Study of Magma Sources, Mantle Structure and the Differentiation of the Earth from Variations of ¹⁴³Nd/¹⁴⁴Nd in Igneous Rocks

Citation

DePaolo, Donald James (1978) Study of Magma Sources, Mantle Structure and the Differentiation of the Earth from Variations of ¹⁴³Nd/¹⁴⁴Nd in Igneous Rocks. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YBQ5-4B95. https://resolver.caltech.edu/CaltechETD:etd-08232006-154851

Abstract

The decay of 147Sm to 143Nd allows 143Nd/144Nd to be used to trace Sm/Nd fractionation in long time-scale geologic processes. Since Sm/Nd is a sensitive indicator of many aspects of the overall chemistry of rocks, the Sm-Nd system provides an excellent tool for the study of the chemical evolution of the earth's crust and mantle.

143Nd/144Nd has been measured in terrestrial rock samples of different ages to establish the characteristics of Nd isotopic evolution in the crust and mantle. The evolution of 143Nd/144Nd in the mantle indicates Sm/Nd equal to that of chondrites, and implies a chondritic REE distribution for the earth. Young basalts show a significant dispersion in 143Nd/144Nd indicating the existence of distinct mantle reservoirs with characteristic 143Nd/144Nd. 143Nd/144Nd in average crustal rocks today is much lower than in mantle samples and reflects their age and low Sm/Nd. Continental flood basalts and mid-ocean ridge (MOR) tholeiitic basalts have distinctly different 143Nd/144Nd which may permit a priori distinction between "continental" and "oceanic" igneous rocks. Ocean island basalts have 143Nd/144Nd intermediate between MOR and continental flood basalts. Initial 143Nd/144Nd of many continental igneous rocks through time fall on a Sm/Nd evolution curve with chondritic REE abundance ratio. Oceanic igneous rocks are derived from a different ancient reservoir which has Sm/Nd higher than chondritic. These observations indicate that many continental igneous rocks are derived from a reservoir with chondritic REE pattern which may represent primary undifferentiated material remaining since the formation of the earth, while oceanic igneous rocks are derived from highly differentiated reservoirs. The mantle beneath the oceans appears to be more depleted in crustal components than is the mantle which is subjacent to the continental crust. In general, basalts are not derived from mantle reservoirs which have been light REE-enriched for long times.

Initial 143Nd/144Nd and 87Sr/86Sr in young basalts from both oceans and continents show a strong correlation suggesting that Sm-Nd and Rb-Sr fractionation events in the mantle may be correlative and caused by the same process. From this correlation Rb/Sr for the earth is inferred to be 0.029.

Initial 143Nd/144Nd in lunar igneous rocks show much more dispersion than in terrestrial rocks of similar age. The data suggest that the earth, unlike the moon, did not undergo an early differentiation event which greatly fractionated the rare earth elements, or if it did, a mixing process operated during the subsequent AE to erase the variation of Sm/Nd produced in this event.

Nd and Sr isotopes indicate that if the earth is made of a mixture of achondrites (low Rb/Sr) and chondrites (high Rb-Sr) that these two components must have been thoroughly mixed. The present-day isotopic heterogeneity of the earth's mantle is unrelated to accretional heterogeneity.

Transport calculations and material balance considerations for simple models of formation of the continental crust indicate that only a small portion of the earth's total budget of Sm and Nd are found in the crust. Highly differentiated mantle reservoirs such as those from which MOR basalts are derived must represent only a small portion of the mantle, perhaps one-fourth to one-sixth or less. The data are consistent with the existence of large volumes of undifferentiated (possibly undegassed) material in the mantle. The data also suggest that the continental crust has a low Rb/Sr (less than 0.10) implying a highly layered structure for the crust, with the lower crust having a much lower Rb/Sr than the upper crust.

Island arc lavas from New Britain and the Marianas have 143Nd/144Nd similar to other oceanic basalts and distinctly different from continental flood basalts and thus appear to be derived from a high Sm/Nd, light-REE-depleted reservoir. Consideration of both Nd and Sr isotopes suggests seawater involvement in the generation of some island arc lavas and thus indicates that they may be derived from altered subducted oceanic crust. Other island arc lavas show no evidence of seawater involvement and may be derived from mantle reservoirs with affinities to the sources of ocean island basalts. Nd and S in some basaltic and ultrapotassic continental lavas and in some Andean volcanics indicate that some magmas in continental regions may be derived from old low-Sm/Nd reservoirs or are heavily contaminated with old continental crustal material. Fish debris from the ocean floor provides an estimate of 143Nd/144Nd in seawater and indicates that light-REE in the marine environment are derived mainly from continents.

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