Galapagos Rise Research Articles

New constraints on deglacial marine radiocarbon anomalies from a depth transect near Baja California

AbstractPrevious studies have shown that radiocarbon activities (Δ14C) in the low‐latitude, middepth Pacific and Indian Oceans were anomalously low during Heinrich Stadial 1 (HS1, ~17.8–14.6 ka) and the Younger Dryas (YD, ~12.8–11.5 ka), coincident with intervals of rising atmospheric CO2 concentration and declining atmospheric Δ14C. However, a full explanation of these events remains elusive due to sparse and sometimes conflicting data. Here we present new 14C measurements on benthic and planktic foraminifera that, in combination with previously published measurements, enable us to reconstruct the Δ14C depth gradient near Baja California. Vertical profiles were similar to present during the Last Glacial Maximum and Bølling/Allerod (14.6–12.8 ka) but display a pronounced middepth (~700 m) Δ14C minimum during HS1 and the YD. The latter observation, along with a comparison to other regional reconstructions, appears to rule out intermediate waters from the north or from directly below as proximate sources of aged 14C‐depleted ocean carbon during deglaciation and point instead to changes in the composition of Equatorial Pacific intermediate waters. Simple mixing constraints require Equatorial Pacific intermediate waters to be only slightly lower in Δ14C than at Baja California, in contrast with previous observations of extremely low Δ14C at Galapagos Rise. While the latter may have been influenced by localized releases of geologic (14C‐dead) CO2, the smaller and more widespread deglacial Δ14C anomalies in the Arabian Sea and North Pacific seem to require a source of aged carbon in the glacial deep Southern and Pacific Oceans for which there is growing evidence.

Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise

New major and trace element and Sr, Nd, and Pb isotope data, together with 39Ar‐40Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid‐ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid‐ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, 87Sr/86Sr, and lower 143Nd/144Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid‐ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger “garnet signature” inferred from Sm/Nd and 143Nd/144Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation‐corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more “depleted” than is generally inferred from the compositions of MORB.

Distribution and provenance of wind-blown SE Pacific surface sediments

The reconstruction of low-latitude ocean–atmosphere interactions is one of the major issues of (paleo-)environmental studies. The trade winds, extending over 20° to 30° of latitude in both hemispheres, between the subtropical highs and the intertropical convergence zone, are major components of the atmospheric circulation and little is known about their long-term variability on geological time-scales, in particular in the Pacific sector. We present the modern spatial pattern of eolian-derived marine sediments in the eastern equatorial and subtropical Pacific (10°N to 25°S) as a reference data set for the interpretation of SE Pacific paleo-dust records. The terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain-size distribution and clay-mineral compositions, respectively, to identify their provenances and transport agents.Dust delivered to the southeast Pacific from the semi- to hyper-arid areas of Peru and Chile is rather fine-grained (4–8μm) due to low-level transport within the southeast trade winds. Nevertheless, wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the Peru–Chile Trench south of ~5°S. Grain-size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub-oceanic ridges and abundant volcanic glass around the Galapagos Islands. Together with the clay-mineral compositions of the clay fraction, we have identified the dust lobe extending from the coasts of Peru and Chile onto Galapagos Rise as well as across the equator into the doldrums. Illite is a very useful parameter to identify source areas of dust in this smectite-dominated study area.

Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget—a combined Pb–Hf–Nd isotope approach

Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center. Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget.

Structural patterns and tectonic history of the Bauer microplate, Eastern Tropical Pacific

