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Nickel-iron spherules in tektites: non-meteoritic in origin
Earth and Planetary Science Letters, 65 (1983) 225-228
225
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [31
N i c k e l - i r o n spherules in tektites" n o n - m e t e o r i t i c in origin R. Ganapathy i and John W. Larimer 2 1 Research Laboratory, J.T. Baker Chemical Company, Phillipsburg~ NJ 08865 (U.S.A.) 2 Department of Geology, Arizona State University, Tempe, A Z 85287 (U.S.A.)
Received May 23, 1983 Revised version receivedAugust 4, 1983
The concentrations of several diagnostic trace elements were determined in two comparativelylarge NiFe spherules extracted from tektites. The purpose of the study was to obtain some clues about the chemistry of the projectile that is presumed responsible for the formation of these tektites. However, the trace element pattern is distinctly terrestrial implying that the spherules are the result of in-situ reduction of the host rock and are not meteoritic in origin.
I. Introduction The discovery of NiFe spherules in tektites from Isabela, Philippine Islands, was first reported by Chao et al. [1] over 20 years ago. Even though the report cautions against too hastily drawing the obvious conclusion, these spherules have generally been regarded as meteoritic in origin. Similar spherules have since been found in a few other tektites [2] and in impact glass from the Aouelloul, Mauritania, crater [3]. If these spherules are in fact meteoritic, then by measuring the concentration of some diagnostic siderophile elements in them it should be possible to ascertain the nature of the impacting meteorite. The general validity of such an approach has been demonstrated in numerous studies of lunar and terrestrial ejecta. But no such attempt has yet been reported to estimate the composition of the projectile that produced the tektites. The only pertinent study of which we are av~are is that of Morgan et al. [4] who examined samples of Aouelloul glass bearing visible spherules. They found the I r / N i ratio to be less than the cosmic value by a factor of about 100. This seemingly rules out a chondritic projectile and, when coupled with the low Ge concentration, also rules out most other known meteorites; the only 0012-821x/83/$03.00
© 1983 Elsevier Science Publishers B.V.
exceptions are the extreme end members of two iron meteorite groups, IIIAB and IIICD. A peculiar feature of m a n y of the spherules in tektites, and the reason for the cautionary remarks by Chao et al. [1], is their detectable but nonetheless very low Ni content, generally 1 - 3 wt.%. This is well below the natural cutoff in meteoritic metal of about 5 wt.% which reflects the most reduced metal possible in cosmic systems where F e / N i - 2 0 . There are only three exceptional iron meteorites known with lower Ni contents; and these extend the range down to only 4 wt.%. In their subsequent study, Chao et al. [2] found spherules in tektites from South Viet N a m with higher Ni contents (9-13% wt.%); these higher Ni contents together with mineralogical (schriebersite and troilite) and textural similarities to spherules from Meteorite Crater, Arizona, led them to conclude that the spherules were indeed meteoritic. However, in the same study spherules with similar mineralogy and textures were found in tektites from the Philippines, all of which contained 2 - 4 wt.% Ni. We decided to pursue the matter further by determining the concentration of several key elements in individual spherules by neutron activation analyses. If they are meteoritic such an ap-
226
proach is likely to provide the most reliable estimates on the composition of the projectile, which remains a fascinating question surrounding the origin of tektites.
2. Discussion
The results on two relatively large, separated NiFe spherules are shown in Table 1. The results on a small spherule imbedded in a piece of glass and a tektite sample with no visible metal spheres are presented in Table 2. The most striking feature in Table 1 is the extraordinarily high concentration of W in the metal spherules. This is particularly noteworthy in comparison to the very low Ir concentrations. These two elements are both highly refractory and strongly siderophile; they occur in nearly cosmic proportions in most samples of meteoritic metal [5-8]. To the extent that they do differ chemically, W is much more readily oxidized than Ir [7]. Thus, if the spherules were partially oxidized then W concentrations should be low while Ir concentrations should be high, just the reverse of what is observed. Nor is there any suggestion that differences in volatility could play a role; much more volatile elements, such as As and Au, are present in concentrations relative to Ir and Ni that are as high or higher than observed in any meteoritic metal. (Voltaile loss has also been shown to be negligible in spherules around craters where the composition of the projectile is known [4].)
