B. Spettel

Fluid- and melt-related enrichment in the subarc mantle: Evidence from Nb/Ta variations in island-arc basalts

The single most distinctive feature of volcanic rocks from convergent-margin settings is a marked depletion of the high field strength elements (HFSE) Nb, Ta, and Ti relative to large ion lithophile and light rare earth elements when compared with basalts from mid-oceanic ridges (MORB) and the oceanic islands. A major impediment to a better understanding of this problem has been a lack of high-quality data for the HFSE (particularly Nb and Ta) that occur in very low concentrations in most volcanic rocks from convergent-margin settings. We report new analyses of Nb and Ta for a suite of island-arc volcanic rocks as well as some sea-floor sediments. Our data show that Nb/Ta values for relatively depleted island-arc volcanic rocks are similar to MORB and essentially chondritic (Nb/Ta ∼ 17), whereas more potassic arc volcanics have substantially higher Nb/Ta values (up to 33). We interpret these high values as due to modification of the subarc mantle source by silicic melts derived from the subducting slab, whereas enrichment of the source regions of the less potassic arc rocks involved a slab-derived fluid.

The solar-system abundances of Nb, Ta, and Y, and the relative abundances of refractory lithophile elements in differentiated planetary bodies

Abstract Analytical data for Nb, Y, and Ho in 8 carbonaceous chondrites were obtained by spark source mass spectrometry (SSMS). In addition, three carbonaceous chondrites were analyzed for Ta by radiochemical neutron activation analysis (RNAA). From these data and earlier literature data on the C1 -chondrite Orgueil a consistent set of solar-system abundances is derived for Nb, Y, Zr, Ta, Hf and the REE. Ratios among these elements are constant within analytical uncertainties in all groups of carbonaceous chondrites. In particular we do not find a difference in Zr Hf ratios between C1 and C2 chondrites. The new abundances for C1-chondrites are: Nb (0.246 ppm), Y (1.57 ppm), Ta (0.014 ppm), or 0.696, 4.64, 0.020 atoms/106 Si atoms, respectively. Based on a large number of analytical data on oceanic basalts, it is argued that the relative abundances of these elements are chondritic in the bulk Earth. Ratios such as Zr Hf or Nb Ta are constant and chondritic in oceanic basalts and agree with estimates of the continental crust. The constant but non-chondritic Nb U ratio (47 vs. 30) in oceanic basalts is balanced by a lower Nb U ratio (~ 10) in the crust. The bulk Earth ratio may therefore be chondritic. The Zr Hf and Nb Ta ratios are correlated in lunar rocks. Both ratios vary within a factor of two, similar to the variability in terrestrial oceanic basalts. The Zr Nb and Hf Ta ratios, however, are much more constant on the Moon. The available evidence suggests that refractory lithophile elements in the Earth, the Moon and achondritic meteorites occur in the same proportions as in carbonaceous chondrites. Refractory elements have greatly different volatilities. The same pattern of refractory lithophile elements in chondrites and planets therefore indicates that protoplanetary materials have never been subject to high temperature processes that would fractionate refractory elements from each other. The same ratio of Zr Nb in the three types of carbonaceous chondrites, in the Earth, the Moon and in differentiated meteorites is a good example, since the condensation temperature for Zr is 177 K higher than that for Nb.

Solubility of palladium in silicate melts: Implications for core formation in the Earth

Abstract Palladium solubilities in silicate melts of anorthite-diopside-eutectic composition were determined at a wide range of oxygen fugacities, from pure O 2 to f o2 slightly below the iron-wustite buffer and at temperatures ranging from 1343 to 1472°C. Experiments were performed by heating palladiumloops with silicates inside a gas controlled furnace. Palladium concentrations were determined by neutron activation analysis. Repeated analyses of the glasses after removal of the outer layers and several reversed experiments with initially high Pd in the glass showed that equilibrium was attained in the experiments. At 1350°C concentrations of Pd in silicate melts range from 428 ppm to 1.2 ppm with decreasing palladium content at decreasing oxygen fugacities. The dependence of log Pd on log f o 2 indicates a change in valence of the dominant palladium species in the silicate melt. The data can be explained by the presence of complexes containing Pd 2+ and Pd 0 . Alternatively, a good fit is obtained by assuming mixtures of Pd 2+ , Pd 1+ and Pd 0 in the melt with increasing contributions of the lower valence species at increasingly reducing conditions. Solubilities increase with temperature at fixed oxygen fugacities independent of the absolute fugacity. This is an unexpected result. From the solubility data, metal/silicate partition coefficients were calculated using known activity coefficients of Pd in Fe-metal. Extrapolations were made to higher temperatures and lower oxygen fugacities. A palladium metal/silicate partition coefficient of 1.6 · 10 7 is inferred for 1623 K and IW-2. Extrapolation to 3500 K leads to a partition coefficient of 3.8 · 10 3 . From earlier data on Ir solubilites, a metal/silicate partition coefficient of 2 · 10 8 was estimated for the same conditions. The high absolute metal/silicate partition coefficients for Pd and Ir and the large difference between the two partition coefficients are not compatible with a global core/mantle equilibrium as a source of the highly siderophile elements in the Earth mantle. The data favour models invoking the accretion of a late chondritic veneer after core formation without further metal segregation.