The Bauer microplate was an independent slab of oceanic lithosphere that from 17 Ma to 6 Ma grew from 1.4 × 105 km2 to 1.2 × 106 km2 between the rapidly diverging Pacific and Nazca plates. Growth was by accretion at the lengthening and overlapping axes of the (Bauer-Nazca) Galapagos Rise (GR) and the (Pacific-Bauer) East Pacific Rise (EPR). EPR and GR axial propagation to create and rapidly grow the counter-clockwise spinning microplate occurred in two phases: (1) 17–15Ma, when the EPR axis propagated north and the GR axis propagated south around a narrow (100- to 200-km-wide) core of older lithosphere; and (2) 8–6 Ma, when rapid northward propagation of the EPR axis resumed, overlapping ∼400 km of the fast-spreading Pacific-Nazca rise-crest and appending a large (200- to 400-km-wide) area of the west flank of that rise as a ‘northern annex’ to the microplate. Between 15 and 8 Ma the microplate grew principally by crustal accretion at the crest of its rises. The microplate was captured by the Nazca plate and the Galapagos Rise axis became extinct soon after 6 Ma, when the south end of the Pacific-Bauer EPR axis became aligned with the southern Pacific-Nazca EPR axis and its north end was linked by the Quebrada Transform to the northern Pacific-Nazca EPR axis. Incomplete multibeam bathymetry of the microplate margins, and of both flanks of the Pacific-Bauer and Bauer-Nazca Rises, together with archival magnetic and satellite altimetry data, clarifies the growth and (counter-clockwise) rotation of the microplate, and tests tectonic models derived from studies of the still active, much smaller, Easter and Juan Fernandez microplates. Our interpretations differ from model predictions in that Euler poles were not located on the microplate boundary, propagation in the 15–8 Ma phase of growth was not toward these poles, and microplate rotation rates were small (5°/m.y.) for much of its history, when long, bounding transform faults reduced coupling to Nazca plate motion. Some structures of the Bauer microplate boundary, such as deep rift valleys and a broad zone of thrust-faulted lithosphere, are, however, similar to those observed around the smaller, active microplates. Analysis of how the Bauer microplate was captured when coupling to the Pacific plate was reduced invites speculation on why risecrest microplates eventually lose their independence.

Seismotectonics of mid-ocean ridge propagation in Hess Deep.

Hydroacoustic data from the eastern equatorial Pacific reveal low-magnitude seismicity concentrated at the propagating tip of the Galapagos Rise in Hess Deep. The patterns of seismicity and faulting are similar to those observed in the process zone of laboratory-scale propagating tensile cracks. Because the fracture energy required for propagation scales with crack length and process zone size, it follows that ridges can propagate stably in the brittle crust without exceptional resisting forces as proposed by previous models based on linear elastic fracture mechanics.

The Bauer scarp ridge jump: a complex tectonic sequence revealed in satellite altimetry

We investigate here the ridge jump that led to abandonment of the Galapagos Rise and formation of the Bauer scarp during the initiation of the present day configuration of the East Pacific Rise since the lower Miocene. We use recently available high resolution satellite-derived gravity data to investigate in detail the tectonic structure of the eastern Pacific from the Equator to 20°S. With this data, we identify fracture zones, abandoned spreading ridges, scarps, and other seafloor features that provide evidence for discerning tectonic history. Based on our structural interpretation of the satellite-derived gravity field, we make the following conclusions: (1) The Galapagos Rise spreading center appears to have originated by opening of the Marquesas/Mendaña transform complex as a result of the change in spreading direction following breakup of the Farallon Plate. (2) The Galapagos Rise was not the sole locus of spreading following plate reorganization at ∼ 20 Ma through to the initiation of the Bauer scarp at ∼ 8 Ma, as had been previously hypothesized. Rather, it and a second western spreading axis were likely active concurrently, forming a counterclockwise-rotating Bauer Microplate at a much earlier stage than thought previously. (3) The Bauer scarps are pseudofaults associated with northward rift propagation. Propagation proceeded in several stages. A first propagator emanating from the Garrett transform complex stalled at the future location of the Wilkes transform creating an area of complex morphology near its northern tip. A second propagator, also emanating from the Garrett complex followed in the first's wake and broke through the complex region. At this point the propagation proceeded very rapidly to the northern end of the Bauer Microplate (the Gallego fracture zone, later to become the Yaquina transform fault). Ridge propagation continued north in two more stages, creating the Gofar and Quebrada transforms at the terminus of each stage.