In order to better emphasize the overall comparison of the spherules with meteoritic metal, the data are plotted in Fig. 1. Here the element/Ni ratios in the spherules are normalized to C1 chondrite abundances, which plot as a horizontal line with a value of 1. All the elements plotted occur in nearly the same proportions in all other chondrites, deviating by no more than a factor of 2 or so relative to C1 values. Iron meteorites display a somewhat more varied composition [9]; at one extreme is the I r / N i ratio which varies by almost 1 0 4 and at the other is C o / N i which varies by a factor of only 2. Clearly, the pattern displayed by the spherules is not chondritic and, while there are some iron meteorites with suitably low I r / N i ratios, the W, As and Co abundances do not match any known sample of meteoritic metal. Also plotted in Fig. 1 are the ranges of the elemental ratios observed in terrestrial granites, basalts and shales [10]. Some mixture of these typical crustal rocks has long been regarded as a suitable, and highly probable, parent material for the bulk composition of tektites [11]. As is clearly evident, the elemental ratios for a number of siderophile elements also provide the best match to those observed in the metal spherules. The obvious implication is that the metal spherules result from the in-situ reduction of the host rock, and are not of meteoritic origin. Two other elements, Cr and Fe, could have been plotted in Fig. 1, but were not. As already mentioned, the F e / N i ratio in most spherules examined falls in the range 30-100 (3 to 1 wt.%
TABLE 1 Abundances of elements in the NiFe spherules from tektites found in Philippine Islands Element
Spherule 1 (260/~g)
Spherule 2 (47/Lg)
Cl chondrites
245 _+ 7 5.9 _+ 7.2 1.08 _+ 0.09 0.260_+ 0.008 301 + 48 280 _+ 10 46 _+ 10 560 + 260
0.095 514 1.03 0.048 152 1.84 2650 138
E
W (ppm) Ir (ppb) Ni(%) Co (%) Au(ppb) As (ppm) Cr (ppm) Sb (ppb)
86 _+ 5 11 _ 19 1.60 _ 0.03 0.380+ 0.002 63 _+ 11 275 _+ 9 57 _+ 7 150 +_350
The uncertainties quoted represents 20 counting statistics. Spherule 1 from B.P. Glass, University of Delaware and Spherule 2 from E.C.T. Chao, U.S. Geological Survey, Reston, Virginia.
227 TABLE 2 Elemental abundances in tektite glass and metal spherule plus glass Element
Glass (2535/~g)
Metal + glass (122 ~g)
Metal (estimated) a
Sc(ppm) Cr(ppm) Co (ppm) Ni (ppm) Au (ppb) Ir (ppb) Os (ppb)
14.3+ 0.8 141 _+ 3 10.1_+ 0.2 84 +12 < 2 < 0.1 < 0.1
15.6 _+ 0.8 172 _+ 9 205 _+ 7 1114 +13 5.1 + 0.4 1.38 _+ 0.38 2.2 + 1.1
8340 45,300 207 56 89
a The diameter of the Ni-Fe spherule is somewhere between 100 and 80 ~m. The estimated weight of the spherule is 3_+ 1/~g, based on 9 0 / t m diameter and an assumed density of 7.9 g / c m 3. The estimate is based on the assumption that all siderophiles are in the metal.
Ni) while the natural upper limit in meteoritic material is 20. In these two spherules the ratios are about 60 and 90, much higher than any known sample of meteoritic metal. The Cr content of the metal is low; in fact, when compared to Ni or any of the other elements Cr is strongly depleted. But this is not unexpected; of all the elements considered here, Cr forms the most stable oxides and
!
I0
I
I
I
[
~
lo'i ~g
therefore cannot be regarded as a useful diagnostic siderophile element. In addition to the data obtained on the metal spherules, we also obtained a partial analysis of two pieces of tektite glass, one bearing a visible metal spherule (Table 2). These analyses are incomplete because the samples could not be counted until long after some of the shorter, and interesting
I
Terrestrial Range
Sp.er.lee-
Iron
Meteorites
10 ~ 0 10 I
0
z
lo
o
-~
t~
N
1o 0
lOt
,o t W
Au
As
Co
ELEMENT Fig. 1. The proportions of some diagnostic siderophile elements in metallic spherules found in tektites bear little resemblance to those observed in meteoritic metal Instead, these elements appear to occur in roughly the same proportions as observed in common terrestrial rocks. Most chondritic metal plots close to the horizontal line at a value of 1 (i.e. cosmic ratios); iron meteorites display a more varied composition (shaded region). The rectangular boxes reflect the range of values observed in basalts, granites and shales.
