Rare Earth Element Abundance Patterns Research Articles

Diffusivities of Rare Earth Elements and Ba in Magmatic Silicate Melts at High Pressures.

Diffusivities of 11 rare earth elements (REE) and Ba in jadeite (NaAlSi2O6) and diopside (CaMgSi2O6) melts have been measured at pressures between 10 and 20kbar. In jadeite melt, diffusivities of REE increase regularly with increasing their ionic radii, light REE (e.g., La and Ce) diffuse faster than heavy REE (e.g., Yb and Lu) by about 30%, Eu diffuses faster by factor of 8 to 12 than other REE under reducing conditions, and diffusivities of REE increase with increasing pressure at constant temperature. In diopside melt, diffusivities of REE are nearly the same as that of Ca and decrease with increasing pressure. The present study demonstrates that diffusion process in magma should change the relative abundances of REE and also generate Eu anomalies without involvement of plagioclase feldspar. This diffusion-induced process is a newly recognized mechanism of changing the relative REE abundances in magmas and generating Eu anomalies, and it must be taken into account in interpreting the REE abundance patterns of igneous rocks.

Rare-earth element patterns and carbon isotopic composition of carbonados: implications for their crustal origin.

Carbon isotopic ratios and rare-earth element (REE) contents were measured for six Central African carbonados and two Brazilian ones. The δ13C values of all the Central African samples and one of the Brazilian samples are from –29.7‰ to –24.4‰, which are within the range of organic matters such as petroleum and coal, and are much lower than the typical values for ordinary diamonds of around -5‰. Another Brazilian sample gives diamond-like δ13C value of –8.8‰, suggesting that this is actually not a carbonado but an aggregate of ordinary diamond such as bort and ballas. Samples with a larger amount of impurities show higher REE contents and higher light-REE/heavy-REE ratios, implying the existence of a light-REE-enriched mineral, such as florencite, along grain boundaries between diamond crystals. Chondrite-normalized REE abundance patterns of the samples are similar to crustal materials such as shales rather than to kimberlites and ordinary diamonds, which are much more light-REE enriched than most of the studied samples. The Brazilian sample with a higher δ13C value, however, shows a kimberlite-like REE pattern which is clearly different from that of the other Brazilian sample. From our data, the crustal origin of carbonado is preferable to its genesis in the mantle.

Hornblende-melt trace-element partitioning measured by ion microprobe

Trace-element abundances were measured in situ by ion microprobe in five samples of hornblende and melt ranging from basaltic andesite to high-silica rhyolite. Except for one sample, for which quench overgrowth or disequilibrium is suspected, the abundance ratios show systematic inter-element and inter-sample variations, and probably approach true partition coefficients. Apparent partition coefficients are reported for La, Ce, Nd, Sm, Dy, Er, Yb, Sc, Ti, V, Cr, Sr, Y and Zr. Rare-earth elements (REE) and Y form smooth convex-upward partitioning patterns that rise to higher D-values and become increasingly convex in more evolved samples. Apparent partition coefficients for REE, Y, Ti, V and Cr can be parameterized as functions of the distribution of Ca between hornblende and melt, giving expressions to predict hornblende-melt trace-element partitioning values. These expressions are used to show that heavy REE-enriched hornblende/whole-rock REE abundance patterns in granitoids may result from partial re-equilibration of hornblende and late-stage residual liquids rather than from anomalous partitioning values.

REE abundances and Rb-Sr systematics for Lunar meteorites: Chemical and isotopic characteristics and their genetic implications.

