V. Rama Murthy

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle

Nd and Sr isotopic data on pargasite Iherzolite inclusions, kaersutite megacrysts and their host alkali basalts are presented here to clarify some questions regarding isotopic equilibration during mantle metasomatism and the role of metasomatism in basalt genesis. Five alkali basalts from Nunivak Island within the Aleutian back-arc basin, have87Sr/86Sr ratios of 0.70251–0.70330 and143Nd/144Nd ratios of 0.51289–0.51304. On a Nd versus Sr isotope composition diagram the basalts overlap the fields of MORB and ocean island basalts. Pargasites and mica separated from hydrous nodules found in these basalts have a range in87Sr/86Sr of 0.70256–0.70337 but identical143Nd/144Nd ratios of 0.51302. The metasomatic fluid represented by the pargasite is in isotopic equilibrium, both for Sr and Nd, with the dry mantle as represented by diopside. Eight alkali basalts from the Ataq diatreme, South Yemen, have87Sr/86Sr range of 0.70335–0.70426 and143Nd/144Nd range of 0.51252–0.51305. On a Nd versus Sr isotope composition diagram the basalts from Ataq plot in two distinct fields, (1) within the field of ocean island basalts, and (2) within the range of continental rift basalts but to the left of the Nd-Sr correlation line, somewhat similar to the Skye and Oslo rift basalts. Diopside and pargasite separated from three nodules at Ataq have a more complex history than those at Nunivak. Two nodules contain pargasite and diopside with identical87Sr/86Sr ratios but different143Nd/144Nd ratios. A third nodule contains diopside with a143Nd/144Nd ratio similar to that of other diopsides. The Nunivak basalts are derived from a source with a time-integrated light-REE depletion, in contrast to the light-REE-enriched nature of the basanites. This is best explained by a recent metasomatic event in the source region which increased the LIL element content of the peridotite thus accommodating higher degrees of melting. The Ataq volcanic rocks seem to tap different sources characterized by both light-REE enrichment and depletion, in contrast to the uniform source of the Nunivak basanites. Production of the Ataq basanites is believed to involve anataxis of metasomatically veined continental mantle where local mantle heterogeneities survived the melting event.

Early differentiation of the Earth and the problem of mantle siderophile elements: a new approach.

The long-standing problem of the excess abundances of siderophile elements in the mantle can be resolved by considering an equilibrium core-mantle differentiation in the earth at 3000 to 3500 kelvin. This high-temperature differentiation results in mantle siderophile element abundances that closely match the observed values. Some lithophile (light) elements could enter the core in this process as is necessary to account for its low density. The abundances of siderophile elements in the mantle are consistent with the conclusion derived from the recent physical models that the earth was molten during accretion.

Experimental evidence that potassium is a substantial radioactive heat source in planetary cores

The hypothesis that 40K may be a significant radioactive heat source in the Earths core was proposed on theoretical grounds over three decades ago, but experiments have provided only ambiguous and contradictory evidence for the solubility of potassium in iron-rich alloys. The existence of such radioactive heat in the core would have important implications for our understanding of the thermal evolution of the Earth and global processes such as the generation of the geomagnetic field, the core–mantle boundary heat flux and the time of formation of the inner core. Here we provide experimental evidence to show that the ambiguous results obtained from earlier experiments are probably due to previously unrecognized experimental and analytical difficulties. The high-pressure, high-temperature data presented here show conclusively that potassium enters iron sulphide melts in a strongly temperature-dependent fashion and that 40K can serve as a substantial heat source in the cores of the Earth and Mars.

A Nd isotopic study of the Kerguelen Islands: Inferences on enriched oceanic mantle sources

The origin of the highly differentiated igneous rocks of the Kerguelen Islands and the nature of their source regions have been investigated by a Nd isotopic study. The Nd isotopic compositions of syenites and granites are identical to those of gabbros and basalts and indicate a common source. The isotopic data preclude the involvement ofold continental crustal material in the genesis of these granitic and alkalic rocks. The data from the Kerguelen samples greatly extend the Nd-Sr isotopic correlation observed for uncontaminated basalts from the oceanic mantle. The large Nd isotopic variations in the Kerguelen samples could be explained by mixing of deep mantle material brought up by a plume and the upper oceanic mantle or by heterogeneities in the lower mantle. An important finding of this study is that there are enriched mantle sources under the oceanic regions. These enriched sources may be ancient in age and are compatible with the 2-b.y. age inferred from the Pb isotope data of these samples. Earth models in future must incorporate this feature of the oceanic mantle in a consideration of mantle-crust evolutionary relationships.