228 (187W), isotopes h a d decayed. Of special interest here are the C o a n d N i concentrations. The N i / C o ratio in the glass is a b o u t 8.4 a n d in the spheruleb e a r i n g glass a b o u t 5.4. Such ratios fall close to the range o b s e r v e d in typical terrestrial rocks, 5 - 1 0 , but far below the 21.5 ratio o b s e r v e d in n e a r l y all meteoritic metal.
Acknowledgements W e t h a n k Drs. B.P. Glass a n d E.C.T. C h a o for p r o v i d i n g the N i - F e spherules used in this investigation. Part of the work d o n e b y J.W.L. was s u p p o r t e d b y N A S A g r a n t N S G 7040. W e thank Dr. J.A. O ' K e e f e for valuable discussions, particularly for bringing to o u r a t t e n t i o n the need to r e - e x a m i n e the N i F e spherules in tektites.
3. Conclusions There seems little d o u b t that the metallic spherules we have analyzed are the p r o d u c t of in-situ reduction. Of course, it is i m p o s s i b l e to c o m p l e t e l y rule out the presence of a small a d m i x t u r e of m e t e o r i t i c material. However, if such a c o m p o n e n t exists, then its c o m p o s i t i o n a l characteristics have b e e n diluted to the extent that there r e m a i n s little h o p e of acquiring any u n a m b i g u o u s i n f o r m a t i o n on the c o m p o s i t i o n of the projectile. Also, b a s e d on our extremely limited sample, we c a n n o t rule o u t the p o s s i b i l i t y that some other spherules in tektites are meteoritic in origin. T h e m o r e N i - r i c h spherules obviously are the m o s t likely candidates. But even for these the chances of acquiring una m b i g u o u s chemical clues on the c o m p o s i t i o n of the projectile do n o t a p p e a r good. H i g h l y reducing c o n d i t i o n s w o u l d be required to preserve such spherules, c o n d i t i o n s which w o u l d be very similar to those required to p r o d u c e the spherules studied here. This in turn implies extensive in-situ reduction of the siderophile elements in the target rock which, when they are i n c o r p o r a t e d into a meteoritic spherule, w o u l d a p p r e c i a b l y alter its elemental concentrations and proportions.
References 1 E.C.T. Chao, I. Adler, E.J. Dwornik and J. Littler, Metallic spherules in tektites from Isabela, Philippine Islands, Science 135, 97, 1962. 2 E.C.T. Chao, E.J. Dwornik and J. Littler, New data on the nickel-iron spherules from Southeast Asian tektites and their implications, Geochim. Cosmochim. Acta 28, 971, 1964. 3 E.C.T. Chao, E.J. Dwornik and C.W. Merrill, Nickel-iron spherules from Aouelloul glass, Science 154, 759, 1966. 4 J.W. Morgan, H. Higuehi, R. Ganapathy and E. Anders, Meteoritic material in four terrestrial meteoritic craters, Proc. 6th Lunar Sci. Conf., p. 1609, 1975. 5 A. Amiruddin and W.D. Ehman, Tungsten abundances in meteoritic and terrestrial materials, Geochim. Cosmochim. Acta. 26, 1011, 1962. 6 K. Imamura and M. Honda, Distribution of tungsten and molybdenum between metal, silicate and sulfide phases of meteorites, Geochim. Cosmochim. Acta 40, 1073, 1976. 7 E.R. Rambaldi and M. Cendales, Tungsten in ordinary chondrites, Earth Planet. Sci. Lett. 36, 372, 1977. 8 E.R.D. Scott, Tungsten in iron meteorites, Earth Planet. Sci. Lett. 39, 363, 1978. 9 E.R.D. Scott, Chemical fractionation in iron meteorites and its interpretation, Geochim. Cosmochim. Acta 36, 1205, 1972. 10 K.H. Wedepohl, ed., The Handbook of Geochemistry (5 volumes), Springer-Verlag, Berlin, 1969-1978. 11 S.R. Taylor, Australites, Henbury impact glass and subgreywacke: a comparison of the abundances of 51 elements, Geochim. Cosmochim. Acta 30, 1121, 1966.
