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Minor and trace element distribution in melilite and pyroxene from the Allende meteorite
Earth and Planetary Science Letters, 22 (1974) 141 - 144
© North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
L_L_A
M I N O R A N D T R A C E E L E M E N T D I S T R I B U T I O N IN M E L I L I T E A N D P Y R O X E N E F R O M THE ALLENDE METEORITE BRIAN MASON Mineral Sciences, Smithsonian Institution, Washington, D.C. (USA)
and
PHILIP M. MART1N Research School of Earth Sciences, Australian National University, Canberra, A. C. T. (Australia) Received December 7, 1973 Revised version received March 4, 1974
Melilite and pyroxene were separated from a coarsely crystalline chondrule in the Allende meteorite and analysed by microprobe and spark source mass spectrometer techniques. Elemental abundances in the bulk chondrule are consistent with a mixture of equal a m o u n t s of the two minerals, as observed microscopically. The lanthanide distributions are markedly different in the two minerals; relative to chondrite abundances, melilite shows progressive depletion of the lanthanides L a - S m , a positive Eu anomaly, and relatively constant abundances of the heavier lanthanides ( G d - Y b , and Y), whereas pyroxene shows progressive enrichment towards the heavier lanthanides, on which is superimposed a negative Eu anomaly. Both minerals, and the bulk chondrule, show unusual concentrations of the platinum metals, b u t the crystallographic site or sites of these metals remains to be determined.
1. Introduction The Allende meteorite is remarkable for its content of a unique range of distinctive chondrules and aggregates. The most prominent of these are large, coarsely crystalline chondrules consisting largely of melilite and an Al-rich pyroxene (fassaite), usually with minor amounts of spinel and anorthite; one of these chondrules, 25 mm in diameter, was described and illustrated by Clarke et al. [1]. Gast et al. [2] showed that one of these chondrules has a relatively fiat lanthanide distribution pattern at about 15 X chondritic abundances, with a slight positive Eu anomaly, and Grossman [3] has provided additional data on a number of samples; we have confirmed this pattern for ten additional chondrules. 2. Sampling and measurements In order to elucidate the elemental distribution within a single chondrule, we extracted a 10 mm
diameter chondrule from an Allende (NMNH 3529) specimen. Microscopic examination of the chondrule gave the approximate composition 35% melilite, 35% pyroxene, 25% spinel, and 5% anorthite. Microprobe analyses were made of the melilite and the pyroxene, and of a sample of the bulk chondrule after fusion to a glass with lithium tetraborate flux; the results are reported in Table 1. Melilite and pyroxene were density-separated from the crushed chondrute using methylene iodide-acetone mixtures; both mineral separates were unavoidably contaminated with spinel, in the form of minute inclusions. These separates, and a sample of the bulk chondrule, were analysed with an A.E.I. MS7 spark source mass spectrometer using the technique described by Taylor [4]. The results are reported in Table 2. The precision is about + 10%, but varies somewhat, being poorer for the mass region 85-105 than for higher mass numbers. Accuracy for the platinum elements is not well established, because of the lack of suitable standards, but the relative abundances are certainly reliable.
