John A. Philpotts

Partition coefficients of rare-earth elements between igneous matrix material and rock-forming mineral phenocrysts—II

Abstract Solid-liquid partition coefficients between phenocrysts and the host lavas have been measured for rare-earth elements by an isotope dilution technique. Phenocrysts analyzed include clinopyroxenes, orthopyroxenes, olivines, micas, amphiboles, garnet, plagioclases and K-feldspar. The consistency of much of the data suggests that most of the phenocrysts crystallized under equilibrium, or quasi-equilibrium, conditions. There is sufficient data on pyroxenes and feldspars to indicate that relative and absolute partition coefficients can be correlated with the major element composition of the phenocrysts. The size of the positive Eu anomaly in feldspars increases with increasing alkali content of the feldspar.

Phenocryst-matrix partition coefficients for K, Rb, Sr and Ba, with applications to anorthosite and basalt genesis

Abstract Partition coefficients for K, Rb, Sr, and Ba distributed between plagioclase, K-feldspar, clinopyroxene, orthopyroxene, mica, hornblende, garnet, and olivine phenocrysts and their igneous matrix materials, have been determined by a mass-spectrometric stable isotope dilution technique. Typical differentiation of a basalt through fractional crystallization results in higher K, Ba, and Rb concentrations, slightly higher K/Sr, Ba/Sr and Rb/Sr ratios, and slightly lower K/Rb and Ba/Rb ratios in the liquid. K/Ba may be a useful solar-system indexing criterion. The high K/Rb ratio and other trace element characteristics of andesine-labradorite anorthosites reflect their origin as plagioclase cumulates. Partition coefficients have been applied to a simple model of basalt genesis.

Petrogenetic implications of some new geochemical data on eclogitic and ultrabasic inclusions

Some eclogitic and ultrabasic inclusions, their separated minerals, host rocks, and related samples have been analyzed by mass-spectrometric isotope dilution for K, Rb, Sr, Ba and rare-earth elements (RE), and by other techniques for selected major elements. The results of these analyses, considered in terms of measured trace-element phenocryst-matrix partition-coefficients, suggest the following trace-element models. Many inclusions are contaminated with at least K, Rb and Ba. Some Kakanui eclogite phases were equilibrated with liquids similar in trace element concentrations to alkali basalt. Some Roberts Victor eclogite and garnet peridotite phases were in equilibrium with kimberlite. Salt Lake Crater eclogite might represent a low-calcium-pyroxene cumulate from alkali basalt. Some peridotites appear to have equilibrated with tholeiitic basalt; but some of these may have been subsequently metamorphosed. Alkali basalt peridotite nodules are not cumulates or residues directly related to their hosts; they might be contaminated metamorphic rocks. Ancillary conclusions are that kimberlite, carbonatite and nepheline-melilite basalt appear to constitute a quantized trace-element group. Rare-earth concentrations are perhaps the best criteria for distinguishing carbonatites from sedimentary carbonates. Roberts Victor omphacite can have high KRb and BaCe. Clinopyroxene/garnet rare-earth ratios tend to be uniform, as, to a lesser extent, do diopside/orthopyroxene rare-earth ratios for coexisting minerals. Mantle rocks appear to have very low Eu2+Eu3+ ratios; this might indicate oxidizing conditions. Alkali basalts approach uniformity in trace-element concentrations. Various degrees of partial fusion of garnet peridotite could account for the whole spectrum of rare-earth concentrations observed for basalts.

Redox estimation from a calculation of Eu2+ and Eu3+ concentrations in natural phases

Assumption of the equivalence of Eu2+ and Sr interphase partitioning permits the calculation of the Eu2+ and Eu3+ concentrations in each of any two equilibrated phases of known Sr and rare-earth concentrations. The calculation has been applied to a number of porphyritic terrestrial lavas, two lunar samples and two basaltic achondrites. Multiphase rocks generally give internally consistent results. The potential size of the positive Eu anomaly in feldspars appears to increase with decreasing feldspar anorthite content. The potential size of the negative Eu anomaly in clinopyroxenes appears to increase with increasing pyroxene iron content. A mica apparently has a positive Eu anomaly. The typical basalt Eu2+/Eu3+ ratio is about 0.15. No correlation of Eu2+/Eu3+ and FeO/Fe2O3 was found for the terrestrial samples. Calculated and estimated differences in terrestrial and lunar oxygen fugacities appear to be in agreement.

