Ocean Floor Basalts Research Articles

Fragments of the mantle and crust from beneath the Sierra Nevada batholith: Xenoliths in a volcanic pipe near Big Creek, California

A Miocene volcanic pipe that cuts Cretaceous granodiorite is exposed in the central Sierra Nevada near the town of Big Creek, California. The volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent, resulting in a pseudo-andesitic composition. The conduit cut through the entire thickness of the Sierra Nevada batholith, as well as the underlying lower crust and upper mantle, and the magma entrained a diverse, polygenetic assemblage of igneous and metamorphic xenoliths, including peridotites, eclogites and granulites, schists and hornfels, and gabbroids and granitoids. Some xenoliths of the high-grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the Sierra Nevada. Garnet granulites, the most abundant rock type of the mafic high-grade metamorphic assemblage, and eclogites are restricted to the central area of the pipe. These xenoliths are essentially bimineralic, consisting of colorless to pale green clinopyroxene and garnet; eclogites contain brilliant emerald green clinopyroxene. The clinopyroxenes, which have a broad range of compostions [Mg/ (Mg + Fe) = 0.45-0.90], have a >20% jadeite component in eclogites. Garnets also have a range of compositions [Mg/(Mg + Fe) = 0.30-0.70], with those from eclogites being the most Fe rich. One of the xenoliths contains Ca-rich garnet (grs + adr > 90%). Feldspathic granulites contain plagioclase of An 45-70 , and some lack garnet. Hornblende occurs in some granulites, and a few amphibolites are present. Likely protoliths are ocean-floor basalts for the eclogites and Precambrian limestone for the grossular-clinopyroxene rock. The Big Creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western Sierra Nevada.

Geochemistry, mineralogy, and petrogenesis of basalt from the Gorda Ridge

Basalt pillow lava with glassy rims was dredged from 17 sites along the Gorda Ridge between latitudes 43°N and 40.8°N. All samples are low‐K2O mid‐ocean ridge tholeiitic basalt having a narrow compositional range with Mg numbers clustered around 60–62; more primitive and evolved compositions are present but rare. None of the more fractionated samples is as evolved as ferrobasalt from the Juan de Fuca Ridge. Incompatible trace element ratios are typical of normal mid‐ocean ridge basalt but show considerable scatter. The 87Sr/86Sr ratios, ranging from 0.70233 to 0.70267, are near the low end of the range for normal mid‐ocean ridge basalt. The phenocrysts present are abundant plagioclase, less abundant olivine ± spinel, and traces of sulfide. Clinopyroxene is absent in the glasses. The mineral chemistry is typical of ocean floor basalt. However, unusually anorthitic plagioclase in some highly plagioclasephyric samples, complex zoning in plagioclase and olivine, and other disequilibrium features indicate complex petrogenetic processes involving magma mixing. Glass inclusions in olivine and plagioclase indicate primitive trapped melts. The chemical diversity of Gorda Ridge lava appears to have been primarily caused by variable degrees of partial melting, but crystal fractionation and magma mixing were also important processes. Compositional variations along the strike of the ridge indicate that lava from the segment south of the offset at latitude 41.6°N was generated by the smallest percentage of partial melting of a source less depleted than that of the two northern segments. Lava from the central and northern segments was generated by variable percentages of partial melting of a uniformly depleted source. Lava from the northern segment shows the greatest diversity with a large range in percentage of melting and both relatively primitive and evolved compositions in spatially and temporally closely associated lava. The diversity of lava from the northern segment may result from thermal perturbations caused by proximity to the Blanco Fracture Zone. Gorda Ridge lava is more similar to that from the Mid‐Atlantic Ridge at 22°–25°N than to that from other spreading centers in the Pacific Ocean. Magma chambers under the Gorda Ridge appear to be small, discontinuous in time and space and possibly at greater depth than those beneath other Pacific spreading centers.