142 3. Discussion Table 1 shows that the melilite and pyroxene are rather similar in composition for the major elements, the principal differences being the considerable Ti in the pyroxene and the higher Ca in the melflite. However, the crystal structures of the two minerals are quite different. In melilite Si, A1, and Mg are all fourcoordinated with oxygen, and Ca is irregularly coordinated with eight oxygens. In pyroxene Si is in fourcoordination, A1 in both four and six-coordination, Mg in six-coordination, and Ca in eight-coordination. The presence of six-coordination sites in pyroxene and their absence in melilite can account for the concentration of Ti in the pyroxene and its exclusion from melilite, since Ti is normally in six-coordination with oxygen; this is true even if some of the Ti is trivalent. Tile Ca sites are somewhat larger in melilite than in pyroxene. This is reflected in the occurrence of (synthetic) strontium melilite and the absence of strontium pyroxenes, the larger Sr ion being cornpatible with the melilite structure but not with that of pyroxene. This effect is demonstrated in Table 2 by the relative enrichment of the large cations Sr, Rb, and Ba in the melilite. Terrestrial melilites often contain appreciable amounts of Na and Ka, but these elements are practically absent from the meteoritic melilite, because the alkalies are present at low concentrations in the chondrule, and are probably present as rare grains of nepheline and sodalite. The data in Table 2 show that the abundances of the minor and trace elements are closely approxi, mated, for most of the elements determined, by a mixture of equal amounts of pyroxene and melilite, the proportions in which these minerals were observed to occur in the chondrule. This suggests that the spinel inclusions in the separated minerals have had little effect on the measured abundances; spinel, which has only four- and six-coordinate sites, proba- bly does not readily accept lanthanides and larger ions in its structure. The elements for which the abundances calculated for a 1/1 pyroxene-melilite mix do not agree within 25% of the abundances measured in the bulk chondrule are Rb, Nb, Mo, and Pd. For all of these (except Pd), the calculated abundances are less than the measured abundances, which suggests that they may be incorporated in accessory minerals. Rubidium would be concentrated in accessory neph-
B. MASON AND P.M. MARTIN TABLE 1 Analyses of bulk chondrule, and of coexisting melilite and pyroxene Chondrule SiO2 TiO2 A1203 FeO MgO CaO Na20 K20
31.1 1.1 27.3 1.4 12.6 23.2 0.56 0.08
Sum
97.3
Melilite
Pyroxene
36.9 0.08 11.7 < 0.10 10.3 41.0 0.1 < 0.O7
42.7 3.9 16.0 < 0.10 11.9 25.9 < 0.10 < 0.O7
100.0
100.4
<
TABLE 2 Minor and trace elements (ppm) in melilite and pyroxene and in the bulk chondrule*
Rb Sr y Zr Nb Mo Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Hf Th Pt lr Os Pd Ru
Melilite
Pyroxene
Chondrule
0.56 160 8.2 7.8 2.0 7.2 66 3.8 8.6 1.0 4.0 1.0 1.3 1.4 0.24 1.6 0.39 1.1 0.16 1.0 < 0.3 0.28
0.07 30 27 98 3.8 13 18 2.8 9.7 1.6 7.1 2.6 0.33 4.7 0.81 5.7 1.5 4.3 0.56 3.3 2.8 0.45
0.45 110 24 59 4.7 14 49 3.9 10 1.4 6.2 2.0 0.91 3.1 0.50 3.7 0.92 2.5 0.33 2.2 1.2 0.44
0.32 90 18 53 2.9 10 42 3.3 9.2 1.3 5.5 1.8 0.81 3.0 0.52 3.7 0.93 2.7 0.36 2.2 1.4 0.36
5.1 4.0 4.0 0.58 4.5
8.7 6.9 6.6 0.97 8.6
7.7 6.5 6.6 0.51 8.6
6.4 5.5 5.3 0.78 6.6
1 melilite + • 1 pyroxene
* The last column gives the calculated abundances for a mixture of equal amounts of melilite and pyroxene.
MINOR AND TRACE ELEMENTDISTRIBUTIONIN MELILITEAND PYROXENE
2O
u a.
