Part I. Sm-Nd and Rb-Sr Chronology of Crustal Formation. Part II. Ba, Nd and Sm Isotopic Anomalies in the Allende Meteorite

Citation

McCulloch, Malcolm Thomas (1980) Part I. Sm-Nd and Rb-Sr Chronology of Crustal Formation. Part II. Ba, Nd and Sm Isotopic Anomalies in the Allende Meteorite. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hnt7-9586. https://resolver.caltech.edu/CaltechTHESIS:11152023-171559514

Abstract

This thesis is divided into two parts. The first part has as its theme the chronology of crustal formation. The times at which new segments of crust were formed has been ascertained by using Sm-Nd and Rb-Sr isotopic systematics, together with plausible assumptions regarding the geochemical evolution of the crust and mantle. The general approach that has been used is to assume that the differentiation processes which produce magmatic rocks occur with a marked chemical fractionation of the Sm/Nd and Rb/Sr ratios relative to the source region. It is then the time of this chemical fractionation which has been dated. This approach is valid insofar as the dominant contribution to the crust comes from the emplacement of magmatic rocks derived from a "uniform" mantle reservoir.

Sm-Nd studies of early Archean complexes from the Minnesota River Valley, Labrador, and the Pilbara of Western Australia have shown that these rocks were derived at ~ 3.6 AE from a mantle with a time­ average Sm/Nd ratio approximately equal to that in chondrites (CHUR). These results, together with a 3.6 AE Sm-Nd model age from a tonalitic gneiss in northern Michigan, confirm that this time period marks one of the earliest episodes of major continental crustal formation on the earth.

In contrast to the apparent uniformity of Sm/Nd in the mantle of the earth, many lunar magmatic rocks have initial ¹⁴³Nd/¹⁴⁴Nd ratios which indicate variability in the Sm/Nd ratios of their source reservoirs of up to 60%. These highly fractionated reservoirs must have formed within about 200 m.y. after the accretion of the moon. The Sm-Nd data on highland breccias and KREEP samples indicate a low Sm/Nd ratio for these samples which was also plausibly established early in lunar history and is complementary to the early production of high Sm/Nd mare basalt source regions. The contrast between the lunar and terrestrial Nd isotopic data implies a grossly different early differentiation history for these two planets.

A study of the Nd, Sr, and O isotopic characteristics of the Samail Ophiolite is also presented. This complex represents oceanic crust that was abducted onto the Arabian continental margin during late Cretaceous times. Using the Sm-Nd method, internal isochrons were obtained from three gabbros establishing a Cretaceous crystallization age for the Samail Ophiolite. These results appear to be highly consistent in spite of extensive hydrothermal exchange which has altered the primary ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O ratios. Gabbros from Ibra give an age of 130 ± 12 m.y., and a gabbro in the northern part of Oman gives an age of 100 ± 20 m.y. These results show that the Sm-Nd technique can be used to determine crystallization ages and initial Nd ratios of relatively young, Mesozoic and Cenozoic mafic complexes.

In part two of this thesis, isotopic anomalies are reported for Ba, Nd, and Sm in two inclusions from the Allende meteorite. These inclusions are typical Ca-Al-rich objects associated with early condensates from the solar nebula but have distinctive O and Mg isotopic anomalies of the FUN type. Sample Cl shows ¹⁴⁴Sm enrichments and a depletion in ¹³⁵Ba of 2 parts in 10⁴ and normal Nd. Sample EKl-4-1 shows large positive excesses in the unshielded isotopes ¹³⁵Ba and ¹³⁷Ba of 13.4 and 12.3 parts in 10⁴, respectively, The Nd and Sm isotopic composition in EKl-4-1 is highly aberrant in at least five isotopes The Ba, Sm, and Nd anomalies in EKl-4-1 can be explained by a model of r-process addition. From the observations of ¹⁴⁴Sm isotopic anomalies in Cl, it is inferred that the p-processes is decoupled from the r-process. All these anomalies are found to be uniform between coexisting mineral phases. These observations show the existence of substantial isotopic anomalies in refractory elements in the neighborhood of Xe and extend the range of elements showing isotopic effects to O, Ne, Si, Mg, Ca, Sr, Kr, Xe, Ba, Sm, and Nd. These observations, in conjunction with the presence of ¹⁰⁷Pd (τ½ = 6.5 x 10⁶ yrs) and ²⁶Al (τ½ = 7 x 10⁵ yrs), are interpreted as the result of a nearby supernova explosion which produced elements over a wide mass range and injected them into the early solar nebula shortly before condensation.

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