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The origin of type B Ca-Al-rich inclusions in carbonaceous chondrites: An ion microprobe study
30 REFRACTORY TRACE ELEMENT ABUNDANCES AS INDICATORS OF SOIAR SYSTEM FORMATION PROCESSES
THE T H E R M A L R E C O R D OF A V I G A R A N O
W.V. BOYNTON. (Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, U.S.A.).
C. CAILLET, J.I. G O L D S T E I N , D. VELDE, and A. E L GORESY. ( U n i v e r s i t @ P. et M. Curie, P a r i s VI, L a b o r a t o i r e de P @ t r o l o g i e M i n @ r a l o g i q u e , France)
The abundances of refractory trace elements in samples extracted from meteorites provide important information on processes that took place as our solar system was forming. Among the refractory elements, the rare-earth elements (REE) are particularly important because they have volatilities that span many orders of magnitude, yet their ability to be incorporated into host phases is very similar and is a smooth function of size. Thus, the primary nebular processes can be distinguished from later planetary processes. Often found in Ca,Al-rich inclusions (CAI), for example, is a peculiar type of REE pattern, called Group II, with a large abundance anomaly at Tm and smaller anomalies at other REE. Thermodynamic calculations indicate that the REE pattern was established in a gas-solld fractionation event (i.e. vaporization or condensation). In fact, the pattern in the Group II CAI are the more volatile REE and therefore must have been the gaseous component in the fractionation event. The knowledge that these REE were in the gas phase sets a lower limit of about 1400K on the temperature where the inclusion formed. Explaining this observation has been a challenge for theorists. The REE have also been useful in suggesting that the thin (50~m) ubiquitous rims on CAI's formed as a result of a rapid heating of the underlying CAI. This observation places important constraints on an early heating event in the solar nebula. It requires an energy input of over 300 W/cm 2 with a duration of less than a few seconds. This combination of energy and timescale seems difficult, if not impossible, to provide in the nebular environment. Nevertheless, the rims are found on virtually all CAI's and some intense heat source seems to be required. It looks llke another challenge for the theorists.
ANALYTICAL ELECI~ON MIC~OSCOFY OF INTERPLANETARY DUST PARTICLES (IDP's) J.P. BRADLEY, D.E. ~ROWNLE~, M.S. G~IRblANI, N. DIETZ (McCrone AssociaTes, Inc. Wes~ont, Ii. 60559 U S A ) InTerplaneTery dust particles (IDP's) are routinely c o l l e c ~ from T/qe sZFatospbet~ by U-2 aircraft. Among l~ne particles are a subset known as "chomaritic". These IDP's are typically 5-15 p m aggregates of mineral grains, glass, and carbonaceous material. Many of them conlmin solar flare tracks, which confirms ~neir recent exposure to space as small (<100 pro) objects. The analytical elec~rom microscope (AEM) is wiaely useS for studying IDP's De~use it o~Ters the necessary spatial resolu.tion ~o study finegralnea mineral aggregsTes. We h~ve develope~ a n~thod ior producing electron transparant T/nin-sections of individual IDP's using an ultramicroTome equipped wiT/n s diamomd knife. These sections ara iaeal for microstructu~al and petrographic studies Ln ~qe ~ as well as quantitative T2ain-film x-ray_ analyses wi~n spaZial resolution DefTer ~nan 500 ~. Using an equipped wi~h digital beam control, several hundred analyses can De eutomatically c o l l e c ~ from s single thin-sectian over a 6-8 hour period. • Four classes of chondritic particles ~8ve been recognized. They are referred to as "pyroxene", "olivine", "smectiTe", ana "serpentine '~ IDP's after imporlmnt mafic silicaTes characteristic of each class. "Pyroxene" and "olivine" IDP's exhibit high (>50%) porosity expected for cometary particles, Dut only ~qe "pyrcxene" particles are mineralogically similar ~ 5 come~ Halle~. "Serpentine" and "smecZiTe" particles are low porosity objects that resemble the fine-g~alneG matrices of c a r D o n a c e c u s ~ F L T e s . .