The genesis of refractory melts in the formation of oceanic crust

Refractory, primary liquids arising in various oceanic plate tectonic settings are characterized by high MgO, SiO2, Ca/Na, low TiO2 and generally low incompatible element abundances relative to primary liquids parental to MORB. We propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to MORB. A compositional range in the ‘second-stage’ melts is expected, depending on the extent of previous depletion of the peridotite, the temperature and pressure of melt segregation, and the possible influence of volatile phases (C-H-O) present during melting. An example of a second stage melt is of magnesian quartz tholeiite composition, identified from among the Upper Pillow Lavas, Troodos ophiolite, Cyprus. Experimental studies determine that this composition has appropriate liquidus phases to have segregated from depleted upper mantle peridotite at about 25 km, 1360° C leaving a harzburgite residue. The experimental studies are applied to interpretation of cooling histories and water contents of specific Upper Pillow Lavas. Magma batches are estimated to have contained 0.5–1.0% H2O. Picritic lavas quenched from olivine +liquid at <5 kb. Magnesian, pyroxene-phyric lavas exhibit intratelluric crystallization at ∼5 kb, 1270° C (Mg88 pigeonite and Mg89 orthopyroxene). These and other second-stage melts will crystallize extremely refractory minerals identical to many found in cumulate sequences in ophiolites, in plutonic rocks dredged and drilled from ocean basins, and occurring as xenocrysts in ocean floor basalts. Multistage melting of upper mantle peridotite, with and without presence of water, reconciles some of the present difficulties in relating ophiolite and ocean floor basalt compositions, and is an important process in ocean crust formation in a variety of different oceanic settings (mid-ocean ridges, marginal basins, and island arcs).

Observation of shrinkage cracks in ocean floor titanomagnetites

A characteristic of low-temperature ocean floor titanomagnetite oxidation is the formation of irregular, curved cracks, which are caused by shrinkage of the spinel lattice. The spatial distribution of the shrinkage cracks within individual titanomagnetite grains, magnetically extracted from different ocean floor basalts, was studied by Scanning Electron Microscopy. The cracks do not develop in grains smaller than approximately 5 microns in diameter. Under ocean floor conditions the critical titanomagnetite oxidation state z for the onset of cracking is between z = 0.23 and z = 0.37. Shortly before the onset of cracking internal stress reaches a maximum, which if reflected in the magnetic coercivity of the material.

O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs

Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ 18O = +5.7 to +6.6 (‰ SMOW) andδ 34S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ 18O = +6.3 ± 0.2 are slightly enriched in 18O with respect to unfractionated basalts of<53%SiO 2 with meanδ 18O = +6.0 ± 0.1. This 18O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ 18O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ 18O SiO 2 trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO 2; therefore, the meanδ 34S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region. The enrichment of 18O and 34S in Mariana Arc parental magmas relative to ocean floor basalts withδ 18O ca. + 5.7‰ andδ 34S = ca.0.3‰ is attributed to the recycling of 18O- and 34S-rich crustal components (sediment withδ 18O = ca. + 25‰ and seawater sulfate withδ 34S = ca. +20‰ into the upper mantle source region for these arc magmas. This interpretation is consistent with enrichments of radiogenic Sr and Pb in the same lavas relative to ocean-floor basalts erupted either side of the arc, which are presumed to share a common upper mantle source. This enrichment is considered to reflect the mixing of two components, one having a typical upper mantle composition and the other having a more radiogenic character similar to that of western Pacific pelagic sediments.

Geochemistry, Petrogenesis and Tectonic Significance of the Early Proterozoic Loch Maree Group Amphibolites of the Lewisian Complex, NW Scotland