Distribution of K, Rb, Sr and Ba in some minerals relevant to basalt genesis

Abstract The abundance levels of K, Rb, Sr and Ba in pyroxenes, olivines and garnets of ultramafic rocks are too low to yield a satisfactory composition for an upper mantle composed of these phases. This deficiency can be corrected by incorporation of a combination of plagioclase, hornblende and phlogopite, all of which have been shown previously to be stable in upper mantle environments. Concentrations of these elements in the above minerals, as determined in the present work, have been employed to calculate the abundances in various model mantles. Empirically determined distribution coefficients have then been used to calculate the trace element characteristics of liquids derived by partial melting of these model mantles. Comparison of these calculated partial melts with oceanic tholeiitic and alkali basalts indicates that the presence of 2–5 % hornblende and trace amounts ( 1 2 %) of phlogopite in the upper mantle are necessary for the generation of these basalts. Alkali basalts may be derived through 5–10 % partial melting of this mantle, and 20–30 % melts resemble oceanic tholeiites. Such an upper mantle will satisfy many other geophysical and geochemical requirements.

The chemical composition of the Earth's core: Possibility of sulphur in the core

Abstract An evaluation of the abundances of several volatile trace-elements shows that sulphur is depleted in the Earths mantle and crust by almost two orders of magnitude more than the rare-gases, halogens and water. This deficiency does not appear to be due to a preterrestrial depletion process or volatalization during accretion. It is suggested that the missing sulphur was segregated into the core as part of the lowest melting components in the FeSO system. A feature common to most thermal history models of the Earth is that the temperature of the outermost 100–200 km never exceeds the melting point of the FeFeS eutectic. This leads to the retention of some metallic FeNi and iron sulphide in the upper mantle, which was subsequently oxidized and mixed with the lower depleted mantle. The excess abundances of some chalcophile and siderophile elements in the upper mantle can therefore be attributed to this feature. An important consequence of having sulphur as a major element in the Earths core is that the composition of no single meteorite type satisfactorily resembles the major element composition of an Earth with a FeNiS core and a mantle similar to pyrolite.

Isotopic and trace element composition of the upper mantle beneath a young continental rift: Results from Kilbourne Hole, New Mexico

Abstract Clinopyroxenes (cpx) separated from discrete spinel lherzolite xenoliths from Kilbourne Hole, New Mexico, are compositionally and isotopically heterogeneous. On a Nd-Sr isotope correlation diagram, the cpx plot largely within the mantle array, from near Bulk Earth to depleted MORB values. None of the bulk xenoliths are equivalent to primitive mantle; all have undergone one or more depletion events and some have been enriched in incompatible elements. The lherzolites as well as the cpx show significant variations in incompatible element ratios including Sm/ Nd and Sr/Nd; moreover, 147 Sm 144 Nd and 143 Nd 144 Nd ratios of the cpx are positively correlated and suggestive of an 0.4 Ga fractionation event. Cpx from spinel pyroxenite dikes and lherzolite wallrocks of composite xenoliths are relatively homogeneous (isotopically) and similar to OIB and some MORB. The wallrocks are isotopically equilibrated with the pyroxenites or nearly so, and have negative Nd model ages; we infer that the pyroxenite-forming event caused enrichment in incompatible elements in the contiguous wallrock. The pyroxenite parent magma was probably a primitive basanite characterized by low Hf/Sm and Ti/Sm ratios relative to primitive mantle as a consequence of residual garnet. Our data confirm the presence of a MORB-related component in the mantle beneath Kilbourne Hole. This component is fertile with respect to basaltic constituents, is relatively LREE-depleted, and is isotopically similar to MORB; it is probably derived from asthenosphere. The isotopic heterogeneity of the discrete lherzolites requires a second, enriched component characterized by relatively low 143 Nd 144 Nd and high 87 Sr 86 Sr ratios and unsupported 143 Nd 144 Nd ratios. Old but disparate Sr and Nd model ages require that MORB-related spinel lherzolites have had a complicated history and differentiated from primitive mantle more than 1 b.y. ago. Two plausible models, one involving more than one depletion event and the second involving mixing of mantle components depleted at distinct times, can explain the common observation that Sr model ages are older than Nd model ages.