Segmentation of the Pacific‐Nazca Spreading Center, 1°N–20°S

A continuous Sea Beam swath along 2500 km of the crest of the fast‐spreading East Pacific Rise (EPR) defines the location and morphology of seven systems of spreading centers. They are bounded by six transform fault systems, where reconnaissance swaths mapped additional short axes of plate accretion. The spreading center systems are subdivided by nontransform offsets, most of which step right on the central part of the plate boundary (12°S–4.5°S) and left on the northern and southern parts. These staircases of small lateral offsets were probably created by changes in spreading direction, the right steps by a counterclockwise rotation as the 12°S–4.5°S EPR matured after 6 Ma from the propagating western boundary of the Bauer microplate. Because the evolution of this microplate also eliminated transform offsets inherited from the Pacific‐Farallon boundary (the pre‐24‐Ma ancestor of the Pacific‐Nazca EPR), led to segmentation of the (fossil) Galapagos Rise, and created the present right‐offset transform fault systems on the EPR, it is identified as the principal event in the development of the mapped plan pattern. A recent small clockwise rotation in Pacific‐Nazca relative motion is inferred to be responsible for most of the small left steps along the boundary and for segmentation of the transforms into systems of en echelon fault zones linked by short intratransform spreading centers. The plan segmentation that results from the plate and microplate histories obstructs the shallow horizontal distribution of magma along the rise crest, allowing spatial variations in the rate of vertical magma delivery to be topographically expressed by the variable cross section of the axial ridge and by the along‐strike relief of the neovolcanic zone. The pattern of variations is much more complex than that predicted by models of transform‐bounded asthenosphere upwelling and regularly spaced melt diapirs. For example, the shallowest spreading segments, as well as the deepest, are short links between long fault zones in transform fault systems, and some long straight parts of the rise crest have humped long profiles consistent with a diapir‐fed model, whereas others have a remarkably flat crest line with no topographic evidence for spreading cells.

Flow rates and reaction rates in the Galapagos Rise spreading center hydrothermal system as inferred from 228 Ra/ 226 Ra in vesicomyid clam shells

The (228)Ra/(226)Ra ratios in a previously dated vesicomyid clam shell were used to determine that seawater was in contact with mid-oceanic-ridge basalt glass for 22-45 years prior to arrival to the surface at 350 degrees C at the Galapagos Rise Spreading Center. The minimum rate of reaction for the 45-year sojourn time, based on a water/rock ratio of 2.8 derived from (226)Ra concentrations, is 8 g of basalt altered per kg of seawater per year.

Hydrogen isotope systematics of submarine basalts

The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in δD and H 2O + from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between δD and H 2O + in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH 4 and H 2. A good correlation between δD values and H 2O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having δD values as low as −100. δD values vary with H 2O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary δD values were similar to those of submarine lavas. Extrapolations to possible unaltered δD values and H 2O contents indicate that the primary δD values of most thoteiite and alkali basalts are near −80 ± 5: the weight percentages of water are variable, 0.15–0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary δD values of −80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth.

Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas

Summary Electron-microprobe analyses of submarine basalt glass and coexisting olivine from the Galapagos Rise and the Mid-Atlantic Ridge (60–63°N) reveal that olivine phenocrysts are virtually unzoned and olivine compositions are identical to those predicted by assuming that the olivine crystallized at the liquidus temperature of the host glass. The absence of chemical zoning of olivine indicates that their host magmas must have cooled by less than 15°C and the MgO/FeO of the magma must have been nearly constant (less than 3 wt% olivine removed) during the time that olivine phenocrysts existed in the magma. Olivine phenocrysts re-equilibrate with their host magma relatively quickly and any changes in temperature or composition that occurred in the 24 h previous to eruption would be recorded in the olivine composition. The settling rates of 1-mm olivine phenocrysts are too slow compared to magma ascent rates to allow early-formed olivine to settle out. There, many magmas that erupt at mid-ocean-ridge crests must ascend from their final reservoir nearly isothermally and isochemically, or alternatively they ascend at superliquidus temperatures and cool below the liquidus just prior to eruption. In either case the magmas identify the temperature (±15°C) and the composition of the magma reservoir immediately prior to eruption. The depth of these magma reservoirs may be estimated through experimental and empirical determination of the pressure-temperature phase diagrams of oceanic basalts. For Galapagos Rise and Mid-Atlantic Ridge basalts these reservoirs are at 3–25 km beneath the ridge crust and in some cases individual reservoirs may be located within ±3 km.