Summary The early Proterozoic Loch Maree Group (LMG) is a supracrustal sequence occupying two belts, 5 km and 3.5 km wide respectively, at Gairloch and Loch Maree in the southern region of the Scottish Lewisian Complex. The LMG at Gairloch consists of about one-third metasediments, mainly metagreywackes, and two-thirds amphibolite. The LMG amphibolites are of igneous origin, possess a tholeiitic chemistry, and can be divided into two petrogenetic groups. Group A consists of the main amphibolite sheets, believed to represent lavas, which show little or no LREE enrichment and flat incompatible element profiles, with some LILE depletion. Their chemical signatures are similar to those of ocean-floor (MORB) basalts. Group B consists of thin amphibolites and biotite-hornblende schists, believed to be sills, which show LREE and LILE enrichment trends quite different from group A and more similar to those of the well-documented Lewisian Scourie dykes. It is suggested that the voluminous group A amphibolites represent primitive basalts extruded rapidly, directly from the mantle, after a period of extensional rifting had caused significant crustal thinning. Subsequently, after the cessation of extension and the filling of the rift basin, later magmas, represented by the group B amphibolites, became subject to crustal contamination before being emplaced as sills within the supracrustal pile. An early, pre-rifting stage of extension is represented by the Scourie dykes, which possess normal continental basalt geochemistry.

Discrimination among tectonic settings using trace element abundances of basalts

A wealth of chemical information may provide a false sense of security to the petrologist interested in extracting petrogenetic information from a set of major and/or trace element abundances. Such data are said to be closed, as each analysis sums to a constant (100%). Failure to sum to a constant results from a combination of errors of commission and omission. A correlation coefficient between a pair of closed components has a contribution from the linear association between the components and a contribution from the effects of closure itself. Standard statistical procedures cannot separate these two sources; therefore, the investigator cannot tell if a strong correlation (as revealed by a nearly linear trend on a binary scatter diagram, for example, is due to a strong linear association between the components or due to closure. Techniques developed by Aitchison (1984a, b) appear to be capable of providing a framework within which the user can begin to assess the relationship among closed components. These techniques are applied to a set of 35 TiO2, Zr, Y, and Sr analyses of basalts. A statistical analysis permits rejection of Aitchison's (1984a, b) hypothesis of complete subcompositional independence, indicating that there is a degree of dependency within the data set. The first two principal components extracted from the covariance matrix of the log‐centered form of these data account for more than 90% of the total variation, and three major tectonic‐related fields can be clearly recognized in the space defined by the first two principal components: ocean floor basalts, within‐plate basalts and arc‐related basalts. Analyses from six additional data sets taken from the literature were plotted on this diagram, and all reclaim the tectonic settings stated in the literature. Simple modeling reveals that the addition or subtraction of Sr from an analysis results in a linear locus of points which is parallel to the boundary between the ocean floor and the within‐plate basalts, suggesting that such a diagram may prove useful in the analysis of basalts that have been subjected to low‐ to moderate‐grade metamorphism/metasomatism.

The origin of the fractionation of platinum-group elements in terrestrial magmas

The platinum-group elements (PGE's), when chondrite normalized, have been found to be fractionated in order of descending melting point (Os, Ir, Ru, Rh, Pt, Pd and Au). Mantle-derived material (garnet lherzolite and spinel lherzolite xenoliths and alphine peridotites) have essentially unfractionated PGE patterns. Periotitic komatiites have mildly fractionated patterns ( Pd Ir = 10 ), pyroxenitic komatiites are slightly more fractionated ( Pd Ir = 30 ). Both continental and ocean-floor basalts are highly fractionated ( Pd Ir = 100 ). Data from intrusive rocks show a large range in PGE fractionation from Pd-depleted chromities of ophiolites ( Pd Ir = 0.1 ) to the extreme Pd enrichment in the JM Reef of the Stillwater Complex ( Pd Ir = 865 ). Some possible mechanisms for the origin of this fractionation are: alteration, partial melting and crystal fractionation. Carbonate alteration affects Au and Pt and hydrothermal alteration mobilizes Pd. Solid substitution of Ir (and associated Os and Ru) into olivine and chromite, during crystal fractionation or partial melting is rejected as a mechanism of fractionating the PGE's. It is suggested; that the major factor in PGE fractionation is the differences in solubility of the PGE's in a silicate magma, that Pd, Pt and Rh are more soluble than Os and Ir, which form an alloy and Ru which forms laurite. These differences in PGE solubility could fractionate the PGE's during partial melting or crystal fractionation. During crystal fractionation prior to Fe-Ni-Cu sulphur saturation the low solubility of Os, Ir and Ru leads to the formation of Os-Ir alloys and RuS 2 in the magma. These may then be settled out of the magma by whatever phase is crystallizing and the remaining magma becomes fractionated in PGE's.