10
.o_
I La
' Ce
I Pr
Nd
I Sm
I I:u
g d
I Tb
1 Dy
I Ho
I Er
' Ib Tm Y
Fig. 1. Abundances of the lanthanides, normalized to the average chondfitic abundances, in the bulk chondrite (B) and the separated melilite (M) and pyroxene (P). eline and sodalite. The platinum metals (and Mo) present a special case. Grossman [3] has recorded high Ir contents in melilite-pyroxene chondrules from the Allende meteorite, and discussed their significance, which he links with the high condensation temperature of Ir; however, he did not determine the mineralogical location of the Ir. Our data confirm his results for It, and extend them to the order platinum metals, and Mo (also an element with a high condensation temperature). However, Pd acts anomalously, being markedly depleted relative to Ru (these elements having approximately equal abundances in carbonaceous chondrites); this is probably explained by the fact that Pd has a considerably lower condensation temperature than any of the other platinum metals. (We were unable to determine Rh because of interferences on the 103Rh line). The site of the platinum metals remains something of an enigma. It seems unlikely that they would readily enter any of the lattice sites available in the silicate minerals. They may well be present as minute, possibly submicroscopic grains of native metal. Perhaps the most significant comparisons can be made for the lanthanide elements, as illustrated in Fig. 1. Here again crystallochemical factors evidently play an important role. The pyroxene shows a higher concentration than melilite for the individual lanthanides, except for La and Eu. It thus appears that under the conditions of crystallization most of the lanthanides were more readily accommodated in the smaller Ca sites of pyroxene than in the larger Ca sites in melilite. Exceptions are the largest trivalent lanthan-
143
ide ion, La and Eu, almost certainly present as the Eu 2 ion, with a radius close to that of Sr. The geochemical coherence of Eu and Sr in this meteorite is illustrated by the relative constancy of the Sr/Eu ratio: 120 in the melilite, 89 in the pyroxene, and 120 in the bulk material. The structural control of lanthanide distribution is also manifested in the relative enrichment of the heavier lanthanides (with smaller ionic radius) in the pyroxene (La/Yb = 0.85) and their depletion in the melilite (La/Yb = 3.8). The pattern of lanthanide distribution in melilite is very similar to that for meteorite anorthite, as determined by Allen and Mason [5] and others. The significance of the minor and trace element data for the origin of these peculiar chondrules remains to be fully explored. The data we have obtained from the analyses of material from ten melilite chondrules shows a remarkably uniform trace element pattern; all of them give a lanthanide distribution pattern similar to the bulk sample in Fig. 1 - a relatively fiat pattern at 10 20 times chondrific abundances, with a moderate positive Eu anomaly [6]. This pattern persists despite considerable differences in the properties and compositions of the principal minerals, melilite and pyroxene; compositional data on these minerals have recently been published by Gray et al. [7]. It thus appears that the absolute amounts of the minor and trace elements in each of these chondrules was established during initial accretion. If subsequent melting and recrystallization has taken place the distribution of these elements has been controlled by crystallochemical factors. This contrasts to a situation in which the elemental abundances in the chondrule were controlled by the compositions of the individual minerals formed in the initial condensation.
Acknowledgements Parts of the costs of this investigation have been covered by a grant (NGR-09.015-170) from the National Aeronautics and Space Administration. We would like to express our appreciation to Mrs. P. Muir and Mr. J. Nelen for assistance in the analytical work, and to Mr. M.P. Gorton for his assistance with computer techniques. One of us (B.M.) is indebted to the Australian National University for an appointment as Honorary Fellow in the Research School of Earth
144 Sciences, w h i c h e n a b l e d h i m to p a r t i c i p a t e in t h e spark source mass s p e c t r o m e t e r analyses.
References 1 R.S. Clarke, Jr., E. Jarowewich, B. Mason, J. Nelen, M. Gomez and J.R. Hyde, The Allende meteorite shower, Smithsonian Contr. Earth Sci. 5 (1970) 1. 2 P.W. Gast, N.J. Hubbard and H. Wiesman, Chemical composition and petrogenesis of basalts from Tranquillity Base, Proc. Apollo 11 Lunar Sci. Conf. 2 (1970) 1143. 3 L. Grossman, Refractory trace elements in Ca- Al-rich inclusions in the Allende meteorite, Geochim. Cosmochim. Acta 37 (1973) 1119.
B. MASON AND P.M. MARTIN 4 S.R. Taylor, Geochemical application of spark source mass spectrography, Geochim. Cosmochim. Acta 35 (1971) 1187. 5 R.O. Allen, Jr. and B. Mason, Minor and trace elements in some meteorite minerals, Geochim. Cosmochim. Acta 37 (1973) 1435. 6 P.M. Martin and B. Mason, Major and trace element abundances in components of the Allende meteorite, Nature 247 (1974) in press. 7 C.M. Gray, D.A. Papanastassiou and G.J. Wasserburg, The identification of early condensates from the solar nebula, Icarus 20 (1973) 213.