Chemical and isotopic diversity in basalts dredged from the East Pacific Rise at 10°S, the fossil Galapagos Rise and the Nazca plate

We present petrographic, chemical and isotopic data for fresh lava samples dredged from three regions: (1) the fossil Galapagos Rise; (2) an elongate volcano near this extinct spreading center; and (3) the East Pacific Rise at 10°S. The samples from the Galapagos Rise are among the first samples from any fossil spreading center to be analyzed. Alkalic picrites from the elongate seamount and transitional basalts from the East Pacific Rise are both somewhat unusual rock types considering their respective tectonic environments. The dredges from the East Pacific Rise at about 10°S recovered unusual transitional, light rare-earth element (LREE) enriched basalts which show a range of fractionation. On the basis of their chemical and isotopic abundances, it is unlikely that the lavas are related by a single simple process of magmatic differentiation. We suggest that the mantle source region of these basalts was chemically and isotopically heterogeneous. The chemistry of LREE-depleted tholeiitic basalt dredged from near the axis of the extinct Galapagos Rise indicates complex petrogenesis and differentiation. The presence of tholeiitic basalts here indicates that unlike the Guadalupe and Mathematician fossil ridges, the Galapagos Rise has not been the site of voluminous post-abandonment alkalic volcanism. Alkalic basalts of picritic bulk composition dredged from an elongate seamount near the Galapagos Rise do not represent liquid compositions. Instead, we suggest that these alkalic liquids contain added olivine and plagioclase xenocrysts. Although most of the samples analyzed are very fresh, a few have been altered. The latter exhibit characteristic chemical and isotopic effects of seawater alteration.

Evidence for two fossil spreading ridges in the southeast Pacific

Two different sets of extinct spreading ridge segments (Pacific-Farallon and Pacific-Nazca) are identified from bathymetric and magnetic data in the southeast Pacific. The older set is composed of several topographically subdued segments of a fossil ridge (Pacific-Farallon) that trends northwest, parallel to and about 175 to 550 km distant from the younger side of anomaly 7 (26 m.y. B.P.) The younger set trends north-northeast subparallel to the present East Pacific Rise (EPR), at an angle of about 45° to the older fossil ridge. The younger ridge forms the Galapagos Rise and includes a set of west-northwest–trending fracture zones. This was the original site of the EPR before it jumped westward to its present position. Oligocene magnetic anomalies (7–12) were generated at the older northwest-trending ridge rather than from the younger northeast-trending Galapagos Rise. The change in ridge orientation from the older Pacific-Farallon to the younger Pacific-Nazca direction is likely to be associated with the birth of the Cocos-Nacza spreading ridge system, about 20 to 25 m.y. ago.

Hydraulic Piston Coring in Equatorial Pacific--Preliminary Results from DSDP Site 503 (Leg 68) Indicate Continuous Section of Undisturbed Late Noegene and Quaternary Sediment: ABSTRACT

Deep Sea Drilling Project Site 503 was cored, using the Hydraulic Piston Corer, at the location of Site 83 on the north flank of the Galapagos Rise. Two holes were cored to a total depth of 235 m and a section was recovered which represents approximately the past 8 m.y. The upper 100 m (Holocene through mid-early Pliocene) is virtually complete whereas below 100 m (to late Miocene) about 70% of the section was recovered. The sediment is a uniformly siliceous calcareous ooze with carbonate and color cycles throughout. These cycles have periodicities from 20,000 to 40,000 years per cycle. Clay content remains fairly constant at low percentages to 226 m but then abruptly increases to greater than 25%. This increase is thought to be the result of hydrothermal activity or weat ering of the igneous basement or both. The section is highly undercompacted, which is thought to result from a significant percentage of diatoms. Aboard ship we were able to identify most magnetostratigraphic boundaries above the Gauss-Gilbert. Sedimentation rates are about 4 cm/k.y. from lower Pliocene through mid-Pliocene but decrease to 1.5 to 2.5 cm/k.y. for the mid-Pliocene through Quaternary. Calculated bulk accumulation rates steadily decrease from late Miocene to late Quaternary with a distinct event of low accumulation rates in mid-Pliocene. This event may be a reflection of the emergence of the Isthmus of Panama. The high accumulation rate and high quality of these cores provide the most detailed stratigraphic record of the past 8 m.y. available from the equatorial Pacific. End_of_Article - Last_Page 711------------

