Large Ion Lithophile Element Abundances Research Articles

Geochemical and palaeomagnetic characteristics of the Vestfold Hills mafic dykes in the Prydz Bay region: implications of a Paleoproterozoic connection between East Antarctica and Proto-India

AbstractThe Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts north–south-, east–west-, NE–SW- and NW–SE-trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The orientations of dykes do not show a direct correlation with the dyke geochemistry. Instead the dykes can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type is more siliceous, large ion lithophile elements (LILEs), high field strength elements (HFSEs) and light REEs enriched crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILEs and REEs, that indicates derivation from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly island arc basalt (IAB) affinity. Palaeomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in the Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic palaeolatitudes.

Petrogenesis, Ore Mineralogy, and Fluid Inclusion Studies of the Tagu Sn–W Deposit, Myeik, Southern Myanmar

Most of the granite-related Sn–W deposits in Myanmar are located in the Western Granitoid Province (WGP) of Southeast Asia. The Tagu deposit in the southern part of the WGP is a granite related Sn–W deposit. The biotite granite is composed of quartz, feldspars (plagioclase, orthoclase, and microcline), and micas (muscovite and biotite) and belongs to S-type peraluminous granite. Abundances of large-ion lithophile elements (LILEs), such as Rb, K, and Pb, coupled with the deficiency of high-field-strength elements (HFSEs), such as Nb, P, and Ti, indicate that the parental magma for the Tagu granite was derived from the lower continental crust at syn-collisional setting. Mineralized veins consist of early-formed oxide ore minerals, such as cassiterite and wolframite, which were followed by the formation of sulfide minerals. Three main types of fluid inclusions were distinguished from the mineralized quartz veins hosted by granite and metasedimentary rocks: Type-A—two phases, liquid (L) + vapor (V) aqueous inclusions; Type-B—two phases, vapor (V) + liquid (L) vapor-rich inclusions; And type-C—three phases, liquid + CO2-liquid + CO2-vapor inclusions. Quartz in the veins hosted in granite corresponding with earlier deposition contains type-A, type-B, and type-C fluid inclusions, whereas that in the veins hosted in metasedimentary rocks corresponding with later deposition contains only type-A fluid inclusions. The homogenization temperatures of type-A inclusions range from 140 °C to 330 °C (mode at 230 °C), with corresponding salinities from 1.1 wt.% to 8.9 wt.% NaCl equivalent for quartz veins hosted in metasedimentary rocks, and from 230 °C to 370 °C (mode at 280 °C), with corresponding salinities from 2.9 wt.% to 10.6 wt.% NaCl equivalents for quartz veins hosted in granite. The homogenization temperatures of type-B vapor-rich inclusions in quartz veins in granite range from 310 °C to 390 °C (mode at 350 °C), with corresponding salinities from 6.7 wt.% to 12.2 wt.% NaCl equivalent. The homogenization temperatures of type-C H2O–CO2–NaCl inclusions vary from 270 °C to 405 °C (mode at 330 °C), with corresponding salinities from 1.8 wt.% to 5.6 wt.% NaCl equivalent. The original ascending ore fluid was probably CO2-bearing fluid which evolved into two phase fluid by immiscibility due to pressure drop in the mineralization channels. Furthermore, the temperature and salinities of two-phase aqueous fluids were later most likely decreased by the mixing with meteoric water. The salinities of the type-B vapor-rich inclusions are higher than those of the type-C CO2-rich inclusions, which may have resulted from CO2 separation from the fluids. The escape of gases can lead to an increase in the salinity of the residual fluids. Therefore, the main ore-forming mechanisms of the Tagu Sn–W deposit are characterized by fluid immiscibility during an early stage, and fluid mixing with meteoric water in the late stage at a lower temperature.

Back-arc basin development: Constraints on geochronology and geochemistry of arc-like and OIB-like basalts in the Central Qilian block (Northwest China)

The Lajishan belt of the Central Qilian block was a back-arc basin during Early Paleozoic. The basaltic magmatism and temporal evolution in this basin provide an opportunity to study the development of back-arc basin in an active continental margin. In this study, we carry out an integrated study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic arc-like and OIB-like basalts. The Lajishan arc-like basalts are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies whereas the OIB-like basalts have high LILE abundances and show positive Nb and Ta anomalies. The arc-like basalts have initial 87Sr/86Sr values of 0.7050–0.7054 and εNd(t) values of +0.51–+2.63, and the OIB-like basalts have initial 87Sr/86Sr values of 0.7049–0.7050 and εNd(t) values of +0.66–+1.57. The geochemical and Sr-Nd isotopic compositions suggest that the arc-like basalts are derived from partial melting of a depleted mantle source metasomatized by slab-derived components at shallow depth levels, and the OIB-like basalts also originated from a metasomatized mantle wedge source. U-Pb zircon dating yielded the ages of ~494 ± 4 Ma for the arc-like basalts and ~468 ± 6 Ma for the OIB-like basalts. We argue that the arc-like basalts are products of back-arc extension before the back-arc rifting initiated in earlier stage, resulting from the northward subduction of the Qaidam-West Qinling oceanic slab, while the OIB-like basalts represent products of further back-arc spreading in response to rollback of the Qaidam-West Qinling oceanic lithospheric slab. The association of arc-like and OIB-like basalts in the Lajishan belt records the development of back-arc basin from initial rifting to subsequent spreading, offering insight into how basaltic magmatism generates in the formation of back-arc basin in subduction zone setting.

