PDF (Hi-Res) - Smithsonian Institution Libraries

More documents

Recommendations

Info

NUMBER 19 91 nium is enriched in Groups I, II, and III, but not in Group IV. Thorium shows a similar pattern. Discussion Several other investigators have published data on minor and trace elements in materials from the Allende meteorite, and it is of interest to correlate their data with those presented here. Unfortunately, in most instances no mineralogical or petrological description accompanies the chemical data, which makes the assignment of type (chondrule or aggregate) uncertain. However, the large melilite chondrules are the most prominent and most easily extracted inclusions in the Allende meteorite, and it is evident that sampling has been biased towards these (as is true for the present research also). Gast, Hubbard, and Wiesmann (1970) provided K, Rb, Sr, Ba, and lanthanide data for what they described as a Ca-rich inclusion from Allende. The FIGURE 3.—The relationship between Zr and Y in individual samples. lanthanide distribution matches that for Group I, as does the high Sr and Ba contents, and their material was probably a melilite chondrule. Tanaka and Masuda (1973) have published Ca, Al, Sr, Ba, and lanthanide data for the Allende meteorite and some selected components. Their Ca/Al-rich chondrule was a melilite chondrule, and its lanthanide distribution pattern closely parallels that from Group I. They analyzed two samples of a finegrained aggregate; the patterns are closely similar and agree with that for Group II. An olivine chondrule gave an unfractionated lanthanide distribution pattern at approximately twice chondritic abundance, consistent with Group IV. Only one of their analyzed samples does not fit readily within the grouping proposed here; their inclusion O has a lanthanide distribution pattern rather similar to Group III, but the negative Eu and Yb anomalies are much less pronounced. The most extensive set of data so far published
92 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES is that of Grossman (1973) on 16 specimens of Ca/Al-rich inclusions selected by Dr. E. A. King, Jr., of the University of Houston. Grossman determined Fe, Mn, Na, Sc, La, Sm, Eu, Yb, Co, Ir, and Au in these specimens and on two samples of the bulk meteorite. Through the kindness of Dr. King, we received splits of specimens 1, 4, and 8 analyzed by Dr. Grossman. These had been selected as covering a wide compositional range, and our analyses show that sample 1 belongs to Group I, sample 4 to Group III, and sample 8 to Group II. Grossman's data for the remaining samples indicate that they all belong to Group I. Perhaps the most intriguing feature revealed by the present research is the occurrence of at least three different lanthanide distribution patterns in the materials comprised under the description of Ca/Al-rich inclusions. Although Groups I, II, and III do show characteristic differences in major element composition, these differences are relatively much less than those for the trace elements. The presence of marked Yb anomalies in Groups II and III, and of a Tm anomaly in Group II appears to be unique to these Allende materials; to our knowledge Yb and Tm are closely coherent with the neighboring lanthanides both in terrestrial and in lunar materials. It is remarkable at first glance that in Group II a negative Eu anomaly is associated with a positive Yb anomaly, whereas in Group III a negative Eu anomaly is associated with a negative Yb anomaly. However, when the Eu and Yb abundances in individual samples in these two groups are compared, one finds that the Eu/Yb ratio is relatively constant (Table 3). Figure 1 shows that the Yb anomaly is positive in Group II because it is superimposed on a fractionated lanthanide pattern with rapidly diminishing abundances of the heavier elements, whereas it is negative in Group III because it is superimposed on an unfractionated lanthanide pattern in which all the other elements (except Eu) are strongly enriched. Thus in Groups II and III Eu and Yb show a geochemical coherence unrelated to the other lanthanides. After Eu, Yb is the lanthanide most