Surface sediments of the Peru-Chile continental margin and the Nazca plate

Surface sediment samples from 86 locations on the Peru-Chile continental margin and the Nazca plate have been analyzed for bulk chemistry and texture to evaluate the factors influencing sediment formation on continental slope and adjacent abyssal plain depositional environments. Sand-sized calcareous microfossils are abundant above the carbonate compensation depth (CCD), but their removal by dissolution leaves fine-grained deposits in the deeper basins. Terrigenous silts are found seaward of the Peru-Chile Trench, especially south of 7°S, and may be transported there by wind, mid-water turbid layers, or bottom waters flowing north through the deep ocean basins. Sediments become finer grained away from shore as coarse terrigenous particles settle out. The more humid climate of northern Peru produces finer fluvial sediments there than to the south. This textural change also appears in adjacent marine sediments. In the Peru Basin, bottom nepheloid layers are found where clay-sized terrigenous material dominates the sediments. On the adjacent Galapagos Rise, however, nepheloid layers are absent, and eolian-derived sedimentary components in the R-mode factor analysis has outlined sediment constituents from the geochemical data. Terrigenous components dominate the slowly accumulating clay-sized sediments of the Peru and Chile Basins, and the coarser, more rapidly forming margin deposits south of 14°S. Biogenic deposits form at intermediate rates on topographic highs on the Nazca plate. Very slow hydrogenous sedimentation occurs in the seaward portions of the deep basins. Sediments rich in organic components are concentrated in rapidly forming deposits along the margin north of 14°S, beneath centers of strong coastal upwelling. Several distinct sediment accumulations have been mapped on the continental shelf and upper slope. An upper-slope deposit is anomalously fine grained and organic rich; it lies between 10.5°S and 13.6°S, in the region of most intense upwelling. Preservation of this body is enhanced by the inclusion of fine inorganic material in biogenic fecal pellets, and by the impingement of the shallow-water oxygen minimum layer on the slope at the same level.

Gabbro, serpentinite, and mafic breccia from the East Pacific.

Gabbro, gabbroic anorthosite, basaltic breccia, diabase and basalt samples were dredged from a fracture zone of the Galapagos Rise, a fossil spreading center on the Nazca plate. Serpentinite was dredged from the Ecuador fracture zone of the Galapagos Spreading Center. Although the rocks are variably altered, minerals and chemistry of gabbroic rocks indicate that extensive fractionation occurred throughout the suite. Basaltic breccias have textures indicating prefracture zone brecciation.

Geophysical surveys on the East Pacific Rise -- Galapagos Rise system

The East Pacific Rise at 12–15° S is topographically smooth with a crestal horst or linear volcanic peak marking the present axis of spreading. The Galapagos Rise at 14–17° S is topographically rough with a possible central graben marking the extinct spreading axis. The seafloor spreading magnetic anomalies on the East Pacific Rise are of low amplitude, but fracture-zone anomalies at 13–14° S have amplitudes of up to 1250 nT. Anomalies of this amplitude at the magnetic equator must be formed within the fracture zone by some combination of block reversal boundaries, anomalously-high magnetic intensities, and/or anomalously-large thicknesses of the magnetic layers within the fracture zone. Magnetization and major-element chemical analyses of basalts dredged from four locales along the fracture zone indicate that the large magnetic-anomaly amplitudes are caused by the high iron and titanium content of these ferrobasalts. The magnetic-anomaly profiles from the Galapagos Rise and its fracture-zone system are of normal amplitude and are extremely difficult to correlate internally or with the geomagnetic timescale. Eighty-one heat-flow measurements indicate that the values measured are controlled by sediment thickness. Where the thickness of the sediment blanket is greater than 100 m, high heat flow is measured and possibly is representative of the total heat transfer at the seafloor. Where the sediment thickness is less than 100 m, seawater circulation in the oceanic crust is thought to remove most of the heat convectively; thus causing low conductive heat-flow values to be measured by the usual heat-flow apparatus. The heat loss by convective processes is probably a function also of topographic roughness and sediment permeability.