225
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [31
N i c k e l - i r o n spherules in tektites" n o n - m e t e o r i t i c in origin R. Ganapathy i and John W. Larimer 2 1 Research Laboratory, J.T. Baker Chemical Company, Phillipsburg~ NJ 08865 (U.S.A.) 2 Department of Geology, Arizona State University, Tempe, A Z 85287 (U.S.A.)
Received May 23, 1983 Revised version receivedAugust 4, 1983
The concentrations of several diagnostic trace elements were determined in two comparativelylarge NiFe spherules extracted from tektites. The purpose of the study was to obtain some clues about the chemistry of the projectile that is presumed responsible for the formation of these tektites. However, the trace element pattern is distinctly terrestrial implying that the spherules are the result of in-situ reduction of the host rock and are not meteoritic in origin.
I. Introduction The discovery of NiFe spherules in tektites from Isabela, Philippine Islands, was first reported by Chao et al. [1] over 20 years ago. Even though the report cautions against too hastily drawing the obvious conclusion, these spherules have generally been regarded as meteoritic in origin. Similar spherules have since been found in a few other tektites [2] and in impact glass from the Aouelloul, Mauritania, crater [3]. If these spherules are in fact meteoritic, then by measuring the concentration of some diagnostic siderophile elements in them it should be possible to ascertain the nature of the impacting meteorite. The general validity of such an approach has been demonstrated in numerous studies of lunar and terrestrial ejecta. But no such attempt has yet been reported to estimate the composition of the projectile that produced the tektites. The only pertinent study of which we are av~are is that of Morgan et al. [4] who examined samples of Aouelloul glass bearing visible spherules. They found the I r / N i ratio to be less than the cosmic value by a factor of about 100. This seemingly rules out a chondritic projectile and, when coupled with the low Ge concentration, also rules out most other known meteorites; the only 0012-821x/83/$03.00
© 1983 Elsevier Science Publishers B.V.
exceptions are the extreme end members of two iron meteorite groups, IIIAB and IIICD. A peculiar feature of m a n y of the spherules in tektites, and the reason for the cautionary remarks by Chao et al. [1], is their detectable but nonetheless very low Ni content, generally 1 - 3 wt.%. This is well below the natural cutoff in meteoritic metal of about 5 wt.% which reflects the most reduced metal possible in cosmic systems where F e / N i - 2 0 . There are only three exceptional iron meteorites known with lower Ni contents; and these extend the range down to only 4 wt.%. In their subsequent study, Chao et al. [2] found spherules in tektites from South Viet N a m with higher Ni contents (9-13% wt.%); these higher Ni contents together with mineralogical (schriebersite and troilite) and textural similarities to spherules from Meteorite Crater, Arizona, led them to conclude that the spherules were indeed meteoritic. However, in the same study spherules with similar mineralogy and textures were found in tektites from the Philippines, all of which contained 2 - 4 wt.% Ni. We decided to pursue the matter further by determining the concentration of several key elements in individual spherules by neutron activation analyses. If they are meteoritic such an ap-
226
proach is likely to provide the most reliable estimates on the composition of the projectile, which remains a fascinating question surrounding the origin of tektites.
2. Discussion
The results on two relatively large, separated NiFe spherules are shown in Table 1. The results on a small spherule imbedded in a piece of glass and a tektite sample with no visible metal spheres are presented in Table 2. The most striking feature in Table 1 is the extraordinarily high concentration of W in the metal spherules. This is particularly noteworthy in comparison to the very low Ir concentrations. These two elements are both highly refractory and strongly siderophile; they occur in nearly cosmic proportions in most samples of meteoritic metal [5-8]. To the extent that they do differ chemically, W is much more readily oxidized than Ir [7]. Thus, if the spherules were partially oxidized then W concentrations should be low while Ir concentrations should be high, just the reverse of what is observed. Nor is there any suggestion that differences in volatility could play a role; much more volatile elements, such as As and Au, are present in concentrations relative to Ir and Ni that are as high or higher than observed in any meteoritic metal. (Voltaile loss has also been shown to be negligible in spherules around craters where the composition of the projectile is known [4].)