© North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
L_L_A
M I N O R A N D T R A C E E L E M E N T D I S T R I B U T I O N IN M E L I L I T E A N D P Y R O X E N E F R O M THE ALLENDE METEORITE BRIAN MASON Mineral Sciences, Smithsonian Institution, Washington, D.C. (USA)
and
PHILIP M. MART1N Research School of Earth Sciences, Australian National University, Canberra, A. C. T. (Australia) Received December 7, 1973 Revised version received March 4, 1974
Melilite and pyroxene were separated from a coarsely crystalline chondrule in the Allende meteorite and analysed by microprobe and spark source mass spectrometer techniques. Elemental abundances in the bulk chondrule are consistent with a mixture of equal a m o u n t s of the two minerals, as observed microscopically. The lanthanide distributions are markedly different in the two minerals; relative to chondrite abundances, melilite shows progressive depletion of the lanthanides L a - S m , a positive Eu anomaly, and relatively constant abundances of the heavier lanthanides ( G d - Y b , and Y), whereas pyroxene shows progressive enrichment towards the heavier lanthanides, on which is superimposed a negative Eu anomaly. Both minerals, and the bulk chondrule, show unusual concentrations of the platinum metals, b u t the crystallographic site or sites of these metals remains to be determined.
1. Introduction The Allende meteorite is remarkable for its content of a unique range of distinctive chondrules and aggregates. The most prominent of these are large, coarsely crystalline chondrules consisting largely of melilite and an Al-rich pyroxene (fassaite), usually with minor amounts of spinel and anorthite; one of these chondrules, 25 mm in diameter, was described and illustrated by Clarke et al. [1]. Gast et al. [2] showed that one of these chondrules has a relatively fiat lanthanide distribution pattern at about 15 X chondritic abundances, with a slight positive Eu anomaly, and Grossman [3] has provided additional data on a number of samples; we have confirmed this pattern for ten additional chondrules. 2. Sampling and measurements In order to elucidate the elemental distribution within a single chondrule, we extracted a 10 mm
diameter chondrule from an Allende (NMNH 3529) specimen. Microscopic examination of the chondrule gave the approximate composition 35% melilite, 35% pyroxene, 25% spinel, and 5% anorthite. Microprobe analyses were made of the melilite and the pyroxene, and of a sample of the bulk chondrule after fusion to a glass with lithium tetraborate flux; the results are reported in Table 1. Melilite and pyroxene were density-separated from the crushed chondrute using methylene iodide-acetone mixtures; both mineral separates were unavoidably contaminated with spinel, in the form of minute inclusions. These separates, and a sample of the bulk chondrule, were analysed with an A.E.I. MS7 spark source mass spectrometer using the technique described by Taylor [4]. The results are reported in Table 2. The precision is about + 10%, but varies somewhat, being poorer for the mass region 85-105 than for higher mass numbers. Accuracy for the platinum elements is not well established, because of the lack of suitable standards, but the relative abundances are certainly reliable.