COMPOSITION AND ORIGIN OF REFRACTORY-METAL-RICH ASSEMBLAGES IN A CA, AL-RICH ALLENDE INCLUSION

Metal-oxide-sulfide assemblages, henceforth opaque assemblages (OA) or Fremdlinge, with high contents of refractory metals (Ir, Os, Ru, Pt, etc.) were separated from the Allende Egg 6 Ca, Al-rich inclusion (Type B1). Instrumental neutron activation analysis (INAA) was applied to seven OAs including Zelda, described in an earlier paper by Armstrong et al. (1987). After INAA, polished sections were prepared of four OAs for petrographic and mineralogical studies. In addition, several INAA analyses of the bulk Egg 6 inclusion and mineral separates were performed. The bulk inclusion shows enrichments in refractory lithophile elements (Ca, Al, Se, REEs, etc.) and refractory metals, typical of coarse-grained CAIs from Allende. The combined study of OAs and the parent Egg 6 inclusion indicates that the major host phases of refractory metals in this inclusion, and probably in many others, are not the large isolated OAs analyzed here but numerous smaller submicroscopic refractory metal-rich alloys finely dispersed in the major minerals of the inclusion. n nThe Egg 6 OAs have refractory metal contents qualitatively compatible with an origin by condensation. Detailed calculations, however, demonstrate that the abundances of both refractory and non-refractory metals in the OAs cannot be matched by simple condensation models. In addition, differences among OAs in absolute and relative abundances of refractory metals require formation of individual OAs under slightly different conditions. The observed distribution of refractory metals within OAs reflects redistribution accompanying sulfurization and oxidation of the primary refractory metal-rich assemblages. Platinum and Rh are primarily concentrated in NiFe, Os, Ru, and Re in tiny, micron-sized OsRu-nuggets formed by exsolution from a more homogeneous alloy. Iridium partitions between FeNi and OsRu-nuggets, while Mo is exclusively concentrated in MoS2. The two major sulfide phases, pyrrhotite and pentlandite, have less than 0.04% of any of the refractory metals, except for about 0.15% Mo. Tungsten is very low in the bulk OAs, although condensation models predict high W. It is demonstrated that W-contents in silicates surrounding OAs are unusually high, indicating redistribution of W after incorporation of OAs in the inclusion in aggreement with laboratory simulation experiments. A model for the origin of the Fremdlinge must involve several stages: (1) Formation of refractory metal alloys by condensation, (2) incorporation into existing CAIs at elevated temperatures, and (3) oxidation and sulfurization of OAs and extensive redistribution of elements including loss of volatile W-compounds and Mo-compounds.

Precursor lithologies and metamorphic history of granulitic breccias from north ray crater, station 11, Apollo 16

Abstract Two distinctly different types of granulitic breccias in the ejecta of North Ray crater, Station 11, Apollo 16 have been analyzed for their modal and chemical composition. Samples 67485, 67488, 67615, 67749, 67947, and 67566 are characterized by a fine-grained granoblastic to poikiloblastic matrix and abundant lithic and mineral clasts predominantly derived from ferroan anorthosites. The mineral clasts in sample 67566 were derived from ferroan anorthosites and Mg-rich lithologies. The fine-grained granulitic breccias are compositionally almost identical to the feldspathic microporphyritic melt breccias (fmmbs) of Station 11, Apollo 16 which are older than, and therefore are possible precursors of, the fine-grained granulitic breccias. The second type of granulitic lithology (67746) is medium-grained, anorthositic-noritic in composition, and contains well equilibrated minerals derived from Mg-rich rocks. The precursors of both granulitic lithologies were KREEP-free and according to their Ir Au ratio were old lunar highland rocks. A comparison of major, minor, and trace element contents shows that the two types of granulites are unrelated. The different recrystallization textures of the two types of granulitic lithologies imply different metamorphic histories. Metamorphism of the fine-grained granulitic breccias occurred near the lunar surface and the heat source very likely was a superheated impact melt. The texture and the degree of equilibration in the minerals of 67746 requires a much more intense metamorphism with slow cooling. Metamorphism of lunar rocks and breccias was a wide-spread process that occurred several times in lunar history.