readily reduced to the divalent state, and this provides a possible explanation for this coherence. However, this explanation requires highly reducing conditions at some stage in the origin of Group II and III materials, more reducing, for example, than for lunar rocks, which show Eu anomalies but do not have Yb anomalies. In this connection, too, it is puzzling to find no Yb anomalies either in Group I materials or their constituent minerals (Mason and Martin, 1974). Boynton (1975) comments that it seems unlikely that divalent Yb can explain the Yb anomalies, since Sm is nearly as easily reduced as Yb, and there is no evidence of an Sm anomaly. He points out that the condensation of lanthanides from the solar nebula may be controlled by thermodynamic equilibrium between gas and condensed solids, and that highly fractionated lanthanide patterns may result if condensates are removed from the gas before condensation is complete. Both Yb and Eu are predicted to be extremely depleted in the early condensate without the requirement of condensation in the divalent state. According to Boynton's model, the Group II inclusions may be a condensate from a previously fractionated gas rather than from a gas of solar composition. Thus the cosmochemical properties of Eu and Yb (determined by gas-solid equilibria) may be quite different from crystallochemical properties (determined by liquid-solid and solidsolid equilibria), and may allow an unambiguous determination of which process is yielding a specific lanthanide pattern. The most remarkable and thought-provoking anomaly is the positive Tm anomaly in Group II samples. Group II samples 37 and 3598 were analyzed in 1971, when no attempt was made to measure Tm because of possible interference on the 169 mass number. In 1973 it was realized that the 169 mass number line in many Allende samples was much too strong to be due to the multiple carbon interference, and it was found that in Group I samples Tm values were obtained that fell on the smooth chondrite-normalized curve linking Er and Yb. However, the Tm values for Group II samples are anomalously high in comparison to Er and Yb (Figure 1). A similar Tm anomaly in some Allende aggregates has recently been reported by Conrad, Schmitt, and Boynton (1975). The cause of this Tm anomaly remains to be elucidated. Boynton (1975) has discussed rare earth anomalies in terms of fractional condensation in the solar nebula, and this may be invoked as a possible cause. Another possibility, suggested by the search for evidence for
Page 1 and 2:
SMITHSONIAN CONTRIBUTIONS TO THE EA
Page 3 and 4:
Contents MINERALOGY COMPOSITION OF
Page 5 and 6:
Constituent SiO TiO Al 0 fl 2 3 Cr
Page 7 and 8:
PER CENT CBLCIUM FIGURE 3.—The ca
Page 9 and 10:
Constituent TiO,, 2 2 3 FeO* MnO Mg
Page 11 and 12:
20 Cfl 40 .60 SMITHSONIAN CONTRIBUT
Page 13 and 14:
35'50' I L 6.. 3.. 2.. r FOKT DEFIA
Page 15 and 16:
12 SMITHSONIAN CONTRIBUTIONS TO THE
Page 17 and 18:
14 20 SMITHSONIAN CONTRIBUTIONS TO
Page 19 and 20:
16 Constituent Si0 2 TiO2 A12O3 Cr
Page 21 and 22:
18 20 40 £0 SMITHSONIAN CONTRIBUTI
Page 23 and 24:
20 40 20 SMITHSONIAN CONTRIBUTIONS
Page 25 and 26:
Abyssal Basaltic Glasses as Indicat
Page 27 and 28:
24 Electron-M icroprobe Analysis An
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
30 Hill in the NE Atlantic, and fro
Page 35 and 36:
32 cores the arrangement is stratig
Page 37 and 38:
Page 39 and 40:
36 ATLANTIC OCEAN LAT. LONG. DEPTH
Page 41 and 42:
Page 43 and 44: 40 SMITHSONIAN CONTRIBUTIONS TO THE
Page 75 and 76: TABLE 1.—Chemical composition in
Page 85 and 86: 82 Literature Cited Bostrom, K., an
Page 87 and 88: Geochemical Differences among Compo
Page 93: 90 SMITHSONIAN CONTRIBUTIONS TO THE
Page 99 and 100: The St. Mary's County, Maryland, Ch
Page 103 and 104: 100 SMITHSONIAN CONTRIBUTIONS TO TH
Page 107 and 108: Mineralogical and Chemical Composit
Page 111 and 112: 108 Sampling Inclusion 1 was taken
Page 115 and 116: 112 15 10 32 3.6 40 44 48 •fl-r O
Page 123 and 124: 2.0- 1.5- 1.0 0.8 : 0.6: 0.4- 0.2-
show all

PDF (Hi-Res) - Smithsonian Institution Libraries

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?