Volcanism from fissure zones and the Caldeira central volcano of Faial Island, Azores archipelago: geochemical processes in multiple feeding systems

AbstractMagmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian^Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian^Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc.To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr^Nd^ Hf^Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankaramites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subductingWoodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Yup to c. 200), and high-Mg andesites (MgO45 wt %, Sr/Y c. 11^46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. Sr/Sr and eNd values along the arc range from 0 7029 to 0 7052 and from þ5 8 to þ8 3, respectively.The Sr^Nd values partially overlap the compositions of oceanic basalts from the Indian^Australian plate. Measured eHf values range from þ10 5 to þ14 6. If corrected for contributions from subduction components, combined eHf^eNd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display eHf^eNd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (46 Myr old) and (2) from the more recently subducted Indian^ Australian plate. Combined Hf^Nd^Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.

Geochemical and Sr–Nd–Pb isotopic characteristics of Paleocene plagioleucitites from the Eastern Pontides (NE Turkey)

In the southern zone of the Eastern Pontides, an isolated small (~ 1.5 km 2) outcrop of plagioleucitites occurs in the Aşutka thrust sheet. Field relations constrain the timing of volcanism between Maastrichtian and late Paleocene. The plagioleucitites consist of clinopyroxene, analcime (former leucite), Ti-magnetite, plagioclase, sanidine, apatite and accessory biotite. The rocks are represented by three geochemical and petrographic varieties: Types I and II are mostly blocks within epiclastic debris and type III are lava flows. All the rocks are considerably altered, whereby leucite is almost totally analcimized, apart from leucite inclusions within clinopyroxene that are remote from cracks. Furthermore, calcite and chlorite/smectite have been formed. Chemical effects of alteration are primarily reflected in an unsystematic variation of whole-rock K 2O/Na 2O ratios (0.12 to 1.71) and of the abundances of large-ion lithophile elements, such as Cs and K, and perhaps also Rb. Both the high modal abundance of (former) leucite and the composition of clinopyroxene indicate an ultrapotassic nature of the primary rocks. In primitive mantle-normalized element concentration diagrams, all samples are characterized by negative anomalies of Nb–Ta, Zr–Hf and Ti and a positive anomaly in Pb, testifying the orogenic nature of the volcanics. The rocks cover restricted ranges in all initial isotope ratios with 87Sr/ 86Sr(i) (60 Ma) ranging from 0.70537 to 0.70568, 143Nd/ 144Nd(i) from 0.512529 to 0.512585, 206Pb/ 204Pb(i) from 18.65 to 18.83, 207Pb/ 204Pb(i) from 15.65 to 15.66 and 208Pb/ 204Pb from 38.64 to 38.88. There is no obvious relationship between the degree of analcimization and the isotopic composition. In Pb–Pb variation diagrams, the rocks plot above the Northern Hemisphere Reference Line (NHRL). Chondrite-normalized rare earth element (REE) patterns show no significant Eu anomaly, but a strong enrichment of the LREE over the HREE with (La/Yb) cn = 11.6–14.2, whereby normalized concentrations of Er to Lu are nearly similar. The Everek Hanları plagioleucitites represent the youngest products of Cretaceous to Paleocene arc magmatism of the Eastern Pontides, thus documenting the last stages of the Neotethyan subduction.

Petrotectonic evolution and melt modeling of the Penon Blanco arc, central Sierra Nevada foothills, California

The Penon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Penon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Penon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Penon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Penon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Penon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Penon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Penon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Penon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions

Hills, Wyoming, USA. The two main groups of lamproites, madupitic lamproites and phlogopite lamproites, are geochemically distinct and cannot be related to one another by either fractional crystallization or crustal contamination. It seems likely that the geochemical differences between these two rock types are related to variations in source mineralogy and depth of partial melting. The high Mg-number and large ion lithophile element abundances and negative eNd values of the lamproites indicate a mantle source that has experienced stages of both depletion and enrichment. The negative Nb, Ta and Ti anomalies in mantle-normalized trace element diagrams and low time-integrated U/Pb, Rb/Sr and Sm/ Nd ratios of both lamproite groups and other Cenozoic igneous rocks from the Wyoming Archean Province indicate an ancient metasomatic enrichment (>1.0 Ga) of the mantle source associated with the subduction of carbonate-bearing sediments. Other chemical characteristics of the Leucite Hills lamproites, especially their high K2O and volatile contents, are attributed to more recent metasomatism (<100 Ma) involving influx from upwelling mantle during back-arc extension or plume activity.