In order to better emphasize the overall comparison of the spherules with meteoritic metal, the data are plotted in Fig. 1. Here the element/Ni ratios in the spherules are normalized to C1 chondrite abundances, which plot as a horizontal line with a value of 1. All the elements plotted occur in nearly the same proportions in all other chondrites, deviating by no more than a factor of 2 or so relative to C1 values. Iron meteorites display a somewhat more varied composition [9]; at one extreme is the I r / N i ratio which varies by almost 1 0 4 and at the other is C o / N i which varies by a factor of only 2. Clearly, the pattern displayed by the spherules is not chondritic and, while there are some iron meteorites with suitably low I r / N i ratios, the W, As and Co abundances do not match any known sample of meteoritic metal. Also plotted in Fig. 1 are the ranges of the elemental ratios observed in terrestrial granites, basalts and shales [10]. Some mixture of these typical crustal rocks has long been regarded as a suitable, and highly probable, parent material for the bulk composition of tektites [11]. As is clearly evident, the elemental ratios for a number of siderophile elements also provide the best match to those observed in the metal spherules. The obvious implication is that the metal spherules result from the in-situ reduction of the host rock, and are not of meteoritic origin. Two other elements, Cr and Fe, could have been plotted in Fig. 1, but were not. As already mentioned, the F e / N i ratio in most spherules examined falls in the range 30-100 (3 to 1 wt.%
TABLE 1 Abundances of elements in the NiFe spherules from tektites found in Philippine Islands Element
Spherule 1 (260/~g)
Spherule 2 (47/Lg)
Cl chondrites
245 _+ 7 5.9 _+ 7.2 1.08 _+ 0.09 0.260_+ 0.008 301 + 48 280 _+ 10 46 _+ 10 560 + 260
0.095 514 1.03 0.048 152 1.84 2650 138
E
W (ppm) Ir (ppb) Ni(%) Co (%) Au(ppb) As (ppm) Cr (ppm) Sb (ppb)
86 _+ 5 11 _ 19 1.60 _ 0.03 0.380+ 0.002 63 _+ 11 275 _+ 9 57 _+ 7 150 +_350
The uncertainties quoted represents 20 counting statistics. Spherule 1 from B.P. Glass, University of Delaware and Spherule 2 from E.C.T. Chao, U.S. Geological Survey, Reston, Virginia.
227 TABLE 2 Elemental abundances in tektite glass and metal spherule plus glass Element
Glass (2535/~g)
Metal + glass (122 ~g)
Metal (estimated) a
Sc(ppm) Cr(ppm) Co (ppm) Ni (ppm) Au (ppb) Ir (ppb) Os (ppb)
14.3+ 0.8 141 _+ 3 10.1_+ 0.2 84 +12 < 2 < 0.1 < 0.1
15.6 _+ 0.8 172 _+ 9 205 _+ 7 1114 +13 5.1 + 0.4 1.38 _+ 0.38 2.2 + 1.1
8340 45,300 207 56 89
a The diameter of the Ni-Fe spherule is somewhere between 100 and 80 ~m. The estimated weight of the spherule is 3_+ 1/~g, based on 9 0 / t m diameter and an assumed density of 7.9 g / c m 3. The estimate is based on the assumption that all siderophiles are in the metal.
Ni) while the natural upper limit in meteoritic material is 20. In these two spherules the ratios are about 60 and 90, much higher than any known sample of meteoritic metal. The Cr content of the metal is low; in fact, when compared to Ni or any of the other elements Cr is strongly depleted. But this is not unexpected; of all the elements considered here, Cr forms the most stable oxides and
!
I0
I
I
I
[
~
lo'i ~g
therefore cannot be regarded as a useful diagnostic siderophile element. In addition to the data obtained on the metal spherules, we also obtained a partial analysis of two pieces of tektite glass, one bearing a visible metal spherule (Table 2). These analyses are incomplete because the samples could not be counted until long after some of the shorter, and interesting
I
Terrestrial Range
Sp.er.lee-
Iron
Meteorites
10 ~ 0 10 I
0
z
lo
o
-~
t~
N
1o 0
lOt
,o t W
Au
As
Co
ELEMENT Fig. 1. The proportions of some diagnostic siderophile elements in metallic spherules found in tektites bear little resemblance to those observed in meteoritic metal Instead, these elements appear to occur in roughly the same proportions as observed in common terrestrial rocks. Most chondritic metal plots close to the horizontal line at a value of 1 (i.e. cosmic ratios); iron meteorites display a more varied composition (shaded region). The rectangular boxes reflect the range of values observed in basalts, granites and shales.