Evolution of the East Pacific Rise between 3°S and 13°S since the Middle Miocene

Since Middle Miocene time seafloor spreading activity in the southeast Pacific has shifted westward from the Galapagos Rise, then the last active segment of the Farallon Ridge system in the South Pacific, to the East Pacific Rise. This shift was accomplished by three large jumps of fracture‐zone‐bound axial sections which progressed from south to north. The jumps spanned an average distance of 750 km and occurred 8.2, 6.4 and 5.7 m.y. ago respectively. Positioning of the newly‐formed axes along the EPR trend apparently was controlled by asthenospheric rather than lithospheric phenomena.

Fractionation and mantle heterogeneity in basalts from the Peru-Chile Trench

Five separate exposures of oceanic basalts were dredged in the vicinity of the Peru-Chile Trench between 9° and 27°S latitude. Each dredge is dominated by abundant pillow basalts. Approximately ten of the most unaltered, glassy and fine-grained samples were selected for detailed chemical and petrographic analyses from each dredge area. All basalts recovered in the Peru-Chile Trench are olivine and quartz-normative tholeiites that are believed to have formed at the now extinct Galapagos Rise 30–50 m.y. ago. Detailed chemical analyses of the basalts, including major and selected trace and rare earth elements, indicate that considerable compositional variability exists both within each of the dredged areas as well as between areas. Most of the inherent chemical variability observed within particular basement sections appears consistent with the concept of temporal evolution of magma bodies at a former spreading center by shallow-level fractional crystallization involving primarily plagioclase and olivine. In contrast, important chemical differences between the dredged areas suggest compositional heterogeneities in the mantle source regions. Our results indicate that although shallow-level fractionation has brought about large changes in composition of basalts in each area, compositional trends are distinct and appear to reflect original mantle-derived compositional differences.

Abyssal tholeiites from DSDP leg 34: The Nazca Plate

Basalts recovered from the Nazca Plate at sites 319A and 321 of DSDP (Deep-Sea Drilling Project) leg 34 are hypersthene normative abyssal tholeiites with chemical compositions similar to abyssal tholeiites of worldwide occurrence. These basalts, among the freshest cored by the DSDP, may be the products of ocean ridge volcanism along the extinct Galapagos rise at about 14 and 40 m.y. B.P. The basalt section at site 319A includes two major flow units of porphyritic intersertal to ophitic plagioclase tholeiites. Within these and other minor flow units at this site, plagioclase (An88–24 and Or ≤ 1) and clinopyroxene (augite to pigeonite) exhibit complex optical and chemical zoning, the result of metastable crystallization. Basalts from site 321 are porphyritic to aphyric and vesicular and intersertal; and they exhibit complex zoning in plagioclase and clinopyroxene similar to those at site 319A. Chemical compositions of the leg 34 basalts indicate evolution from a more primitive magma generated in a peridotitic upper mantle. The evolved character of the leg 34 basalts is a consequence of varying degrees of fractionation of olivine and plagioclase from the ascending primary magmas. The site 321 magmas are more fractionated than site 319A magmas. This is indicated by their higher TiO2, lower average Sr, higher REE (rare-earth element) concentrations, lower Mg/(Mg + Fe2+) ratios, and more sodic plagioclase core compositions (An71) and the presence of clinopyroxene microphenocrysts in the site 321 basalts. Whole rock and mineral chemical data from this study support the idea of a chemically similar worldwide source region of abyssal tholeiites through time.