Petrology and Geochemistry of Boninite-Series Volcanic Rocks, Chichi-Jima, Bonin Islands, Japan

An Eocene submarine boninite-series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types—boninite, bronzite andesite, dacite, quartz dacite, and rhyolite—were distinguished within the boninite volcanic sequence on the basis of petrography and geochemistry. Boninites contain high Mg, Ni, and Cr contents indicative of primitive melts, but have high Si contents relative to other mantle-derived magmas. All boninite-series rocks contain very low incompatible-element concentrations, and concentrations of high-field-strength elements in primitive boninites are less than half of those found in depleted mid-ocean ridge basalts. Abundances of large-ion lithophile elements are relatively high in boninite-series rocks, similar to the enrichments observed in many island-arc lavas. Trends for both major- and trace-element data suggest that the more evolved members of the boninite magma series were derived primarily through high-level fractional crystallization of boninite. Textural features, such as resorption and glomeroporphyrocrysts, and reverse chemical zonations suggest that magma mixing contributed to the development of the quartz dacites.

Timing of Magmatism following Initial Convergence at a Passive Margin, Southwestern U.S. Cordillera, and Ages of Lower Crustal Magma Sources

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali‐calcic Fe‐ and Sr‐enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal‐miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo‐Triassic intrusion ages, and a prolonged postemplacement thermal history within the long‐lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc.

Petrogenesis of two groups of pyroxenite from Tungchihsu, Penghu Islands, Taiwan Strait: implications for mantle metasomatism beneath SE China

Type II pyroxenite xenoliths are a common form of mantle material found in Miocene basaltic pyroclastic rocks at Tungchihsu in the Penghu Islands, Taiwan Strait. Two groups are identified from mineralogy, petrology and chemistry: Group I have mineral assemblages of clinopyroxene+amphibole (mosaic, generally kaersutite)±apatite±Ti–Fe oxide mineral±iron sulfide. These xenoliths are characterized by high abundances of large-ion lithophile elements (including Ba, K, Rb, Sr and Th), high field strength elements (Zr, Nb and Ti), and light rare-earth elements. Chemical compositions of this group of pyroxenite resemble those of Penghu alkali basalts. Likewise, the Sr–Nd isotopic compositions ( 87Sr/ 86Sr=0.70370–0.70385; 143Nd/ 144Nd=0.51285–0.51295) of clinopyroxene, amphibole and apatite separates from this group falls within the field for late Cenozoic Penghu basaltic rocks and megacrysts, and it is suggested that this group of pyroxenite was formed by the crystallization of alkali basaltic magma in the upper mantle. The second group pyroxenites (group II) contain clinopyroxene, garnet, spinel, amphibole (interstitial, generally pargasite/hastingsite) and/or orthopyroxene and a Ti–Fe oxide mineral. They have low Na 2O, K 2O, TiO 2, LREE, and high Al 2O 3 relative to group I pyroxenites, and compositionally are similar to tholeiitic picrites. From major- and trace elements evidence, we suggest that the second group of pyroxenites may have formed by crystallization of tholeiitic picritic magmas under high pressure. Isotopic differences between clinopyroxene, amphibole and garnet or apatite separates from the two groups (group II: 87Sr/ 86Sr=0.70460–0.70640; 143Nd/ 144Nd=0.51269–0.51290) indicate the two groups of pyroxenite are not related. The coexisting mineral assemblages (clinopyroxene, amphibole and garnet) of group II pyroxenite are not in isotopic equilibrium suggesting that the minerals crystallised from different melt phases, possibly in a magma chamber undergoing fractionation and recharge or by progressive crystallisation in a vein system. The pyroxenite thus formed was then subsequently fragmented and/or re-cemented by a subsequent volatile bearing-melt that crystallised the interstitial amphibole.

