A.A. Ulyanov

ION MICROPROBE STUDIES OF EFREMOVKA CAIS : II. POTASSIUM ISOTOPE COMPOSITION AND 41CA IN THE EARLY SOLAR SYSTEM

The potassium isotope composition of refractory phases in Efremovka CAIs has been measured by an ion microprobe. Extensive studies of terrestrial standards were carried out to ensure that the experimental procedures adopted for potassium isotope studies provide accurate 41K/39K ratios of the analysed phases. Excess 41K is found to be present in refractory phases with high Ca/K ratios (>3 × 105) in all the four Efremovka CAIs analysed in this study. The excess 41K in these phases correlates well with their 40Ca/39K ratios. Several possible causes for this excess have been considered and it is concluded that in situ decay of 41Ca can best explain our observations. The results obtained in this work substantiate the conclusion drawn from our initial study (Srinivasan et al., 1994) and confirm the presence of the short-lived nuclide 41Ca (τ ~ 0.15 Ma) in the early Solar System with an initial 41Ca/40Ca value of (1.41 ±0.14) × 10−8 at the time of formation of the Efremovka CAIs. We have considered several processes that may lead to the presence of 41Ca in the early Solar System. These include, production by energetic particles from an active early Sun, low energy particle induced reactions in a molecular cloud complex of which the proto-solar cloud was a part and freshly synthesized material from suitable stellar source(s). The last alternative turns out to be the best one from plausibility considerations. The Efremovka CAIs with excess 41K also have excess 26Mg that can be attributed to the decay of the short-lived nuclide 26Al(τ ~ 1 Ma) within these objects. The presence of both 26Al and 41Ca in Efremovka CAIs is used to infer the most probable stellar site(s) for the synthesis of these nuclides in a self consistent manner. Our observations coupled with predicted stellar production rates suggest an asymptotic giant branch (AGB) star to be a plausible source. However, we cannot completely rule out a supernova or a Wolf-Rayet star as being responsible for the synthesis and subsequent injection of these nuclides to the solar nebula. In spite of this inability to pinpoint the exact stellar source, our result constrains the time interval between the injection of freshly synthesized 41Ca and 26Al to the solar nebula and the formation of first Solar System solids (CAIs) to less than a million year.

Multiple Generations of Refractory Inclusions in the Metal-Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

Ca,Al-rich inclusions (CAIs) are believed to have formed by evaporation, condensation, and melting of the pre-existing solids during the earliest stages of the solar system evolution. Most CAIs in unmetamorphosed chondrites contain detectable excesses of 26Mg(26Mg*), a decay product of the short-lived radionuclide 26Al ( -->T1/2 ~ 730,000 yr), that correspond to an initial 26Al/27Al ratio of ~ -->(4–7) × 10−5. It is suggested that 26Al was injected into the protosolar molecular cloud or protoplanetary disk by a nearby core-collapse supernova (SN Type II) and uniformly distributed in the solar system; CAI formation started shortly after injection of 26Al and lasted less than 20,000 yr . Here we show that CAIs from the metal-rich carbonaceous chondrites Acfer 214 (CH) and Isheyevo (CH/CB-like) have a bimodal distribution of 26Mg*. Most CAIs composed of grossite (CaAl4O7), hibonite (CaAl12O19), Al-rich pyroxene, perovskite (CaTiO3), and gehlenitic melilite (Ca2Al2SiO7-Ca2MgSi2O7) show either unresolvable or small 26Mg* corresponding to an initial 26Al/27Al ratio of ~ -->4 × 10−7. Some of the grossite-rich CAIs and the less refractory inclusions composed of melilite, spinel (MgAl2O4), Al,Ti-pyroxene, and anorthite (CaAl2Si2O8) have large 26Mg* corresponding to the initial 26Al/27Al ratio of ~ -->5 × 10−5. The 26Al-poor and 26Al-rich CAIs are characterized by 16O-rich (Δ -->17O < − 20‰) compositions typical of CAIs. We suggest that the 26Al-poor and 26Al-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of 26Al into the solar system, respectively. Model yields of 16O,17O, and 18O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both 26Al-poor and 26Al-rich CAIs plot on a three-isotope oxygen diagram (δ17O vs. δ18O) along a single line with a slope of ~1 are consistent with injection of 26Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.

ION MICROPROBE STUDIES OF EFREMOVKA CAIS. I: MAGNESIUM ISOTOPE COMPOSITION

Five coarse-grained Ca-Al-rich refractory inclusions (CAIs) from the Efremovka CV3 chondrite are analysed for their magnesium isotopic composition using an ion microprobe. The analysed inclusions represent the three common CAI types (A, B1, and B2). Data for magnesium isotopic composition of the major mineral phases (melilite, spinel, fassaite, and anorthite) in these inclusions are used to determine intrinsic magnesium isotopic fractionations and to identify nonlinear effect in 26Mg due to the decay of extinct 26Al. The magnesium isotopic mass fractionation, F(Mg), generally favours the heavier isotopes, as is expected in the case of coarse-grained CAIs. The F(Mg) data for two of the inclusions, however, show contrasting trends. Sympathetic behaviour found in both isotopic and petrographic data in one type B1 inclusion suggests that the parent melt of this inclusion experienced a short-duration volatilization event prior to crystallization. The isotopic data also suggest possible presence of relict spinel in this inclusion that constrains the cooling rate of the parent melt to >70°C/h during crystallization. Hot and dense nebular settings that allow for such fast cooling can only be localized in nature. The pristine nature of the Efremovka CAIs, as evident from the lack of secondary alteration products in them, is well supported by the isotopic data. Three of the five inclusions yield well-behaved Mg-Al isochrons with (26Al27Al)0 close to the canonical value of 5 × 10−5. The anorthite data for another inclusion suggest minor disturbance in the isotopic systematics and yield a nearly well-behaved isochron with a slightly lower value of (26Al27Al)0. Data for melilite and anorthite in the fifth inclusion define apparent isochrons that suggest partial re-equilibration and exchange of magnesium isotopes between these two phases. The Efremovka data are suggestive of a relatively homogeneous distribution of 26Al in the solar nebula.

