G.W. Lugmair

Lunar initial 143Nd/144Nd: Differential evolution of the lunar crust and mantle

The Sm-Nd evolution of Apollo 15 green glass is discussed. The ICE age (intercept with chondritic evolution) of 3.8 ± 0.4 AE overlaps the range of reported 39Ar-40Ar ages (T2) and implies a distinct source region for green glass, characterized by very low and unfractionated REE abundances. We present evidence that LINd (lunar initial Nd) is compatible with a “chondritic” type Nd isotopic evolution as observed in the Juvinas meteorite. Using this normalization we studied the Sm-Nd system of various lunar rock types. The results obtained from a limited number of rocks clearly indicate differential Sm-Nd evolution for the lunar crust and mantle. High-Ti basalts returned by the Apollo 11 and 17 missions were derived from distinct source regions. The 143Nd evolution in KREEP requires a source region of negative e(T) which is clearly distinct from any mantle reservoir.

Age and isotopic relationships among the angrites Lewis Cliff 86010 and Angra dos Reis

Results of a wide-ranging isotopic investigation of the unique Antarctican angrite LEW-86010 (LEW) are presented, together with a reassessment of the type angrite Angra dos Reis (ADOR). The principal objectives of this study are to obtain precise radiometric ages, initial Sr isotopic compositions, and to search for the erstwhile presence of the short-lived nuclei 146Sm and 26Al via their daughter products. The isotopic compositions of Sm, U, Ca, and Ti were also measured. This allows a detailed appraisal to be made of the relations between, and the geneology of, these two angrites. LEW proves to be severely contaminated with modern terrestrial Pb, which is shown to result from terrestrial weathering. Nevertheless, concordant Pb-Pb model ages of pyroxene separates were obtained (2σ): 4.55784 ± 52 Ga for LEW and 4.55780 ± 42 Ga for ADOR. Uranium isotopic compositions are normal within error. The inferred initial Pb isotopic composition is within error of primordial Pb, as defined by Canon Diablo troilite. Because of the extreme UPb ratios of both angrites, this places an upper limit of ~2 Ma on the time between volatile element-loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates. Initial 87Sr86Sr ratios were found to be indistinguishable in LEW, ADOR, and the cumulate eucrite Moore County. The derived initial 87Sr86Sr for all three meteorites is 0.698970 ± 15 (2σ). A 147Sm-143Nd isochron for LEW was obtained, yielding an age of 4.553 ± 34 Ga with an initial 143Nd144Nd of 0.506682 ± 49 (2σ). 146Sm142Nd isotope systematics were also measured. Anomalies on 142Nd arising from extinct 146Sm were clearly resolved, resulting in an initial 146Sm144Sm of 0.0071 ± 17 with ϵNd142 = −2.57 ± 0.62 at the time of isotopic closure. Both of these Sm-Nd methods imply derivation of LEW from a reservoir with chondritic SmNd ratio; they are also quite consistent with the previously reported systematics for ADOR. Overall, the age and isotopic similarities between LEW and ADOR are striking; it suggests almost simultaneous production on the same asteroid, even though recent experimental studies imply that the two are not comagmatic. Calcium and titanium do not exhibit enrichments in the n-rich isotopes, in contrast to CAIs, making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula. No evidence was found for live 26Al in LEW. When combined with the Pb-Pb age and initial 87Sr86Sr data, this firmly excludes 26Al being an important heat source in the early solar system; heat supplied for, for example, (1) the differentiation of the angrite and eucrite parent bodies, and (2) metamorphism of the chondrite parent bodies must come from some other source. The debate over live vs. extinct 26Al in the earliest solar system remains unresolved by this new data from LEW, however. Both the angrite and eucrite parent bodies have extremely low RbSr and high UPb ratios compared to the solar nebula. Therefore, the common initial 87Sr86Sr of 0.698970 can be firmly associated with the U-Pb angrite age above, and the formation of both bodies. This enables an absolute chronology of the early solar system to be established. Based upon 87Sr86Sr differences, the oldest CAIs are a minimum of 11 ± 4 Ma older than the angrites, i.e., 4.569 ± 5 Ga. The age and isotopic constraints are discussed with respect to current collapse, condensation, and accretion timescales calculated for the solar nebula.

Columbia River volcanism - The question of mantle heterogeneity or crustal contamination

Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. 143Nd144Nd ranges from 0.51303 to 0.51208 and is strongly correlated with variations in 87Sr86Sr. This correlation suggests mixing between two end member compositions, one characterized by 143Nd144Nd > 0.51303 and 87Sr86Sr 0.715. The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4AE, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust.