Pb, Sr and 10Be isotopic studies of volcanic rocks from the Northern Mariana Islands. Implications for magma genesis and crustal recycling in the Western Pacific

We report Sr and Pb isotope analyses for an extensive suite of volcanic rocks from the N. Mariana arc together with Sr and Pb isotope analyses of sediments from the nearby Mariana and Nauru basins. In addition ten of the most recent volcanic samples were analysed for 10Be. The Sr isotope compositions cluster tightly around 87Sr 86Sr = 0.7035 being slightly but significantly higher than the Pacific ocean floor basalts on either side of the arc and agreeing well with previous data. In contrast, the large number of new Pb isotopic data presented significantly extends the observed range of Pb isotope compositions for volcanic rocks from the Mariana arc to more radiogenic compositions. The concentrations of 10Be were very low (< 0.5 × 10 6 atom g −1). These new data require either that the Pb and Sr isotopic compositions of the Mariana sub-arc mantle be substantially different from those of the mantle source of ocean floor basalts on either side of the arc, or that the enrichment in radiogenic Pb and Sr relative to the values observed in Pacific ocean floor basalts be related to the subduction process. We prefer the latter hypothesis in which radiogenic Sr and Pb in ocean floor sediments are added to M.O.R.B. type mantle either by direct assimilation of the sediments in partial melts or, more probably, by transfer in a fluid phase into the zone of magma production. The low 10Be concentrations observed suggest the removal of at least the top few metres of sediment during subduction.

Petrology of the amphibolitized eclogites of Gorduno, Lepontine Alps, Switzerland

The amphibolite-facies gneisses outcropping west of Bellinzona, from Gorduno up to Alpe Arami, include boudins and layers of amphibolite with relics of eclogite. Well-preserved eclogites crop out at the margin of the garnet—peridotite of Alpe Arami and are included within the chlorite—peridotite bordering it. The chemical compositions of eclogites and relic-bearing amphibolites are so similar as to suggest isochemical retrograde metamorphism. However, a minor change can be traced, because the chemical affinity slowly trends from clearly tholeiitic to alkali-basaltic, while the trace elements still suggest derivation from “ocean-floor basalts”. Garnets in the eclogites are homogeneous almandine—pyrope (Alm 41Pyr 36Gro 23) but with increasing amphibolitization they become rimmed by more ferrous compositions (Alm 53Pyr 27Gro 20). Clinopyroxenes are omphacites with cores richer in Jd (Jd 35–40) than the rims (Jd 23–26). Symplectites contain diopside (Jd 6–8) in equilibrium with plagioclase (An 15–30). Jd-rich omphacites display the ordered P2/n symmetry, whereas Jd-poor omphacites are disordered C2/c. Primary brown amphiboles show extreme alumino-ferroanpargasitic compositions; green and colourless amphiboles are either hastingsites or Mg-hornblendes. The petrogenetic evolution of these rocks is bracketed between a maximum P— T condition of 720°C, 16 kbar and a minimum of 500°C, 4 kbar. The latter condition reflects declining stages of the amphibolite facies, later than the climax of the Lepontine phase. The former suggests derivation from oceanic mafic material subducted during the Eo-alpine phase, rather than from cognate mafic layers of upper-mantle origin. The geochemical evidence supports the possibility that they are neither mantle-derived nor in situ eclogites but tectonically emplaced foreign slices of oceanic material.

Episodic volcanism during break-up of the North Atlantic: evidence from the East Greenland plateau basalts

The lower Tertiary (upper Paleocene/lower Eocene) plateau basalts in the Scoresby Sund region of East Greenland formed during three distinct magmatic episodes. The activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence (lower sequence). The activity of the second episode was centred to the southeast, offshore the present Atlantic coast, and this also produced a voluminous regional basalt sequence (upper sequence). The third episode was centred on the Atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting. The lavas are all tholeiitic basalts. After an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern: a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height, from TiO 2-rich “FeTi” basalts to TiO 2-“poor” basalts, and ending with a reversal to TiO 2-rich basalts. Each of these cyclic patterns apparently resulted from a temporary rifting episode, and much of the compositional variation found can be explained by magma development in open magma chambers. The dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts, and their TiO 2/P 2O 5 ratios indicate formation at a spreading ridge centre. The pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region, and the volcanism itself is closely related to this process.

