergotty has a ratio of 4 .2 whereas the highest eucrite value is 3 .8. Angra
J.W.MORGAN
142
good agreement with the slope of the line for the high 18 0 achondrites alone . If the high 180 achondrites are related as members of an igneous differentiatior, series, a possible acidic end member could well satisfy the Th/U ratio criterion for potential tektite parent material, especially for the Australasian tektites. to E
0 0
Acknowledgements
E i
O t
Professor W.D.Ehmann critically read the manuscript and made several useful suggestions. This work was supported in part by a research grant from the Australian Institute of Nuclear Science and Engineering, and by National Aeronautics and Space Administration contract NAS 9-8(117 . w.t 1 8.01
at
Uranium, ppm
10
lo
Fig. 1 . Regression line of log uranium and thorium abundances in Nakhla, Shergotty and Angra dos Reis (stone), illustrating the relationship to tektite abundances.
dos Reis (stone) possesses a ratio of 4.9, but Stannern, with a slightly higher uranium abundance has a ratio of 3 .2. The mos' noticeable featur of the uranium and dzcrium abundances in tektites is the unusually high rb./U ratio [ 1 ] , the mean value of which is 5.8 t 1 .3. Tfds suggests that the correlation line tl tough the high 18 0 achondrites might fit the teR titre abundances closely without the need to postulate , rbitrary , ~h .ges in abundance during the proi :f ss which formed the tektites. In fig. 1 is shown the regres :don line calculated. from duplicate uranium and thorium analyses of the three high 18 0 achondrites, taking uranium as the independent variable. On a logarithmic abundance scale the slope_at the line is 1 .15 with an intercept of log Th pp.n = 1 .630 a . log U ppm - 1 .000. If the high ` 8 0 achondrite abundances are pooled with 15 selected tektites values [ 1] the intercept is virtually unchanged (1 .63E) but the slope is reduced to 1 .08 . A test of the difference between the two slopes,indicates that it is not significant at the 95% confidence level (t=1 .07). If the log abundances for Australasian tektites only are pooled with the high 19C achondrites, the sl :)pe of the line becomes 1 .11 which is in very
References [ ) j J.W.Morgan, Uranium and thorium in tektites, Earth
Planet . Sci. Letters 7 (1969) 53 . 121 K.S .Heier and J.J .W.Rogers, Radiometric determination of thorium, uranium and potassium in basalts and in two magmatic differentiation series, Geochim. Cos mochim . Acts 27 (1963) 137. 13] K.S .Heier, W.Compston and I.McDougall, Thoriur>> and uranium concentrations and the isotopic composi .ion of strontium in the differentiated Tasmanian dolerites, Geochim. Cosmochim. Acts 29 (1965) 643. [41 S.Epstein and H.P.Taylor Jr ., 18 0/ 16 0, a0 Si/ 28 Si, D/H and 13C/ 12C studies of lunar rocks and minerals returned by the Apollo 11 expedition, Science 167 (1970) 533. [51 N.Onuma, R.N.Clayton and T.K .Mayeda, Oxygen isolopic fractionation between minerals, and an estimate of the temperature of formation, Science 167 (1970) 536. [61 %Friedman, J .R .O'Niel, L.H .Adami, J.D .Gleason and K.Hardcastle, The water, hydrogen, deuterium, carbon, 13C and 18 0 content of selected lunar materials, Science 167 (1970) 538. [71 B.Mason, Meteorites (John Wiley, New York, 1962) pp . 115-116. [81 J .A .Philpotts and C .C .Schnetzler, Apollo 11 lunar samples : Potassium, rubidium, strontium, barium and rare earth concentrations in some rocky and separated phases, Science 167 (1970) 493. [91 J.W.Morgan and J.F.Lovering, Uranium and thorium abundances in stony meteorites 2, The achondritic meteorites, J. Geophys. Res. 69 (1964) 1989 . [ 1Gj J.F .Lovering and J.W .Morgan, Uranium and thorium abundance-, in achondritic meteorites (in preparation) .
URANIUM AND THORIUM IN TEKTITES: AN ADDITIONAL COMMENT J.VV .MORGAN
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA Received 10 February 1970
The relationship between uranium and thorium in tektites and achondrites is reconsidered in the light of the oxygen isotopic composition of lunar rocks . A correlation is found between the uranium and thorium abundances in three high 180 achondrites and those in tektites which is particularly strong in the case of the Australasian group .
