Isotope Dilution MC-ICP-MS Rare Earth Element Analysis of Geochemical Reference Materials NIST SRM 610, NIST SRM 612, NIST SRM 614, BHVO-2G, BHVO-2, BCR-2G, JB-2, WS-E, W-2, AGV-1 and AGV-2

We present data for the concentrations of eleven rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu) in eleven international geochemical reference materials obtained by isotope dilution multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). We have analysed both rock powders and synthetic silicate glasses, and the latter provide precise data to support the use of these as reference materials for in situ trace element determination techniques. Our data also provide precise measurements of the abundance of mono-isotopic Pr in both glasses and powders, which allows more accurate constraints on the anomalous redox-related behaviour of Ce during geochemical processes. All materials were analysed in replicate providing data that typically reproduce to better than one percent. Sm/Nd ratios in all these materials also reproduce to better than 0.2% and are accurate to < 0.2% and can thus be used as calibrants for Sm-Nd geochronology. Our analyses agree well with existing data on these reference materials. In particular, for NIST SRM 610, USGS BHVO-2, AGV-1 and AGV-2, our measured REE abundances are typically within < 2% (and mostly 1%) of REE concentrations previously determined by isotope dilution analysis and thermal ionisation mass spectrometry, consistent with the higher degree of precision and accuracy obtained from isotope dilution techniques. Close agreement of results between basaltic glass reference materials USGS BHVO-2G and BCR-2G and the BHVO-2 and BCR-2 powders from which they were created suggests that little fractionation, concentration or dilution of REE contents occurred during glass manufacture.     

A micro-scale investigation of melt production and extraction in the upper mantle based on silicate melt pockets in ultramafic xenoliths from the Bakony–Balaton Highland Volcanic Field (Western Hungary)

Mantle xenoliths in Neogene alkali basalts of the Bakony–Balaton Highland Volcanic Field (Western Hungary) frequently have melt pockets that contain silicate minerals, glass, and often carbonate globules. Textural, geochemical and thermobarometric data indicate that the melt pockets formed at relatively high pressure through breakdown of mainly amphibole as a result of temperature increases accompanied, in most cases, by the influx of external metasomatic agents. New elemental and Sr–Nd–Pb isotope data show that in several xenoliths the external agent was either a LIL-enriched aqueous fluid or a CO₂-rich fluid, whereas in other xenoliths the melt pockets were additionally enriched in LREE and sometimes HFSE, suggesting metasomatism by a silicate melt. The compositional character of the external agents might have been inherited by melting of a hydrated and probably carbonated deeper lithospheric component, which itself was metasomatized by melts with significant slab-derived components. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the earth’s oldest oceanic crustal sequence

Despite superimposed metamorphic overprinting and metasomatic alterations, primary volcanic features remain preserved in low-strain domains of mafic volcanic sequences in the western Isua supracrustal belt (ISB, West Greenland). These basaltic successions represent the hitherto oldest known fragments of oceanic crust on Earth. Early Archean metasomatic fluids, rich in light rare earth elements (LREE), Th, U, Pb, Ba, and alkalies, invaded the supracrustal package and distinctively altered the basaltic sequences. Field relationships, source characteristics traced by Pb isotopes, and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the ISB between 3.81 and 3.74 Ga ago. Subsequent early Archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks. Allanite occurs as the most characteristic and critical secondary metasomatic-metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration, even in domains of low strain. Because of its high concentration of LREE, Th, and U, this secondary mineral accounts for much of the disturbances recorded by the Sm-Nd and Th-U-Pb isotope systematics of the pillowed metabasalts.The supracrustal sequences were tectono-metamorphically affected to varying degrees during a late Archean, ∼2.6- to 2.8-Ga-old event, also recognized in the adjacent gneiss terranes of the Isuakasia area. The degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite-hosted parent-daughter elements to the overall whole-rock mass budget of the respective isotope systems. Although low-strained (initially only weakly metasomatized) pillow basalts remained more or less closed with respect to the U-Pb and Rb-Sr systems since ∼3.74 Ga, the Sm-Nd system appears to have been partially opened on a whole-rock scale during the late Archean event. This diversified behavior of the whole-rock isotope systems with respect to late Archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late Archean recrystallization processes with associated renewed fluid flow. In reactivated zones of high strain, where primary metasomatic alteration is most prominently developed, late Archean partial resetting also of the U-Pb isotope system on a whole-rock scale occurred. This is consistent with an apparent late Archean age of kyanite, which initially crystallized during the early Archean metamorphism. Its age is controlled by the U-Pb systematics of allanite inclusions, which have exchanged their isotopic properties during the tectono-metamorphic event that overprinted the oceanic crustal sequence at Isua more than 1000 myr later.These results underline the need for care in the interpretation of whole-rock geochemical data from polymetamorphic rocks in general, and from the Isua oceanic crustal sequences in particular, to constrain isotopic models of early Earth’s evolution. Likewise, this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from Isua to infer models for geotectonic settings relevant for their formation.