Concentrations and isotope ratios of carbon, nitrogen and sulfur in ocean-floor basalts

Fresh submarine basalt glasses from Galapagos Ridge, FAMOUS area, Cayman Trough and Kilauea east rift contain 22 to 160 ppm carbon and 0.3 to 2.8 ppm nitrogen, respectively, as the sums of dissolved species and vesicle-filling gases (CO 2 and N 2). The large range of variation in carbon content is due to combined effect of depth-dependency of the solubility of carbon in basalt melt and varying extents of vapour loss during magma emplacement as well as in sample crushing. The isotopic ratios of indigenous carbon and nitrogen are in very narrow ranges,−6.2 ± 0.2% relative to PDB and +0.2 ± 0.6 %. relative to atmospheric nitrogen, respectively. In basalt samples from Juan de Fuca Ridge, however, isotopically light carbon ( δ 13 C = around −24%.) predominates over the indigenous carbon; no indigenous heavy carbon was found. Except for Galapagos Ridge samples, these ocean-floor basalts contain 670 to 1100 ppm sulfur, averaging 810 ppm, in the form of both sulfide and sulfate, whereas basalts from Galapagos Ridge are higher in both sulfur (1490 and 1570 ppm) and iron (11.08% total iron as FeO). The δ 34 S values average +0.3 ± 0.5%. with average fractionation factor between sulfate and sulfide of +7.4 ± 1.6%.. The sulfate/sulfide ratios tend to increase with increasing water content of basalt, probably because the oxygen fugacity increases with increasing water content in basalt melt.

Segregation vesicles of ocean floor basalts: 2. Their use as paleovertical indicators

Pillow lava samples dredged from the rift valley of the Mid‐Atlantic Ridge (FAMOUS area) are oriented according to their cooling position on the seafloor (paleovertical) by the segregation vesicle technique (Bideau et al., 1977). Assuming that the spatial distribution of the oriented vesicles corresponds to a Fisherian distribution, it is possible to compute the dispersion coefficient K from the characteristics of the planar distributions (Von Mises distributions). From a theoretical point of view a high probability of finding crescent‐shaped segregation vesicles in thin sections of ocean floor basalts is suggested, and the distribution of the oriented crescents should have higher precision parameter k in sections approaching the paleovertical. However, from a set of 100 rocks, only 27 samples exhibit enough segregation vesicles to provide meaningful directional averages. The paleovertical indicated by segregation vesicles is usually poorly defined in spite of the large number (commonly, ∼100) of angular determinations for each rock. The mean precision parameter K is low (∼3), the λ 95% confidence semi‐angles are large (typically, 15°). Moreover, the calculated magnetic inclinations are almost randomly distributed, which suggests that either the preferential orientation of the segregation vesicles does not correspond to the paleovertical or that the rocks rotate between the temperature of segregation (&gt;1000°C) and the temperature of magnetization (&lt;300°C, generally). Within one of our pillows the occurrence of rotations at high temperature is demonstrated by a progressive rotation in the average direction of the axes of segregation vesicles from the margin toward the interior of the pillow. The amount of rotation, as measured in successive parallel thin sections, amounts to 35° between two sections only 9 cm apart. In an effort to select the samples the most suitable for reorientation purposes a subset of samples was selected, based on petrological criteria, where the segregation vesicles probably developed under the quietest conditions. The preferential orientation of the segregation vesicles of these samples may have been controlled by gravity alone (no systematic effect of flow‐induced rotation) during the segregation process. A second subset of rocks was independently selected using statistical criteria in order to select the more precise directional data. Only seven samples compose the intercept of these two subsets. These rocks show less scatter in inclinations, but two of them have negative inclinations (−23°, −42°), although sampled within the limits of the Bruhnes magnetic anomaly. The most plausible interpretation of these negative inclinations seems to be the occurrence of rotations before the rocks were magnetized but after segregation vesicle solidification. This study shows that the use of segregation vesicles for reorienting ocean floor basalts for paleomagnetic studies has to be critically reconsidered.