The early chemical history of the earth: Some critical elemental fractionations

Thermodynamic data for reactions relevant to the upper parts of the earth satisfactorily explain the observed geochemical behavior of trace elements. Reactions in a primitive earth undergoing core formation through FeS liquid segregation indicate that the alkali elements K, Rb and Cs, in contrast to their surficial behavior, will be strongly chalcophile and thus withdrawn into the core. Chondritic models for the earth need not be discarded because of the apparent depletion of the alkalies from the silicate fraction of the earth. Segregation of K into the core will decouple the heat production of40K from that of U and Thearly in the earths history. The subsequent thermal and differentiation history would contrast markedly with that obtained in presently existing models which employ a coherent relationship between K and U in the earth. The present model can satisfactorily explain the evolution of the radiogenic isotopes of Pb and Sr in the earth, in addition to providing sufficient40K heat energy in the core to account for the geomagnetic field.

Core formation and chemical equilibrium in the Earth—I. Physical considerations

Current models of planetary formation suggest a hierarchy in the size of planetesimals from which planets were formed, causing formation of a hot magma ocean through which metal-silicate separation (core formation) may have occurred. We analyze chemical equilibrium during metal-silicate separation and show that the size of iron as well as the thermodynamic conditions of equilibrium plays a key role in determining the chemistry of the mantle (silicates) and core (iron) after core formation. A fluid dynamical analysis shows that the hydrodynamically stable size of iron droplets is less than ∼ 10−2 m for which both chemical and thermal equilibrium should have been established during the separation from the surrounding silicate magma. However, iron may have been separated from silicates as larger bodies when accumulation of iron on rheological boundaries and resultant large scale gravitational instability occurred or when the core of colliding planetesimals directly plunged into the pre-existing core. In these cases, iron to form the core will be chemically in dis-equilibrium with surrounding silicates during separation. The relative role of equilibrium and dis-equilibrium separation has been examined taking into account of the effects of rheological structure of a growing earth that contains a completely molten near surface layer followed by a partially molten deep magma ocean and finally a solid innermost proto-nucleus. We show that the separation of iron through a completely molten magma ocean likely occurred with iron droplets assuming a hydrodynamically stable size (∼ 10−2 m) at chemical equilibrium, but the sinking iron droplets are likely to have been accumulated on top of the partially molten layer to form a layer (or a lake) of molten iron which sank to deeper portions as a larger droplet. The degree of chemical equilibrium during this process is determined by the size of droplets which is in turn controlled by the size and frequency of accreting planetesimals and the rheological properties of silicate matrix. For a plausible range of parameters, most of the iron that formed the core is likely to have been separated as large droplets or bodies and chemical equilibrium with silicate occurred only at relatively low temperatures and pressures in a shallow magma ocean or in their parental bodies. However, a small portion of iron that separated as small droplets was in chemical equilibrium with silicate at high temperatures and pressures in a deep magma ocean during the later stage of core formation. Therefore the chemistry of the core is mostly controlled by the chemical equilibrium with silicates at relatively low temperatures and pressures, whereas the chemistry of the mantle controlled by the interaction with iron during core formation is likely to have been determined mostly by the chemical equilibrium with a small amount of iron at high temperatures and pressures.

Trace element geochemistry of Archean volcanic rocks

Abstract K, Rb, Sr, Ba and rare earth elements of some Archean volcanic rocks from the Vermilion greenstone belt, northeast Minnesota, were determined by the isotopic dilution method. The characteristics of trace element abundances, supported by the field occurrences and major element chemistry, suggest that these volcanic rocks were formed in an ancient island arc system. A felsic rock is suggested to be derived by partial melting of a basaltic source, presumably in an ancient subduction zone. It is well known that the distribution coefficients (liquid/source) for the above trace elements are almost invariably greater than one. Continuous extraction of volcanic liquids from the upper mantle through geologic time would result in depletion of these elements in the upper mantle. However, all trace element abundances in many Archean volcanic rocks are almost identical to their modern equivalents. This gross constancy of trace element concentration in rocks of different geologic age raises some important questions as to the evolution of the upper mantle. It is proposed that the trace elements have been repeatedly and fully recycled in a restrictive and closed system of crust and upper mantle during the last three billion years (recycled mantle), or the trace elements have been replenished from the lower part of the mantle by some undefined process (replenished mantle). It is believed that interplay of both recycling and replenishment have been responsible for crust-mantle evolution in geological history.