CAI
N u m e r o u s m e t a l g r a i n s w e r e f o u n d in a t y p e B V i g a r a n o CAI. T h e m e t a l s c o n s i s t of k a m a c i t e (4.9 % Ni) and t a e n i t e (33-48 % Ni). B o t h m e t a l s c o n t a i n s e v e r a l p e r c e n t s of Os, Ir, Ru, and Re in s o l i d solution. In some cases, the r e f r a c t o r y m e t a l s are p r e s e n t as e x s o l u t i o n s at the k a m a c i t e - t a e n i t e interface. The N i - c o n c e n t r a t i o n p r o f i l e s from k a m a c i t e to t a e n i t e d o e s not s h o w M - p r o f i l e . In few cases, m a g n e s i o w ~ s t i t e ( P e r i c l a s e 69 W u s t i t e 31) was found w i t h the m e t a l s b o r d e r e d by s t o i c h i o m e t r i c spinel. This s u g g e s t s f o r m a t i o n b e f o r e c a p t u r e in the CAI. O u r p r e s e n t a p p r o a c h is b a s e d on experimentally determined diffusion parameters of Ni in k a m a c i t e and taenite. T h i s is t h e first a t t e m p t to use t h e s e p a r a m e ters to d e t e r m i n e the t h e r m a l h i s t o r y of a CAI. Our i n v e s t i g a t i o n s i n d i c a t e t h a t k a m a c i t e and t a e n i t e w e r e in e q u i l i b r i u m n e a r 500°C. U s i n g the N i - d i f f u s i o n p a r a m e t e r s in k a m a c i t e and a p p l y i n g the b a n d w i d t h i n d i c a t e either: (I) fast c o o l i n g to 500°C. t h e n a p e r i o d of g r o w t h for a b o u t 105 years, or (2) fast c o o l i n g to low t e m p e r a t u r e s f o l l o w e d by r e h e a t i n g to 500°C. T h e l a t t e r m o d e l r e q u i r e s a p e r i o d of 104 years. The m i n o r a m o u n t s of ref r a c t o r y m e t a l s m a y c o m p l i c a t e the system. N o n e t h e l e s s , ~ r a n s f o r m a t i o n t i m e s b e t w e e n I0 v and 107 y e a r s a p p e a r to be a reasonable estimation.
THE ORIGIN OF TYPE B CA-AL-RICH INCLUSIONS IN CARBONACEOUS CHONDRITES: AN ION MICROPROBE STUDY GHISLAINE CROZAZ and GLENN J. MACPHERSON (Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, Mo 63130, USA). USNM 5241, a complex Type B1 inclusion from the Allende meteorite, is a spherical inclusion (radius -8mm) consisting of a melilite mantle ( - 2 a m thick) surrounding a core of fassaite, spinel, melilite and rare anorthite. Spinelfree islands, interpreted to be xenoliths, are disseminated throughout the core (El Goresy et al., 1985). Rare earth element (REE) concentrations and relative abundances of a number of major, minor and trace elements were measured in individual melilite and fassaite crystals from these three distinct areas. Preliminary data were presented by MacPherson et al. (1987); the more extensive measurements and the REE modelling presented here lead to a somewhat different but simple and coherent picture of the evolution of this inclusion. The REE data can be explained by a simple closed-system fracfonal crystallization model in which the mantle and core of the inclusion formed sequentially out of a single melt, confirming some earlier models.for Type B1 inclusion formation (MacPherson and Grossman, 1981; Stolper, 1982). However, there is no evidence that surface volatilization played any significant role in the formation of the melilite mantle. REFERENCES: El Goresy A. et al. (1985) Geochim. Cosmochim. Acta 49, 2433; MacPherson G. J. and Grossman L. (1981) Earth Planet. Sci. Lett. 52, 16; MacPberson G. J. et al. (1987) Lunar Planet. Sci. XVIII, 590; Stolper E. (1982) Geochim. Cosmochim. Acta 46, 2159.