Segregation vesicles of ocean floor basalts: 1. Petrological study of the segregation products

Thin section studies of about 100 rock samples recovered from the Mid‐Atlantic Ridge (FAMOUS area, near 37°N) show that the occurrence of segregation vesicles is restricted to particular textural zones corresponding to the inner portion of the pillows. A detailed study of an aphyric olivine basalt shows that the mineralogy of the segregated material is mainly clinopyroxene (salite or Ca‐augite) and plagio‐clase (An 50 to An 60) intergrowths associated with titanomagnetite grains. Olivine and plagioclase microlites (An 65 to An 75) crystallized in the groundmass prior to the formation of the segregation vesicles. Scanning microprobe analyses of the segregated product show an enrichment in Si, Fe, Ca, Na, Ti, P, and K and a depletion in Mg and Al with respect to the bulk rock composition. The compositional variation of the segregated material across the sample suggests that the degree of segregated melt differentiation increases toward the center of the pillow. The formation of segregation vesicles is a progressive phenomenon which occurs during a period of the cooling time when viscous deformation is still possible. This study provides criteria for recognizing portions or loose fragments of pillows which may be oriented by the method of Bideau et al. [1977]. This method is helpful for paleomagnetic studies of ocean floor basalt dredged samples.

Sulfur isotope study of Quaternary volcanic rocks from the Japanese Islands Arc

The sulfide and sulfate contents and their δ 34 S values were determined in Quaternary volcanic rocks from the Japanese Islands Arc. The total sulfur contents are much lower (less than 40 ppm) and the δ 34 S values are higher (+4.4 ± 2.1) than those of ocean-floor basalts (800 ± 100 ppm and +0.8 ± 0.5, respectively; Moore and Fabbi, 1971; Sakai et al., 1982). Lateral variations of both sulfur content and δ 34 S values were observed in the four volcanic belts in Japan. In the Northeast Japan belt, the sulfur content (30 ± 10 ppm) of the rocks in the inner zone (the Japan Sea side) is 3 to 5 times that in the outer zone (the Pacific side), although the δ 34 S values of the two zones are almost the same (+4.3 ± 1.0). The δ 34 S values for the two belts in West Japan are on the average 2%. higher than those of East Japan. This study suggests that the primary magmas that formed the island arc volcanic rocks are initially depleted in sulfur (<120 ppm) and enriched in 34S ( δ 34S: +5 ∼ +7 ) compared to ocean-floor tholeiitic basalts which formed at mantle under oceanic region. This indicates that the upper-mantle is heterogeneous in sulfur content and isotope composition.

The depletion of siderophile elements in the Earth's mantle: new evidence from molybdenum and tungsten

We have recently measured the concentrations of W and Mo in a large number of terrestrial samples using a new neutron activation analysis method and from these data we have estimated the abundance of these elements in the mantle. The new Mo mantle abundance of 59 ppb is much lower, the W mantle abundance of 10 ppb is somewhat lower than previous estimates. The concentrations of W in some ocean floor basalts are much lower than previously reported. The good correlation of W with U confirms the highly incompatible behavior of W and the good correlation of Mo with Nd indicates a moderately incompatible nature for Mo. The new data on W and Mo provide important constraints regarding the possible mechanisms of core formation and accretion because W and Mo are refractory elements under reducing conditions which would have accreted in the Earth in chondritic proportions, unaffected by volatility. The Mo/W ratio of 5.9 in the mantle is less than a factor of two lower than the chondritic ratio of 9.8. The ratio of Mo to W is a sensitive indicator for metal or sulfide fractionation, because Mo is more siderophile and more chalcophile than W. This tightly limits the amount of metal or sulfide segregation from the mantle to less than 0.1% since the end of accretion. The data for the moderately siderophile elements Mo, W, Co and Ni suggest that core formation in the Earth was essentially complete after 85–95% of the Earth had accreted.