228 (187W), isotopes h a d decayed. Of special interest here are the C o a n d N i concentrations. The N i / C o ratio in the glass is a b o u t 8.4 a n d in the spheruleb e a r i n g glass a b o u t 5.4. Such ratios fall close to the range o b s e r v e d in typical terrestrial rocks, 5 - 1 0 , but far below the 21.5 ratio o b s e r v e d in n e a r l y all meteoritic metal.
Acknowledgements W e t h a n k Drs. B.P. Glass a n d E.C.T. C h a o for p r o v i d i n g the N i - F e spherules used in this investigation. Part of the work d o n e b y J.W.L. was s u p p o r t e d b y N A S A g r a n t N S G 7040. W e thank Dr. J.A. O ' K e e f e for valuable discussions, particularly for bringing to o u r a t t e n t i o n the need to r e - e x a m i n e the N i F e spherules in tektites.
3. Conclusions There seems little d o u b t that the metallic spherules we have analyzed are the p r o d u c t of in-situ reduction. Of course, it is i m p o s s i b l e to c o m p l e t e l y rule out the presence of a small a d m i x t u r e of m e t e o r i t i c material. However, if such a c o m p o n e n t exists, then its c o m p o s i t i o n a l characteristics have b e e n diluted to the extent that there r e m a i n s little h o p e of acquiring any u n a m b i g u o u s i n f o r m a t i o n on the c o m p o s i t i o n of the projectile. Also, b a s e d on our extremely limited sample, we c a n n o t rule o u t the p o s s i b i l i t y that some other spherules in tektites are meteoritic in origin. T h e m o r e N i - r i c h spherules obviously are the m o s t likely candidates. But even for these the chances of acquiring una m b i g u o u s chemical clues on the c o m p o s i t i o n of the projectile do n o t a p p e a r good. H i g h l y reducing c o n d i t i o n s w o u l d be required to preserve such spherules, c o n d i t i o n s which w o u l d be very similar to those required to p r o d u c e the spherules studied here. This in turn implies extensive in-situ reduction of the siderophile elements in the target rock which, when they are i n c o r p o r a t e d into a meteoritic spherule, w o u l d a p p r e c i a b l y alter its elemental concentrations and proportions.
References 1 E.C.T. Chao, I. Adler, E.J. Dwornik and J. Littler, Metallic spherules in tektites from Isabela, Philippine Islands, Science 135, 97, 1962. 2 E.C.T. Chao, E.J. Dwornik and J. Littler, New data on the nickel-iron spherules from Southeast Asian tektites and their implications, Geochim. Cosmochim. Acta 28, 971, 1964. 3 E.C.T. Chao, E.J. Dwornik and C.W. Merrill, Nickel-iron spherules from Aouelloul glass, Science 154, 759, 1966. 4 J.W. Morgan, H. Higuehi, R. Ganapathy and E. Anders, Meteoritic material in four terrestrial meteoritic craters, Proc. 6th Lunar Sci. Conf., p. 1609, 1975. 5 A. Amiruddin and W.D. Ehman, Tungsten abundances in meteoritic and terrestrial materials, Geochim. Cosmochim. Acta. 26, 1011, 1962. 6 K. Imamura and M. Honda, Distribution of tungsten and molybdenum between metal, silicate and sulfide phases of meteorites, Geochim. Cosmochim. Acta 40, 1073, 1976. 7 E.R. Rambaldi and M. Cendales, Tungsten in ordinary chondrites, Earth Planet. Sci. Lett. 36, 372, 1977. 8 E.R.D. Scott, Tungsten in iron meteorites, Earth Planet. Sci. Lett. 39, 363, 1978. 9 E.R.D. Scott, Chemical fractionation in iron meteorites and its interpretation, Geochim. Cosmochim. Acta 36, 1205, 1972. 10 K.H. Wedepohl, ed., The Handbook of Geochemistry (5 volumes), Springer-Verlag, Berlin, 1969-1978. 11 S.R. Taylor, Australites, Henbury impact glass and subgreywacke: a comparison of the abundances of 51 elements, Geochim. Cosmochim. Acta 30, 1121, 1966.