Submarine basalts: some K, Rb, Sr, Ba, rare-earth, H2O, and CO2 data bearing on their alteration, modification by plagioclase, and possible source materials

Abstract Typical sea-water alteration of submarine basalts apparently effects little change in their Sr and rare-earth concentrations, in general, and enrichments or depletions of Ba of up to about 50%. The trace element data indicate that plagioclase has had an important effect in modifying composition. A simple model indicates eclogite or garnet peridotite as the most likely source rocks of alkali and submarine basalts.

Geochemical aspects of some Japanese lavas.

K, Rb, Sr, Ba and rare-earth concentrations in some Japanese lavas have been determined by mass-spectrometric stable-isotope dilution. The samples fall into three rare-earth groups corresponding to tholeiitic, high alumina and alkali basalts. Japanese tholeiites have trace element characteristics similar to those of oceanic ridge tholeiites except for distinctly higher relative concentrations of Ba. Japanese lavas may result from various degrees of partial fusion of amphibole eclogite.

Genesis of the Calcium-Rich a Chondrites in Light of Rare-Earth and Barium Concentrations

Rare-earth and barium concentrations have been determined by mass-spectrometric isotope-dilution for a chondrite composite, nine calcium-rich achondrites, two plagioclase separates, and two pyroxene separates. The brecciated basaltic achondrites have higher (3–17 times) absolute rare-earth and barium concentrations than those in the average chondrite, but similar relative concentrations. Rare-earth and barium data, petrography, and chemistry indicate that the brecciated achondrites cannot be cumulates, to any great extent, of pigeonite or plagioclase, which have fractionated relative rare-earth and barium concentrations. The brecciated achondrites apparently represent liquids, perhaps with different amounts of phenocrystic hypersthene; the closed-system crystallization of the Juvinas achondrite, which is induced from the whole-rock and mineral europium concentrations, strongly supports this interpretation. Hypabyssal or extrusive (rather than plutonic) crystallization is indicated. Measured plagioclase and clinopyroxene partition coefficients for the rare earths and barium place limits on maximum amounts of plagioclase and pigeonite of approximately 20% each that could have been removed from the achondritic magmas.

Europium anomalies and the genesis of basalt

Abstract Rare-earth abundance data for mafic and plagioclase phenocrysts and their igneous matrix materials indicate that Eu anomalies, while caused by the relative stability of divalent Eu, are controlled largely by crystal-chemistry rather than by the redox conditions in the melt. Eu anomalies in plagioclases and pyroxene fractions from the Juvinas achondrite and a Palisade sill diabase are interpreted in terms of closed-system competition. Eu anomalies in minerals from the southern California batholith are interpreted in terms of Eu depletions in the melts owing to the crystallization of feldspar. The lack of Eu anomalies in most basalts suggests that they probably have crystallized much less than about 18% plagioclase prior to extrusion.

Rare-earth and barium abundances in Ivory Coast tektites and rocks from the Bosumtwi Crater area, Ghana

Abstract Abundances of eight rare-earth elements and barium have been determined by isotope dilution in an Ivory Coast tektite composite, an individual Ivory Coast tektite, two impactite glasses and three country-rocks from the Bosumtwi Crater area of Ghana. The rare-earth abundances are lower, in general, than those in previously analyzed tektites from different geographic localities. However, there is some overlap in the data and the differences may not be significant in view of the Limited rare-earth data on tektites. The rare-earth patterns of the meta-sedimentary country-rocks do not exactly match the patterns of the tektites, although there are general similarities. On the other hand, the Bosumtwi Crater impact glasses have rare-earth and barium abundances that are virtually identical to those in the tektites. This identity is viewed as supporting evidence that the Ivory Coast tektites were produced terrestrially by the impact that formed the Bosumtwi Crater.

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Abstract Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Masons classification) of the Allende inclusions. These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the 26 Al- 26 Mg chronometer.