Isotope and trace element characteristics of a super-fast spreading ridge: East Pacific rise, 13–23°S

Isotopic patterns of Nd, Sr, and Pb are remarkably coherent along the super-fast spreading portion of the East Pacific Rise from 13°S to 23°S. Between 15.8°S and 20.7°S, all three define a broad, smooth peak, which culminates at ∼ 17–17.5°S and is characterized by elevated 87Sr86Sr, 206Pb204Pb, and lower ϵNd (reaching values of 0.70271, 18.64, and +8.9, respectively). To the north and south this peak is flanked by ∼ 300 km long, isotopically homogeneous sections of ridge with higher ϵNd (∼ +10.9) and lower 87Sr86Sr (∼ 0.7024) and 206Pb204Pb (∼ 18.1). Although otherwise similar, these two sections differ from each other slightly in their 207Pb204Pb and 87Sr86Sr ratios. The isotopic peak corresponds to a region of greater axial cross-sectional area, but axial bathymetry and physical segmentation appear generally unrelated to mantle isotopic composition. However, an abrupt break in isotopic ratios does occur at the large, > 3 Ma, southward-propagating overlapping spreading center at 20.7°S, which marks the end of the south limb or flank of the isotopic peak. The peak itself appears to be a manifestation of large-scale binary mixing between material possessing at least mildly plume-like Nd, Pb, and Sr isotopic characteristics (most abundant at ∼ 17–17.5°S) and two slightly different high-δNd mantle end-members equivalent to those north of 15.8°S and south of 20.7°S. Helium isotopes also define a prominent along-axis peak, but it spans a much narrower range of latitude and is offset slightly to the north of those for Nd, Sr and Pb isotopes. The combined results suggest that a discrete mantle heterogeneity may be entering into the melt zone near 15.8°S and migrating southward as far as the 20.7°S overlapping spreading center. Isotopic variability at short length scales is very limited throughout the entire 13–23°S region. It cannot be solely a result of homogenization by petrogenetic processes, because there is a lack of corresponding uniformity in ratios of highly to moderately incompatible elements; also, isotopes do not correlate with major element indicators of degree of partial melting or differentiation, or, in general, with the secondary magmatic segmentation thought to reflect different partial melting domains. Therefore, the subaxial mantle must be isotopically well-mixed relative to the scale of melting. In part, this probably reflects: (1) a larger volume of melting per unit length of ridge; and (2) a greater flow of mantle into the subaxial melt zone at super-fast spreading; but also must represent (3) a reduced amount of real isotopic variability in the shallow asthenosphere, as emphasized by the regional isotopic uniformity north and south of the isotopic peak. Such large-scale homogeneity could be a result of enhanced convective asthenospheric mixing over a long period of time. It could also reflect a low, long-term input of continental, lithospheric, recycled slab, or plume-type material into the regional asthenosphere. Largely independent of the north-south isotopic patterns is a fairly regular, southward depletion in highly incompatible elements such as Rb and Nb, superimposed on which is sizable local variability. Because ratios of highly to moderately incompatible elements show little or no correlation with major-element indicators of degree of melting, much of the variation in highly incompatible elements must be caused by a different (probably larger) volume of mantle than that conferring the major element signatures, or by one (or more) event that preceded the main melting episode in the not too ancient past.

SmNdPu timepieces in the Angra dos Reis meteorite

We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT2 = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.142Nd/144Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 105. This variation is tentatively assigned to a radiogenic contribution from extinct146Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of244Pu with the light REE. Computed ratios244Pu/Nd are the same in pyroxene and phosphate separates, but244Pu/238U and244Pu/232Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio244Pu/Nd= 1.5 × 10−4 (or an atomic ratio244Pu/150Nd= 1.6 × 10−3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the81Kr-83Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio150Sm/149Sm in Angra dos Reis by 6 parts in 104.

The age of ferroan anorthosite 60025: oldest crust on a young Moon?

Abstract Sm Nd isotopic data for mineral separates from the ferroan anorthosite 60025 define a precise isochron of 4.44 ± 0.02Ga age. This age is roughly 110 m.y. younger than the formation of the first large solid objects in the solar nebula, as recorded by the radiometric ages of the differentiated meteorites. In the magma ocean model for early lunar differentiation, ferroan anorthosites are the first crustal rocks to form on the Moon. If the Moon is as old as the oldest meteorites, the relatively young age determined for 60025 implies either that the magma ocean did not form synchronously with lunar formation, or that the magma ocean required over 100 m.y. before reaching the stage of ferroan anorthosite crystallization. Alternatively, we propose that the accumulated body of radiogenic isotope data for lunar rocks permit the Moon to be as young as 4.44–4.51 Ga. If so, isotopic evidence for chemical differentiation on the Earth at about this same time suggests that the formation of the Moon is reflected in the chemical evolution of the Earth. This, in turn, is consistent with the idea that the materials that now make up the Moon were derived from the Earth, perhaps ejected by collision between the Earth and another very large planetesimal during the final stages of accumulation of the terrestrial planets. Terrestrial origin models for the Moon lessen the requirement that the Earth and Moon each have near chondritic relative abundances of the refractory elements and could require that certain chemical and isotopic characteristics of both bodies be considered in the framework of the chemical mass-balance of the combined Earth-Moon system.

