Stein B. Jacobsen

Sm-Nd isotopic evolution of chondrites

The ^(143)Nd/^(144)Nd and ^(147)Sm/^(144)Nd ratios have been measured in five chondrites and the Juvinas achondrite. The range in ^(143)Nd/^(144)Nd for the analyzed meteorite samples is 5.3 e-units (0.511673–0.511944) normalized to ^(150)Nd/^(142)Nd= 0.2096. This is correlated with the variation of 4.2% in ^(147)Sm/^(144)Nd (0.1920–0.2000). Much of this spread is due to small-scale heterogeneities in the chondrites and does not appear to reflect the large-scale volumetric averages. It is shown that none of the samples deviate more than 0.5 e-units from a 4.6-AE reference isochron and define an initial ^(143)Nd/^(144)Nd ratio at 4.6 AE of 0.505828 ± 9. Insofar as there is a range of values of ^(147)Sm/^(144)Nd there is no unique way of picking solar or average chondritic values. From these data we have selected a new set of self-consistent present-day reference values for CHUR (“chondritic uniform reservoir”) of (^(143)Nd/^(144)Nd)_(CHUR)^0 = 0.511836 and (^(147)Sm/^(144)Nd)_(CHUR)^0 = 0.1967. The new ^(147)Sm/^(144)Nd value is 1.6% higher than the previous value assigned to CHUR using the Juvinas data of Lugmair. This will cause a small but significant change in the CHUR evolution curve. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that depleted sources were also tapped early in the Archean. Such a depleted layer may represent the early evolution of the source of present-day mid-ocean ridge basalts. There exists a variety of discrepancies with most earlier meteorite data which includes determination of all Nd isotopes and Sm/Nd ratios. These discrepancies require clarification in order to permit reliable interlaboratory comparisons. The new CHUR curve implies substantial changes in model ages for lunar rocks and thus also in the interpretation of early lunar chronology.

Precise determination of SmNd ratios, Sm and Nd isotopic abundances in standard solutions☆

The methods used for precise calibrations of Sm/Nd ratios and the average isotopic abundances obtained for normal Sm and Nd are given. A mixed Sm-Nd normal solution with a precisely known ^(147)Sm/^(144)Nd ratio close to the nominal average chondritic value is described and the calibration discussed. Aliquots of this standard solution are available on request and may be useful for precise interlaboratory calibration of Sm and Nd.

Sm-Nd isotopic evolution of chondrites and achondrites. II

The ^(147)Sm-^(143)Nd and ^(146)Sm-^(142)Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of (^(143)Nd/^(144)Nd)_(CHUR)^0 = 0.511847 and (^(147)Sm/^(144)Nd)_(CHUR)^0 = 0.1967. Most of the bulk chondrites analyzed have ^(143)Nd/^(144)Nd and ^(147)Sm/^(144)Nd within 0.5 e-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an Sm-Nd internal isochron age of T = 4.55 ± 0.33AE and an initial e_(Nd) = 0.11 ∓ 0.26. Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03 AE and 4.564 ± 0.037 AE were obtained for these meteorites, respectively. The initial e_(Nd) values obtained for Moama and ADOR are 0.03 ∓ 0.25 and 0.14 ∓ 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial e_(Nd) data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial e_(Nd) values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the e_(Nd) vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods. n^(146)Sm-^(142)Nd systematics of ADOR and Moama are supportive of the hypothesis that ^(146)Sm was present in the early solar system and suggests a ^(146)Sm/^(144)Sm ratio of about 0.01 for the solar system ∼ 4.56 AE ago. This inferred high ^(146)Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm-Nd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initial e_(Nd) = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm-Nd model age of the plagioclase lherzolite total rock is T_(CHUR)^(Nd) = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm-Nd mineral isochron age of T = 435 ± 21 Ma and e_(Nd) = + 6.7 ∓ 0.3. A pyroxenite dike yields an initial e_(Nd) = + 7.3 ± 0.4. The initial e_(Nd) values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb-Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of e_(Sr) ⩽ −21 for the plagioclase lherzolite and e_(Sr) ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm-Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.

Interpretation of Nd, Sr and Pb isotope data from Archean migmatites in Lofoten-Vesterålen, Norway

Sm-Nd data for the Archean granulite and amphibolite facies migmatites of Langoy and Hinnoy in Vesteralen are presented which indicate that their protoliths formed ∼2.6 AE ago. Rubidium and U loss during a granulite facies metamorphism at ∼1.8 AE caused serious disturbance of total-rock U-Pb and Rb-Sr systems. Therefore these systems do not provide any precise age information for the granulite facies migmatites. For the amphibolite facies migmatites of Vesteralen both Sm-Nd, Rb-Sr and Pb-Pb total-rock systems give model ages of ∼2.6 AE. The results on both granulite and amphibolite facies rocks are thus in agreement. Previous interpretations based on Pb-Pb data, which indicated an age of 3.41 AE for the Archean terrane of Vesteralen, are not valid. nOne Sm-Nd model age from the granulite facies migmatites at Moskenesoy in Lofoten indicates that the protoliths of these migmatites formed ∼2.0 AE ago and are thus not related to the Vesteralen migmatites.

Transport models for crust and mantle evolution

The exact solutions for the isotopic compositions and the concentrations of two models for mantle—crust evolution are given for arbitrary rates of crustal growth and of backflow to the mantle. In Model I, continents are derived by melt extraction over the history of the earth from undepleted mantle. In Model II, new additions to the continents are derived from a mantle reservoir which becomes increasingly depleted through time by repeated extraction of melts. The key parameters are the chemical fractionation factors for crustal growth and refluxing and the integrated fractional mass removal rates from the crust and the mantle. The relationships between the mean age of the mass of the continents and the isotopic parameters are given. The models suggest that the pre-Archean era (prior to 3.8 AE) was dominated by rapid refluxing of crust to the mantle which during the early Archean gave way to a regime where transport from the mantle to the crust substantially dominated over refluxing of crust to the mantle. The mean age of the mass of the continents appears to be ~1.8 AE. Even for small amounts (~10%) of refluxing Model II shows that highly incompatible elements have very short residence times in the mantle. Mass balance considerations implies that the continents were derived from only a small fraction (~30%) of the mantle.