Recently [I], selected uranium and thorium abundances in tektites were compared with those of possible source materials. Of the terrestrial rocks and soils considered, granophyres approach the tektites most closely in absolute abundances and Th/U ratio . Even granophyres, however, have too low a Th/U ratio to be tektite parent material, and a fractionation of -20% U and +10% Th would be needed to derive tektite U and Th abundances from these rocks . In support of a possible lunar origin of tektites, it was shown that the uranium and thorium contents of basaltic achondrites were strongly correlated in a manner somewhat analogous to a terrestrial differentiation series [2,31 . If this series developed a granophyre member, from the terrestrial analogy this would be expected to lie on a regression line through the ba%iltic achondrite abundances. Here again it was found that a relative loss of uranium and/or gain of thorium would be necessary to develop the observed tektite abundances from a postulated lunar granophyre . The recent release of results for the Apollo 1 I hinnr rocke hi- nrnritirpll mnnv new nrotimente hnth
for and against a lunar origin, and preliminary results from Apollo 12 indicate that lunar rocks of high silicon abundance do exist . It is not the purpose of this brief communication to consider the significance of the new results, but rather to point out one particular feature of the achondrite uranium and thorium abun-
dances that was omitted in my previous discussion [ 1 ] . Oxygen isotope studies indicate that the Apollo 11 rocks [4-6] have pyroxene 18 0 values similar to terrestrial and cliondritic values, but decidedly different from those of the basaltic and hypersthene achondrites . There are three achondrites, namely Nakhla, Shergotty and Angra dos Reis (stone) for which uranium and thorium data are available but which were not discussed in {1 ] . In their isotopic oxygen abundances the:,c achondrites more closely resemble the chondrites and terrestrial rocks than basaltic achondrites . Angra dos Reis (stone) is of particular interest in view of the apparently widespread Ti enrichment of lunar rocks, in that it is largely composed of titi :naugite [7] . Additionally, it shows rare earth concentrations and a negative Eu anomaly somewhat similar to those of lunar rocks 181 . There is an increase in Th/U ratio in the basaltic and hypersthene achondrites with increasing uranium abundances, and the same is true of the three high 18n ~,Ir nd,,itoc Ti,r lattar °rn~F:n hn ovar hnç mtirh higher Th/U ratios than basaltic achondrites with similar uranium contents 19,101 . For example, Nakhla contains 0.049 ppm 1) and has a Th/U ratio of 3 .9. Frankfort which has a very similar uranium abundance has a ratio of only 2. i . Similarly, Shergotty has a ratio of 4 .2 whereas the highest eucrite value is 3 .8. Angra
J.W.MORGAN
142
good agreement with the slope of the line for the high 18 0 achondrites alone . If the high 180 achondrites are related as members of an igneous differentiatior, series, a possible acidic end member could well satisfy the Th/U ratio criterion for potential tektite parent material, especially for the Australasian tektites. to E
0 0
Acknowledgements
E i
O t
Professor W.D.Ehmann critically read the manuscript and made several useful suggestions. This work was supported in part by a research grant from the Australian Institute of Nuclear Science and Engineering, and by National Aeronautics and Space Administration contract NAS 9-8(117 . w.t 1 8.01
at
Uranium, ppm
10
lo
Fig. 1 . Regression line of log uranium and thorium abundances in Nakhla, Shergotty and Angra dos Reis (stone), illustrating the relationship to tektite abundances.
dos Reis (stone) possesses a ratio of 4.9, but Stannern, with a slightly higher uranium abundance has a ratio of 3 .2. The mos' noticeable featur of the uranium and dzcrium abundances in tektites is the unusually high rb./U ratio [ 1 ] , the mean value of which is 5.8 t 1 .3. Tfds suggests that the correlation line tl tough the high 18 0 achondrites might fit the teR titre abundances closely without the need to postulate , rbitrary , ~h .ges in abundance during the proi :f ss which formed the tektites. In fig. 1 is shown the regres :don line calculated. from duplicate uranium and thorium analyses of the three high 18 0 achondrites, taking uranium as the independent variable. On a logarithmic abundance scale the slope_at the line is 1 .15 with an intercept of log Th pp.n = 1 .630 a . log U ppm - 1 .000. If the high ` 8 0 achondrite abundances are pooled with 15 selected tektites values [ 1] the intercept is virtually unchanged (1 .63E) but the slope is reduced to 1 .08 . A test of the difference between the two slopes,indicates that it is not significant at the 95% confidence level (t=1 .07). If the log abundances for Australasian tektites only are pooled with the high 19C achondrites, the sl :)pe of the line becomes 1 .11 which is in very
References [ ) j J.W.Morgan, Uranium and thorium in tektites, Earth
Planet . Sci. Letters 7 (1969) 53 . 121 K.S .Heier and J.J .W.Rogers, Radiometric determination of thorium, uranium and potassium in basalts and in two magmatic differentiation series, Geochim. Cos mochim . Acts 27 (1963) 137. 13] K.S .Heier, W.Compston and I.McDougall, Thoriur>> and uranium concentrations and the isotopic composi .ion of strontium in the differentiated Tasmanian dolerites, Geochim. Cosmochim. Acts 29 (1965) 643. [41 S.Epstein and H.P.Taylor Jr ., 18 0/ 16 0, a0 Si/ 28 Si, D/H and 13C/ 12C studies of lunar rocks and minerals returned by the Apollo 11 expedition, Science 167 (1970) 533. [51 N.Onuma, R.N.Clayton and T.K .Mayeda, Oxygen isolopic fractionation between minerals, and an estimate of the temperature of formation, Science 167 (1970) 536. [61 %Friedman, J .R .O'Niel, L.H .Adami, J.D .Gleason and K.Hardcastle, The water, hydrogen, deuterium, carbon, 13C and 18 0 content of selected lunar materials, Science 167 (1970) 538. [71 B.Mason, Meteorites (John Wiley, New York, 1962) pp . 115-116. [81 J .A .Philpotts and C .C .Schnetzler, Apollo 11 lunar samples : Potassium, rubidium, strontium, barium and rare earth concentrations in some rocky and separated phases, Science 167 (1970) 493. [91 J.W.Morgan and J.F.Lovering, Uranium and thorium abundances in stony meteorites 2, The achondritic meteorites, J. Geophys. Res. 69 (1964) 1989 . [ 1Gj J.F .Lovering and J.W .Morgan, Uranium and thorium abundance-, in achondritic meteorites (in preparation) .