Lunar meteorites recovered from Antarctica are recognized to have been derived from the lunar highlands. Seventeen samples (rock fragments and clasts) from Yamato-791197 and Yamato-82192 have been analyzed for chemical compositions (rare earth elements and major elements) and Rb-Sr systematics. Those samples show chemical and chronological characteristics resembling the lunar highland samples very much. The lunar highland rocks have been known to show positive Ce anomalies in their REE (rare earth elements) abundance patterns. These anomalies are considered to have formed on the lunar surface, but their genesis has not been discussed definitely. The lunar meteorites, which we analyzed, also show the positive Ce. anomalies. According to our geochemical (major elements and REE abundances) and chronological (Rb-Sr systematics) studies for some lunar meteorites, the Ce anomalies appear to increase with decreasing Al2O3 and CaO contents. Furthermore, the initial 87Sr/86Sr ratios obtained for some samples with ages of around 3.9 Gyr are as low as that for the Juvinas eucrite which has an age of 4.5 Gyr, indicating the removal of radiogenic 87Sr. Samples of a lunar meteorite, which form an isochron with relatively high initial ratio, show smaller positive Ce anomalies than the samples of 2 other lunar meteorites which form isochrons with relatively low initial ratios. As one possibility, these correlations among major element contents, REE abundances, initial 87Sr/86Sr ratios and Ce anomalies suggest that H2O (possibly ice) might have played a role in the alteration having produced the Ce anomalies, and it is suggested that ice may have existed at least locally on the surface of the moon at an early stage of lunar history.

MORB peridotite-sea water interaction: experimental constraints on the behaviour of trace elements, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios

Abstract The geochemical changes associated with the experimental serpentinization of peridotite are due to reaction with sea water and to the growth of new mineral phases. Since the rare earth elements (REE) and Sr are primarily ensconced in clinopyroxene, it is the unreactive nature of this phase in the presence of sea water that determines the REE content, the 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratio of the experimentally produced serpentinites. Serpentinites (after lherzolite and dunite) have chondrite-normalized REE abundance patterns similar to the initial peridotite indicating that the light REE (LREE) are not selectively mobilized by peridotite-sea water interaction at 300°C. However, the serpentinites produced as a result of harzburgite-sea water experiments show an increase in LREE content. Despite the sporadic behaviour of the LREE, the 143 Nd/ 144 Nd ratios in experimentally produced serpentinites (after lherzolite and harzburgite) are identical to primary clinopyroxene in the unaltered peridotite. In contrast, a marked change in the strontium isotopic composition of the peridotites occurs during experimental serpentinization due to the growth of hydrous and Ca-rich phases which facilitates the uptake of sea water Sr. Whereas harzburgite and dunite alter to produce serpentinites with high Sr contents and 87 Sr/ 86 Sr > 0.709, lherzolites tend to alter to serpentinites with 87 Sr/ 86 Sr < 0.709. This behaviour invalidates the use of Rb-Sr data in understanding the origin of oceanic and ophiolitic peridotites, but the relative immobility of the light REE (in clinopyroxene-bearing peridotites) and the low REE content of sea water encourages the careful use of the REE and neodymium isotopes as a petrogenetic indicator for elucidating the origin of serpentinized abyssal peridotites.

Rare earth mobility in young arc-type volcanic rocks from northern New Zealand

Samples of Late Cenozoic arc-type volcanic rocks from the Hauraki Volcanic Region in northern New Zealand show evidence for enrichment of the rare earth elements (REEs). These samples have variable to high La and Y abundances together with REE abundance patterns which are up to an order of magnitude higher than those considered typical of the suite as a whole. Scanning electron microscope imaging and analysis shows the presence of micron scale veins and patches containing REE-bearing phases. These observations are explained as the result of cryptic alteration processes which enrich the REEs but not other chemical components and which are probably the result of low temperature (<250°C) hydrothermal alteration.

Rare earth element scavenging in seawater

Examinations of rare earth element (REE) adsorption in seawater, using a variety of surface-types, indicated that, for most surfaces, light rare earth elements (LREEs) are preferentially adsorbed compared to the heavy rare earths (HREEs). Exceptions to this behavior were observed only for silica phases (glass surfaces, acid-cleaned diatomaceous earth, and synthetic SiO 2). The affinity of the rare earths for surfaces can be strongly affected by thin organic coatings. Glass surfaces which acquired an organic coating through immersion in Tampa Bay exhibited adsorptive behavior typical of organic-rich, rather than glass, surfaces. Models of rare earth distributions between seawater and carboxylate-rich surfaces indicate that scavenging processes which involve such surfaces should exhibit a strong dependence on pH and carbonate complexation. Scavenging models involving carboxylate surfaces produce relative REE abundance patterns in good general agreement with observed shale-normalized REE abundances in seawater. Scavenging by carboxylate-rich surfaces should produce HREE enrichments in seawater relative to the LREEs and may produce enrichments of lanthanum relative to its immediate trivalent neighbors. Due to the origin of distribution coefficients as a difference between REE solution complexation (which increases strongly with atomic number) and surface complexation (which apparently also increases with atomic number) the relative solution abundance patterns of the REEs produced by scavenging reactions can be quite complex.