142 3. Discussion Table 1 shows that the melilite and pyroxene are rather similar in composition for the major elements, the principal differences being the considerable Ti in the pyroxene and the higher Ca in the melflite. However, the crystal structures of the two minerals are quite different. In melilite Si, A1, and Mg are all fourcoordinated with oxygen, and Ca is irregularly coordinated with eight oxygens. In pyroxene Si is in fourcoordination, A1 in both four and six-coordination, Mg in six-coordination, and Ca in eight-coordination. The presence of six-coordination sites in pyroxene and their absence in melilite can account for the concentration of Ti in the pyroxene and its exclusion from melilite, since Ti is normally in six-coordination with oxygen; this is true even if some of the Ti is trivalent. Tile Ca sites are somewhat larger in melilite than in pyroxene. This is reflected in the occurrence of (synthetic) strontium melilite and the absence of strontium pyroxenes, the larger Sr ion being cornpatible with the melilite structure but not with that of pyroxene. This effect is demonstrated in Table 2 by the relative enrichment of the large cations Sr, Rb, and Ba in the melilite. Terrestrial melilites often contain appreciable amounts of Na and Ka, but these elements are practically absent from the meteoritic melilite, because the alkalies are present at low concentrations in the chondrule, and are probably present as rare grains of nepheline and sodalite. The data in Table 2 show that the abundances of the minor and trace elements are closely approxi, mated, for most of the elements determined, by a mixture of equal amounts of pyroxene and melilite, the proportions in which these minerals were observed to occur in the chondrule. This suggests that the spinel inclusions in the separated minerals have had little effect on the measured abundances; spinel, which has only four- and six-coordinate sites, proba- bly does not readily accept lanthanides and larger ions in its structure. The elements for which the abundances calculated for a 1/1 pyroxene-melilite mix do not agree within 25% of the abundances measured in the bulk chondrule are Rb, Nb, Mo, and Pd. For all of these (except Pd), the calculated abundances are less than the measured abundances, which suggests that they may be incorporated in accessory minerals. Rubidium would be concentrated in accessory neph-
B. MASON AND P.M. MARTIN TABLE 1 Analyses of bulk chondrule, and of coexisting melilite and pyroxene Chondrule SiO2 TiO2 A1203 FeO MgO CaO Na20 K20
31.1 1.1 27.3 1.4 12.6 23.2 0.56 0.08
Sum
97.3
Melilite
Pyroxene
36.9 0.08 11.7 < 0.10 10.3 41.0 0.1 < 0.O7
42.7 3.9 16.0 < 0.10 11.9 25.9 < 0.10 < 0.O7
100.0
100.4
<
TABLE 2 Minor and trace elements (ppm) in melilite and pyroxene and in the bulk chondrule*
Rb Sr y Zr Nb Mo Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Hf Th Pt lr Os Pd Ru
Melilite
Pyroxene
Chondrule
0.56 160 8.2 7.8 2.0 7.2 66 3.8 8.6 1.0 4.0 1.0 1.3 1.4 0.24 1.6 0.39 1.1 0.16 1.0 < 0.3 0.28
0.07 30 27 98 3.8 13 18 2.8 9.7 1.6 7.1 2.6 0.33 4.7 0.81 5.7 1.5 4.3 0.56 3.3 2.8 0.45
0.45 110 24 59 4.7 14 49 3.9 10 1.4 6.2 2.0 0.91 3.1 0.50 3.7 0.92 2.5 0.33 2.2 1.2 0.44
0.32 90 18 53 2.9 10 42 3.3 9.2 1.3 5.5 1.8 0.81 3.0 0.52 3.7 0.93 2.7 0.36 2.2 1.4 0.36
5.1 4.0 4.0 0.58 4.5
8.7 6.9 6.6 0.97 8.6
7.7 6.5 6.6 0.51 8.6
6.4 5.5 5.3 0.78 6.6
1 melilite + • 1 pyroxene
* The last column gives the calculated abundances for a mixture of equal amounts of melilite and pyroxene.
MINOR AND TRACE ELEMENTDISTRIBUTIONIN MELILITEAND PYROXENE
2O
u a.