Chemical fluxes and wedge replenishment rates along recent destructive plate margins

The large ion lithophile element (LILE) contents of island-arc tholeiites are dominated by the contribution from the subducted slab, whereas those of continental margin rocks are dominated by that from the mantle wedge. It is therefore argued that the LILE abundances in the destructive-margin rocks that are most depleted in high field strength elements offer the best indication of the LILE flux from the subducted slab, and that the slab contributes ∼25% of the Sr in average new crust. The remainder is derived from the mantle wedge, which is the principal site of crust generation and therefore must be replenished about every 75 m.y. Sr isotope arguments suggest that <5% of the Sr in new crust is from recycled continental crust with average crustal 87Sr/86Sr ratios. In the model outlined here, ≤5% of the Rb, Ba, and K in new crust is from the subducted slab, which is in contrast to previous suggestions that as much as 90% of the K in new crust might be from recycled continental material. As little as 3% of the Sr in subducted crust is required for the slab-derived flux, which further implies that most of the LILE in oceanic crust are recycled deep into the upper mantle.

The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.

Lead isotopic compositions of Andean igneous rocks, latitudes 26 degrees to 29 degrees S; petrologic and metallogenic implications

Lead isotopic data are reported for 12 whole rock samples of Mesozoic to Quaternary plutonic and volcanic rocks from an intensely mineralized transect of the ensialic Central Andean orogen. In contrast to the initial Sr isotopic ratios and large ion lithophile element abundances of these rocks, the Pb isotope ratios show no trend either with decreasing age or with distance from the continental margin.The ratios 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are significantly higher than those of oceanic tholeiite and alkali basalt from the Nazca plate, and we conclude that the lead cannot have been derived from subducted, metamorphosed basalt. However, there is extensive compositional overlap with the Pb isotopic ratios of ocean floor sediments and manganese nodules from the southeast Pacific.Considered in conjunction with the Sr isotope ratios and rare earth elements (and other large ion lithophile element) abundances of these rocks the new lead data are interpreted in terms of a two-stage magma generation model in which, though lead and strontium were derived from layers 1 and 2, respectively, of subducted oceanic lithosphere, the immediate source of the magmas was in the continental mantle peridotite which had become enriched in large ion lithophile elements through interaction with melts or waters rising from the subduction zone. It should be stressed, however, that the Pb isotopic ratios are not incompatible with a source in the sialic crust, although the wholesale anatexis of crust is here a very improbable mechanism for magma generation. Contributions from the continental crust may have occurred directly, through selective transfer to the invading magmas or, more remotely, following subduction and partial melting of material abraded from the leading edge of the continent.The relatively consistent nature of the rock lead compositions is paralleled by the broadly homogeneous metallogenic aspect of this transect, in which copper deposits almost everywhere predominate, and in which the longitudinal metal zonation, considered a characteristic of Andean-type orogens, is not developed. If rock and ore leads, and the economically important metals (Cu, Mo, Ag, Au), are cogenetic, arguments similar to those advanced to constrain the origin of the parent igneous rocks may be applied to delimit the source of the ore metals. Thus, metalliferous sediments such as those of the Nazca plate are improbable sources, whereas the metal-rich pelagic sediments and Mn nodules are promising candidates. However, the ore metals may have been derived from the deeper continental mantle or the sialic crust.

Atlantic ocean floor: Geochemistry and petrology of basalts from legs 2 and 3 of the Deep-Sea Drilling Project

Basalts cored on legs 2 and 3 of the Deep-Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr. Although many of the basalts are highly altered, fresh glass is usually present. Except for site 2–10 the fresh glasses are petrographically and geochemically similar to mid-Atlantic ridge (MAR) axial basalts. There are no systematic compositional differences as a function of distance from the MAR axis. Two sites contain basalts with olivine (Fo90) phenocrysts, high Mg/Mg + ΣFe, high Ni and Cr abundances, and very low large ion lithophile (LIL) element abundances. These basalts are the best candidates for primary magma recovered from the sea floor; fractional crystallization of such basalt may yield the more evolved basalts typical of the MAR. More fractionated basalts with clinopyroxene phenocrysts occur at twp other sites, but they retain low LIL element abundances. Site 2-10 contains titaniferous augite and is relatively enriched in LIL elements. It is unlikely that this basalt was derived by fractional crystallization from LIL element depleted tholeiites; instead, the site 2-10 basalt requires a different mantle source. These results imply that the upper Atlantic Ocean basement is dominantly LIL element depleted tholeiite.

The chemical composition of soil from the Apollo 16 and Luna 20 sites

The concentrations of the rare earth elements (REE), K, Rb, Sr, Ba, U, Zr and Cr for the Luna 20 soil and four different Apollo 16 soils are reported. These trace element abundances imply: (1) that the lunar highlands consist of a mixture of rocks rich in large ion lithophile (LIL) elements and LIL-element impoverished anorthosites; or (2) that the bulk of the aluminum-rich crust did not originate by upward segregation of plagioclase in a primitive liquid shell. The Luna 20 soil is distinguished from the Apollo 16 soil by lower aluminum and LIL element abundances.