THE T H E R M A L R E C O R D OF A V I G A R A N O
W.V. BOYNTON. (Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, U.S.A.).
C. CAILLET, J.I. G O L D S T E I N , D. VELDE, and A. E L GORESY. ( U n i v e r s i t @ P. et M. Curie, P a r i s VI, L a b o r a t o i r e de P @ t r o l o g i e M i n @ r a l o g i q u e , France)
The abundances of refractory trace elements in samples extracted from meteorites provide important information on processes that took place as our solar system was forming. Among the refractory elements, the rare-earth elements (REE) are particularly important because they have volatilities that span many orders of magnitude, yet their ability to be incorporated into host phases is very similar and is a smooth function of size. Thus, the primary nebular processes can be distinguished from later planetary processes. Often found in Ca,Al-rich inclusions (CAI), for example, is a peculiar type of REE pattern, called Group II, with a large abundance anomaly at Tm and smaller anomalies at other REE. Thermodynamic calculations indicate that the REE pattern was established in a gas-solld fractionation event (i.e. vaporization or condensation). In fact, the pattern in the Group II CAI are the more volatile REE and therefore must have been the gaseous component in the fractionation event. The knowledge that these REE were in the gas phase sets a lower limit of about 1400K on the temperature where the inclusion formed. Explaining this observation has been a challenge for theorists. The REE have also been useful in suggesting that the thin (50~m) ubiquitous rims on CAI's formed as a result of a rapid heating of the underlying CAI. This observation places important constraints on an early heating event in the solar nebula. It requires an energy input of over 300 W/cm 2 with a duration of less than a few seconds. This combination of energy and timescale seems difficult, if not impossible, to provide in the nebular environment. Nevertheless, the rims are found on virtually all CAI's and some intense heat source seems to be required. It looks llke another challenge for the theorists.
ANALYTICAL ELECI~ON MIC~OSCOFY OF INTERPLANETARY DUST PARTICLES (IDP's) J.P. BRADLEY, D.E. ~ROWNLE~, M.S. G~IRblANI, N. DIETZ (McCrone AssociaTes, Inc. Wes~ont, Ii. 60559 U S A ) InTerplaneTery dust particles (IDP's) are routinely c o l l e c ~ from T/qe sZFatospbet~ by U-2 aircraft. Among l~ne particles are a subset known as "chomaritic". These IDP's are typically 5-15 p m aggregates of mineral grains, glass, and carbonaceous material. Many of them conlmin solar flare tracks, which confirms ~neir recent exposure to space as small (<100 pro) objects. The analytical elec~rom microscope (AEM) is wiaely useS for studying IDP's De~use it o~Ters the necessary spatial resolu.tion ~o study finegralnea mineral aggregsTes. We h~ve develope~ a n~thod ior producing electron transparant T/nin-sections of individual IDP's using an ultramicroTome equipped wiT/n s diamomd knife. These sections ara iaeal for microstructu~al and petrographic studies Ln ~qe ~ as well as quantitative T2ain-film x-ray_ analyses wi~n spaZial resolution DefTer ~nan 500 ~. Using an equipped wi~h digital beam control, several hundred analyses can De eutomatically c o l l e c ~ from s single thin-sectian over a 6-8 hour period. • Four classes of chondritic particles ~8ve been recognized. They are referred to as "pyroxene", "olivine", "smectiTe", ana "serpentine '~ IDP's after imporlmnt mafic silicaTes characteristic of each class. "Pyroxene" and "olivine" IDP's exhibit high (>50%) porosity expected for cometary particles, Dut only ~qe "pyrcxene" particles are mineralogically similar ~ 5 come~ Halle~. "Serpentine" and "smecZiTe" particles are low porosity objects that resemble the fine-g~alneG matrices of c a r D o n a c e c u s ~ F L T e s . .