10
.o_
I La
' Ce
I Pr
Nd
I Sm
I I:u
g d
I Tb
1 Dy
I Ho
I Er
' Ib Tm Y
Fig. 1. Abundances of the lanthanides, normalized to the average chondfitic abundances, in the bulk chondrite (B) and the separated melilite (M) and pyroxene (P). eline and sodalite. The platinum metals (and Mo) present a special case. Grossman [3] has recorded high Ir contents in melilite-pyroxene chondrules from the Allende meteorite, and discussed their significance, which he links with the high condensation temperature of Ir; however, he did not determine the mineralogical location of the Ir. Our data confirm his results for It, and extend them to the order platinum metals, and Mo (also an element with a high condensation temperature). However, Pd acts anomalously, being markedly depleted relative to Ru (these elements having approximately equal abundances in carbonaceous chondrites); this is probably explained by the fact that Pd has a considerably lower condensation temperature than any of the other platinum metals. (We were unable to determine Rh because of interferences on the 103Rh line). The site of the platinum metals remains something of an enigma. It seems unlikely that they would readily enter any of the lattice sites available in the silicate minerals. They may well be present as minute, possibly submicroscopic grains of native metal. Perhaps the most significant comparisons can be made for the lanthanide elements, as illustrated in Fig. 1. Here again crystallochemical factors evidently play an important role. The pyroxene shows a higher concentration than melilite for the individual lanthanides, except for La and Eu. It thus appears that under the conditions of crystallization most of the lanthanides were more readily accommodated in the smaller Ca sites of pyroxene than in the larger Ca sites in melilite. Exceptions are the largest trivalent lanthan-
143
ide ion, La and Eu, almost certainly present as the Eu 2 ion, with a radius close to that of Sr. The geochemical coherence of Eu and Sr in this meteorite is illustrated by the relative constancy of the Sr/Eu ratio: 120 in the melilite, 89 in the pyroxene, and 120 in the bulk material. The structural control of lanthanide distribution is also manifested in the relative enrichment of the heavier lanthanides (with smaller ionic radius) in the pyroxene (La/Yb = 0.85) and their depletion in the melilite (La/Yb = 3.8). The pattern of lanthanide distribution in melilite is very similar to that for meteorite anorthite, as determined by Allen and Mason [5] and others. The significance of the minor and trace element data for the origin of these peculiar chondrules remains to be fully explored. The data we have obtained from the analyses of material from ten melilite chondrules shows a remarkably uniform trace element pattern; all of them give a lanthanide distribution pattern similar to the bulk sample in Fig. 1 - a relatively fiat pattern at 10 20 times chondrific abundances, with a moderate positive Eu anomaly [6]. This pattern persists despite considerable differences in the properties and compositions of the principal minerals, melilite and pyroxene; compositional data on these minerals have recently been published by Gray et al. [7]. It thus appears that the absolute amounts of the minor and trace elements in each of these chondrules was established during initial accretion. If subsequent melting and recrystallization has taken place the distribution of these elements has been controlled by crystallochemical factors. This contrasts to a situation in which the elemental abundances in the chondrule were controlled by the compositions of the individual minerals formed in the initial condensation.
Acknowledgements Parts of the costs of this investigation have been covered by a grant (NGR-09.015-170) from the National Aeronautics and Space Administration. We would like to express our appreciation to Mrs. P. Muir and Mr. J. Nelen for assistance in the analytical work, and to Mr. M.P. Gorton for his assistance with computer techniques. One of us (B.M.) is indebted to the Australian National University for an appointment as Honorary Fellow in the Research School of Earth
144 Sciences, w h i c h e n a b l e d h i m to p a r t i c i p a t e in t h e spark source mass s p e c t r o m e t e r analyses.
References 1 R.S. Clarke, Jr., E. Jarowewich, B. Mason, J. Nelen, M. Gomez and J.R. Hyde, The Allende meteorite shower, Smithsonian Contr. Earth Sci. 5 (1970) 1. 2 P.W. Gast, N.J. Hubbard and H. Wiesman, Chemical composition and petrogenesis of basalts from Tranquillity Base, Proc. Apollo 11 Lunar Sci. Conf. 2 (1970) 1143. 3 L. Grossman, Refractory trace elements in Ca- Al-rich inclusions in the Allende meteorite, Geochim. Cosmochim. Acta 37 (1973) 1119.
B. MASON AND P.M. MARTIN 4 S.R. Taylor, Geochemical application of spark source mass spectrography, Geochim. Cosmochim. Acta 35 (1971) 1187. 5 R.O. Allen, Jr. and B. Mason, Minor and trace elements in some meteorite minerals, Geochim. Cosmochim. Acta 37 (1973) 1435. 6 P.M. Martin and B. Mason, Major and trace element abundances in components of the Allende meteorite, Nature 247 (1974) in press. 7 C.M. Gray, D.A. Papanastassiou and G.J. Wasserburg, The identification of early condensates from the solar nebula, Icarus 20 (1973) 213.