Young ages of two diogenites and their genetic implications

THE achondrite meteorites known as eucrites and diogenites are thought to be igneous rocks derived from the crust and mantle, respectively, of a common parent planet or asteroid1,2. Several eucrites have been dated at 4.5–4.6 Gyr by the Rb–Sr method4–6, and a Rb–Sr age of 4.45±0.18 Gyr (λ = 1.42x 10−11yr−1) has been reported3 for three diogenites. From these data, one cannot judge whether diogenites and eucrites are the same age. Here we present precise Rb–Sr data for two diogenites (Tatahouine and Johnstown) that yield an age of 4.394 ±0.011 Gyr—significantly younger than our age of 4.52 ±0.15 Gyr for the Juvinas eucrite. The low initial 87Sr/86Sr ratio of these diogenites, and their rare-earth element abundance patterns, are inconsistent with the interpretation that the young ages reflect secondary metamorphism. We suggest that our data raise doubts about a genetically close relationship between eucrites and these diogenites, and that there exist groups of diogenites that are genetically distinct from one another. Our results also suggest that igneous activity in the assumed parent body continued for about one hundred million years after its formation 4.52 Gyr ago. The parent body may thus have had a complicated evolutionary history.

Ca isotopes in refractory inclusions

We report measurements of the absolute isotope abundance of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites. Improved high precision measurements are reported also for 46Ca. We find that nonlinear isotope effects in Ca are extremely rare in these inclusions. The absence of nonlinear effects in Ca, except for the effects in FUN inclusions, is in sharp contrast to the endemic effects in Ti. One fine-grained inclusion shows an excess of 46Ca of (7 ± 1)%., which is consistent with addition of only 46Ca or of an exotic (∗) component with 46Ca ∗ ~ 48Ca ∗. FUN inclusion EK-1-4-1 shows a small 46Ca excess of (3.3 ± 1.0)%.; this confirms that the exotic Ca components in EK-1-4-1 were even more deficient in 46Ca relative to 48Ca than is the case for normal Ca. The Ca in the Ca-Al-rich inclusions shows mass dependent isotope fractionation effects (as deduced from the absolute 40Ca 44Ca ) which have a range from −3.8 to +6.7%. per mass unit difference. This range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. Ca and Mg isotope fractionation effects in the Ca-Al-rich inclusions are common and attributed to kinetic isotope effects which imply the production of the inclusions by complex sequences of condensation, vaporization and recondensation. A correlation was found between Ca and Mg isotope fractionation effects and inclusion type. A possible correlation between isotope fractionation and rare earth element abundance patterns is discussed.

The evolution of early precambrian crustal rocks at Isua, West Greenland — Geochemical and isotopic evidence

Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860 ± 240 m.y. (2 sigma error), but consideration of the initial 87Sr/ 86Sr ratio constrains the possible age of formation of 3710 ± 900 m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m.y. on Isua banded ironstones. Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700–3800 m.y. old Amiˆtsoq gneisses, involving fractionation of garnet during their evolution. A Pb/Pb whole-rock isochron for Amiˆtsoq gneisses from Isua yields an age of 3800 ± 120 m.y. (2σ), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 ± 120 m.y. ago. A two-stage model for the U-Pb system yields an average 238U/ 204Pb (μ 1) value of 9.3 ± 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U-Pb heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700–3800 m.y. ago. Attempts to apply U-Pb whole-rock dating to the Amiˆtsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching. A Rb-Sr whole-rock isochron on a suite of Amiˆtsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 ± 130 m.y. In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900–3700 m.y. ago.