Rare earths in oceanic basalts

Abstract

Rare-earth relative abundance distribution patterns barely altered from that characteristic of chondritic meteorites have been found in three basalts from the mid-Atlantic ridge and in one from the experimental Mohole. Lanthanum has been depleted to about half its normal chondritic relative abundance in all four specimens, and Ce and Pr have been depleted by 10 to 15 per cent in the ridge basalts. Absolute rare-earth concentrations are 10 to 16 times those of chondritic meteorites and are comparable to those of calcium-rich achondrites. It is probable that the whole-earth lanthanide distribution is identical with the chondritic pattern. The presence of this pattern in terrestrial, chondritic, and achondritic matter is chemical evidence in support of the hypothesis that bodies in the solar system derived from a homogeneous source. No time or mechanism for rare-earth enrichment of the basalts or of the calcium-rich achondrites over chondritic matter is indicated, but mechanisms which would fractionate the rare-earth group may be ruled out.A popular working hypothesis is that the solar system developed from a homogeneous nebula, and hence the relative abundances of the nonvolatile chemical elements are the same for all bodies therein. A nonvolatile element in this sense is one which was not measurably vaporized away under the conditions of formation of whatever particular solid bodies are being compared. Appromations to the primordial distribution are often based on analyses of chondritic meteorites [Urey, 1952]. Most abundance data from the spectrum of the sun are consistent with the chondritic pattern. The earth has a zoned structure, presumably due to differentiation accompanying its evolution from this primordial material, so its over-all abundance pattern is not accessible to direct determination. But its total composition, as inferred from geophysical evidence, is compatible with the above hypothesis. And, where expected geochemical fractionation has not occurred, the isotopic ratios of nonvolatile, nonradiogenic elements in the earth's crust match those in meteorites. Difficulties arise, however, when earth models are restricted to precisely chondritic or achondritic elemental abundances [Gast, 1960].In the materials available for chenfical analysis, the elements least likely to have suffered fractionation from their primordial relative abundances are the rare earths. Suess and Urey [1956] suggested that the geochemical coherence of these elements was such that even the processes of planetary evolution might not alter their initial distribution. However, the relative abundance pattern for the lanthanides as determined by Minami [1935] for three terrestrial shale mixtures was not identical with that found by Noddack [1935] for a composite meteorite sample. Because the low rare-earth concentrations taxed the sensitivity of the then-available analytical techniques, the existence of this difference in distribution was considered to be suspect until subsequent analyses with more sensitive neutron-activation metho...

Mantle Heterogeneity in the North Atlantic: Evidence from Leg 49 Geochemistry

“…The early work on the geochemistry of ocean floor basalts suggested that ocean floor basalts form a relatively homogeneous group quite distinct in character from most other basalt types (Engel, Engel, and Havens, 1965;Frey and Haskin, 1964;Gast, 1965;Kay, Hubbard, and Gast, 1970;see references in Cann and Simkin, 1971). Thus, the rare-earth patterns of most basalts from the ocean floor show a characteristic depletion in light rare earths on a Cory ell-Masuda chondrite-normalized plot.…”

Determination of Rare-Earth Elements in Leg 51, Site 417 Samples

“…An overwhelming majority of abyssal tholeiites show light REE-depleted patterns (Frey and Haskin, 1964;Kay et al, 1970;Schilling, 1971;Kay and Hubbard, 1978), but LREE-enriched patterns have been found (Langmuir et al, 1977;Schilling, 1976;Dostal and Muecke, 1977;Puchelt and Emmermann, 1977;Puchelt et al, 1977), as well as V-shaped ones (Schilling, et al, 1977).…”

Rare-earth and barium abundances in the Bununu howardite

“…Rare-earth abundances in oceanic tholeiites (16) and our unpublished data on basic and ultrabasic rocks suggest that a similar "unfractionated sequence" exists for terrestrial rocks.…”