Geochemistry of trace elements in the extraterrestrial matter examined by the X-ray fluorescence method with synchrotron radiation

Abstract X-ray fluorescence analysis with synchrotron radiation (XFA-SR) has been applied for the determination of incompatible trace elements (Rb, Sr, Y, Zr, Nb, Ba, La and Ce) in some extraterrestrial samples. A distribution of these elements in lunar whole rocks and their minerals, in the matrix, chondrules and CaAl-rich inclusions from various chondrites has been investigated. The XFA-SR analytical technique for very small objects ranging from 2 mm to 300 μm has been described. The specific basaltic group of Luna-20, geochemically similar to Luna-24 VLT basalt, has been established. The points of intersection of the HAB-VHA rock trend and the TB rock trend on the bi-element correlation diagrams probably correspond to the concentration levels of these trace elements in the parent magmas. Distribution of trace elements in rock-forming minerals and in their host rocks showed two trends: early and late crystallizations with an inversion of the Y/Zr ratio. The difference in trace element distribution in chondrules, matrix and CaAl-rich inclusions from the chondrites has been shown. This indicates a varied genesis for these objects.

Geochemistry of rare elements in lunar basalt rocks based on SRXFA

Abstract For the first time the distribution of Rb, Sr, Y, Zr and Nb in Apollo-17 magmatic rock series and in new Luna-16 basalt series has been investigated by SRXFA. On the base of bi-element correlation, individual geochemical trends for various petrochemical types of rocks were established. The distinction between magmatic sources of Apollo-17 and Luna-16 rocks and the presence of a new VLT-LT rock group were demonstrated. Common use of SRXFA and INAA methods is promising.

GEOCHEMISTRY OF TRACE ELEMENTS IN THE HIGHLAND LUNAR ROCKS BASED ON SRXFA DATA

Abstract X-ray fluorescence analysis using synchrotron radiation (SRXFA) has been applied for determination of trace elements in the highland lunar rocks (Luna 20 and Apollo 16). On the basis of the distribution of Rb, Sr, Y, Zr, and Nb in 69 lunar highland fragments six geochemical groups of rocks with various aluminium and trace element contents were distinguished for each station. They have various levels of trace element content. This suggests the various petrological processes: cumulation of rock-forming minerals and partial melting of source rocks. The last process lead up to enriching in trace elements in rocks formed from these melts. The distribution of trace elements in highland and mare rocks of the Moon were compared.

Trace element distribution in lunar rocks of various titanium content by SR-XFA data

Abstract The distribution of trace elements (Rb, Sr, Y, Zr, Nb) in various types of lunar mare basalts has been investigated by SR-XFA. The new data of Apollo 11, Apollo 12, and Apollo 15 and also our earlier published data of other mare basalts have been discussed. Geochemical groups and trends for rocks of various titanium content were distinguished on the basis of diagrams of Zr vs. Sr and of Y vs. Zr. The problem regarding the magmatic sources of various types of lunar basalts has been considered.

Some methodical peculiarities of analysis of small-mass samples by SRXFA

Abstract The stability of work of the element analysis station on the storage rings VEPP-3 and VEPP-4 in INP (Novosibirsk, USSR) was demonstrated on the example of three sets of rare element analyses carried out by SRXFA in May 1985, January and May–June 1988. These data show that there are some systematic deviations in the results of measurements of Zr and La contents. SRXFA and INAA data have been compared for the latter element. A false linear correlation on the Rb-Sr plot in one set of analyses has been attributed to an overlapping artificial Sr peak on a Rb peak. The authors proposed sequences of registration of spectra and computer treatment for samples and standards. Such sequences result in better final concentration data.

Trace element identification of very low titanium (VLT) and low titanium (LT) basalts from mare Fecunditatis and mare Crisium regions: SR-XFA and INAA studies of lunar samples

Abstract The very low titanium (VLT), low titanium (LT), and titanium (T) basalts of the Eastern part of the Moon were investigated by SR-XFA and INAA methods. New intermediate VLT-LT basalt groups were distinguished on the basis of titanium contents, REE patterns and trace element distributions. All basalt groups were compared and conclusions about possible sources of these basalts and their origin were drawn.

Geochemical identification of mare-type basalt groups from a lunar highland region (by INAA and SRXFA)

Abstract The distribution of Rb, Sr, Y, Zr and Nb in 15 fragments of lunar mare-type basalt rocks from the Apollonius highland region has been investigated by the SRXFA method. The work has been carried out on the element analysis station of the storage ring VEPP-3. Preliminary identification of lunar rock groups was based on INAA data. Investigation by SRXFA permits to distinguish VLT-LT groups of basalts by geochemical criteria.