Dredged basalts from the western Nazca plate and the evolution of the East Pacific Rise

Ocean-floor basalts and glasses were recovered from three stations along the western Nazca plate, from a sublinear topographic feature believed to represent the proto-East Pacific Rise (EPR), and include abyssal tholeiites, FeTi-basalts and glasses, as well as transitional and little fractionated compositions. When compared with their coexisting fresh glasses, the FeTi-basalts have higher total alkalies, TiO 2 and MgO, and lower FeO *, suggesting that they have also been affected by non-oxidative post-magmatic alteration processes. The FeTi-glasses form a remarkably uniform compositional group through space and time. A little fractionated composition having an Mg-number= 73, similar to those reported from the Mathematician Ridge, has higher Na 2O and TiO 2, and slightly lower CaO than similar compositions from the slowly accreting Mid-Atlantic Ridge. The basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode. Both geochemical and geophysical interpretations support the inference that the EPR grew from Miocene times by the progressive growth and propagation of mantle perturbations, leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto-EPR. Continuing translations and rotations of axial segments are occurring along the EPR, probably in response to self-reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions. The data allow the inference that the youthful rift systems of the eastern Pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes.

The petrology and geochemistry of Cretaceous to Recent volcanism in Colombia: the magmatic history of an accretionary plate margin

The Pacific margin of Colombia records a complex history of subduction-related magmatism from at least Cretaceous times. Cretaceous lavas from the Western Cordillera and part of the Central Cordillera of the Colombian Andes exhibit some chemical characteristics typical of island-arc tholeiites, including low LIL-element abundances and flat to light depleted REE patterns, but also have chemical and geological affinities with deposits from marginal basins, and with transitional type ocean floor basalts. An appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent. A suite of basic rocks from the Coastal Cordillera of Upper Cretaceous to Tertiary age has been interpreted elsewhere as abyssal tholeiites. Following accretion of the Cretaceous sequences on to cratonic S America, volcanism continued in the upper Miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the Western Cordillera. This tholeiitic suite is LREE and LIL-element enriched compared with the Cretaceous lavas, and represents the trenchward portion of an ensialic island-arc sequence. Subduction-related magmatism continued from the Miocene with the formation of composite calc-alkaline strato-volcanoes, mostly along the axis of the Central Cordillera. These Miocene to Recent lavas are dominantly andesitic, with strong LIL and LREE enrichment and 87 Sr/ 86 Sr ratios of 0.7042:0.7047.

Geochemistry and petrogenesis of ophiolites from Northern Pindos (Greece)

The ophiolitic complex of Northern Pindos (Greece) contains ocean-floor basalts and low-Ti mafic rocks. The former rocks are similar to recent mid-ocean ridge basalts with a light REE depletion and a La/Yb ratio < 2. The low-Ti rocks resemble boninites in their high Mg and very low Ti and Zr contents and in their REE patterns which have convex-downwards shape with a slight light REE enrichment. However, their Zr/Ti, Ti/V and Zr/Y ratios are lower than in boninites. Both rock-types could be generated by dynamic partial melting of a rising upper mantle diapir. Slight enrichment in light REE, Sr, Rb and Ba in low-Ti rocks could be the result of either metasomatic or alteration processes. Although a subduction zone origin of the sequence is possible, the geochemical data do not necessarily imply such a setting.

Aqueous low‐temperature oxidation of titanomagnetite

In an attempt to find conditions suitable for aqueous oxidation of titanomagnetite in submarine basalts, synthetic titanomagnetite powders have been exposed to solutions with different pH's and gas fugacities at different temperatures. Analysis of dissolved Fe and Ti, thermomagnetic measurements, and x‐ray measurements all confirm that aqueous oxidation by Femigration does occur. Furthermore, it is, in the first order, pH dependent, implying that ‘maghemitization’ of ocean floor basalts proceeds most favorably under what geochemists call water‐dominated conditions, where basalt‐seawater interaction leads to an increase in acidity.