CAI
N u m e r o u s m e t a l g r a i n s w e r e f o u n d in a t y p e B V i g a r a n o CAI. T h e m e t a l s c o n s i s t of k a m a c i t e (4.9 % Ni) and t a e n i t e (33-48 % Ni). B o t h m e t a l s c o n t a i n s e v e r a l p e r c e n t s of Os, Ir, Ru, and Re in s o l i d solution. In some cases, the r e f r a c t o r y m e t a l s are p r e s e n t as e x s o l u t i o n s at the k a m a c i t e - t a e n i t e interface. The N i - c o n c e n t r a t i o n p r o f i l e s from k a m a c i t e to t a e n i t e d o e s not s h o w M - p r o f i l e . In few cases, m a g n e s i o w ~ s t i t e ( P e r i c l a s e 69 W u s t i t e 31) was found w i t h the m e t a l s b o r d e r e d by s t o i c h i o m e t r i c spinel. This s u g g e s t s f o r m a t i o n b e f o r e c a p t u r e in the CAI. O u r p r e s e n t a p p r o a c h is b a s e d on experimentally determined diffusion parameters of Ni in k a m a c i t e and taenite. T h i s is t h e first a t t e m p t to use t h e s e p a r a m e ters to d e t e r m i n e the t h e r m a l h i s t o r y of a CAI. Our i n v e s t i g a t i o n s i n d i c a t e t h a t k a m a c i t e and t a e n i t e w e r e in e q u i l i b r i u m n e a r 500°C. U s i n g the N i - d i f f u s i o n p a r a m e t e r s in k a m a c i t e and a p p l y i n g the b a n d w i d t h i n d i c a t e either: (I) fast c o o l i n g to 500°C. t h e n a p e r i o d of g r o w t h for a b o u t 105 years, or (2) fast c o o l i n g to low t e m p e r a t u r e s f o l l o w e d by r e h e a t i n g to 500°C. T h e l a t t e r m o d e l r e q u i r e s a p e r i o d of 104 years. The m i n o r a m o u n t s of ref r a c t o r y m e t a l s m a y c o m p l i c a t e the system. N o n e t h e l e s s , ~ r a n s f o r m a t i o n t i m e s b e t w e e n I0 v and 107 y e a r s a p p e a r to be a reasonable estimation.
THE ORIGIN OF TYPE B CA-AL-RICH INCLUSIONS IN CARBONACEOUS CHONDRITES: AN ION MICROPROBE STUDY GHISLAINE CROZAZ and GLENN J. MACPHERSON (Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, Mo 63130, USA). USNM 5241, a complex Type B1 inclusion from the Allende meteorite, is a spherical inclusion (radius -8mm) consisting of a melilite mantle ( - 2 a m thick) surrounding a core of fassaite, spinel, melilite and rare anorthite. Spinelfree islands, interpreted to be xenoliths, are disseminated throughout the core (El Goresy et al., 1985). Rare earth element (REE) concentrations and relative abundances of a number of major, minor and trace elements were measured in individual melilite and fassaite crystals from these three distinct areas. Preliminary data were presented by MacPherson et al. (1987); the more extensive measurements and the REE modelling presented here lead to a somewhat different but simple and coherent picture of the evolution of this inclusion. The REE data can be explained by a simple closed-system fracfonal crystallization model in which the mantle and core of the inclusion formed sequentially out of a single melt, confirming some earlier models.for Type B1 inclusion formation (MacPherson and Grossman, 1981; Stolper, 1982). However, there is no evidence that surface volatilization played any significant role in the formation of the melilite mantle. REFERENCES: El Goresy A. et al. (1985) Geochim. Cosmochim. Acta 49, 2433; MacPherson G. J. and Grossman L. (1981) Earth Planet. Sci. Lett. 52, 16; MacPberson G. J. et al. (1987) Lunar Planet. Sci. XVIII, 590; Stolper E. (1982) Geochim. Cosmochim. Acta 46, 2159.