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with143Nd/144Nd ranging from ∼ 0.51302 to 0.51355 and87Sr/86Sr from ∼ 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path ∼ 2 Ga ago as a consequence of partial melting. The combination of high143Nd/144Nd with high87Sr/86Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity (143Nd/144Nd∼ 0.51256–0.51273,87Sr/86Sr∼ 0.7044–0.7032). An evaluation of SmHf and YbHf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.

Spinel peridotite xenoliths from the Tariat Depression, Mongolia. II: Geochemistry and Nd and Sr isotopic composition and their implications for the evolution of the subcontinental lithosphere

A suite of spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, Tariat Depression (central Mongolia) represents (for major elements) fertile to moderately depleted subcontinental lithosphere. Part of the variation of moderately incompatible trace elements is ascribed to small-scale mineralogical heterogeneities caused by processes like metamorphic differentiation accompanying partial melting or by mechanical segregation. Several bulk lherzolites show a high relative enrichment of the LREE over HREE which can be traced to a grain boundary phase genetically linked to, but not directly representing, the host basanitoid. In Nd and Sr isotopic composition the anhydrous peridotites cover the field of oceanic basalts (143Nd/144Nd = 0.5128–0.5133, 87Sr/86Sr = 0.7020–0.7039). In contrast, a phlogopite peridotite has a high 87Sr/86Sr and also a less radiogenic 143Nd/144Nd. The majority of “dry” lherzolites have Nd and Sr “bulk earth” model ages around 2 Ga. They may be interpreted as dating a small-degree (< ~5%) melting event which would not have severely affected the major element chemistry of the xenoliths. The ~2 Ga model ages may indicate a genetic relation between the lithospheric mantle and the stabilization of the continental crust in Mongolia at that time. Alternatively, if the peridotites are unrelated to the overlying crust, they may be pieces of a young asthenospheric diapir. Coexisting ortho-and clinopyroxenes are in Nd isotopic equilibrium for Iherzolites having equilibrated at temperatures around 950°C at mantle pressures. Disequilibrium melting models of mantle rocks are not supported by our data because for medium to coarse-grained mantle spinel peridotite the Rb-Sr and Sm-Nd isotopic systems close with respect to diffusional exchange at temperatures around 900°C, as indicated by recently published diffusion experiment results and supported by our data.

Nickel and chromium isotopes in Allende inclusions

High-precision nickel and chromium isotopic measurements have been carried out on nine Allende inclusions.62Ni,64Ni excesses are present in at least three of the samples. The magnitudes of the excesses are about 3 e units (1 e unit = 1 part in 10,000) for64Ni and about 1 e unit for62Ni. Chromium isotopes measured for some of the same samples show general excesses on54Cr of about 7 e units while small deficits are observed for53Cr. Considering these isotope anomalies (except those for53Cr) together with the excesses present in the samples of other neutron-rich isotopes of the iron group elements, the most likely mechanism responsible for these anomalies is a neutron-rich statistical equilibrium process. In addition, there is an indication of elevated60Ni in almost every inclusion measured. However, for ALK 111 this excess was clearly resolved by our experimental technique. This effect may be related to the decay of now extinct60Fe but the present data are not sufficient to clearly demonstrate its existence in the early solar system. An upper limit of 1.6 × 10−6 can be calculated for60Fe/56Fe at the time these Allende inclusions crystallized.

60Fe in eucrites

Additional evidence for the existence in the early solar system of live60Fe has been found in the basaltic achondrite Juvinas. The relative abundance of 60Fe/56Fe of x0223C; 4 × 10−10 at the time of meteorite solidification is one order of magnitude lower than that previously observed for the eucrite Chervony Kut. This difference corresponds to a ∼ 4.7 Ma time interval between closure of the60Fe-60Ni isotopic system in the two meteorites. The factor two higher initial 60Ni/58Ni ratio in Juvinas is consistent with this time difference and a common bulk Fe/Ni ratio in the two meteorites. Using the60Fe-60Ni isotope system as a chronometer various models for the evolution of the eucrite parent body of different degrees of complexity are discussed. Regardless of the detailed differences between these models a short time interval of only a few million years between planetary differentiation and basaltic crust formation is indicated.