Geochemistry of volcanic rocks of ODP Holes 126-792E and 126-793B

The Izu-Bonin forearc basement volcanic rocks recovered from Holes 792E and 793B show the same phenocrystic assemblage (i.e., plagioclase, two pyroxenes, and Fe-Ti oxides ±olivine), but they differ in the crystallization sequence and their phenocryst chemistry. All the igneous rocks have suffered low-grade hydrothermal alteration caused by interaction with seawater. As a result, only clinopyroxenes, plagioclases, and oxides have preserved their primary igneous compositions. The Neogene olivine-clinopyroxene diabasic intrusion (Unit II) recovered from Hole 793B differs from the basement basaltic andesites because it lacks Cr-spinels and contains abundant titanomagnetites (Usp38.5-46.4) and uncommon FeO-rich (FeO = 29%) spinels. It displays petrological and geochemical similarities to the Izu Arc volcanoes and, thus, can be considered as related to Izu-Bonin Arc magmatic activity. The titanomagnetites (Usp28.5-33) in the calc-alkaline andesitic fragments of the Oligocene volcaniclastic breccia in Hole 793B (Unit VI) represent an early crystallization phase. The Plagioclase phenocrysts enclosed in these rocks show oscillatory zoning and are less Ca-rich (An78.6-67.8) than the plagioclase phenocrysts of the diabase sill and the basement basaltic andesites. Their clinopyroxenes are Fe-rich augites (Fs ? 19.4; FeO = 12%) and thus, differ significantly from the clinopyroxenes of the Hole 793B arc-tholeiitic igneous rocks. The 30-32 Ma porphyritic, two-pyroxene andesites recovered from Hole 792E are very similar to the andesitic clasts of the Neogene breccia recovered in Hole 793B (Unit VI). Both rocks have the same crystallization sequence, and similar chemistry of the Fe-Ti oxides, clinopyroxenes, and plagioclases: that is, Ti-rich (Usp25.5-30.4) magnetites, Fe-rich augites, and intensely oscillatory zoned plagioclases with bytownitic cores (An86-63) and labradorite rims (An73-68). They display a calc-alkaline differentiation trend (Taylor et al., this volume). So, the basement highly porphyritic andesites recovered at Hole 792E, and the Hole 793B andesitic clasts of Unit VI show the same petrological and geochemical characteristics, which are that of calc-alkaline suites. These Oligocene volcanic rocks represent likely the remnants of the Izu-Bonin normal arc magmatic activity, before the forearc rifting and extension. The crystallization sequence in the basaltic andesites recovered from Hole 793B is olivine-orthopyroxene-clinopyroxene-plagioclase-Fe-Ti oxides, indicating a tholeiitic differentiation trend for these volcanic rocks. Type i is an olivine-and Cr-spinel bearing basaltic andesite whereas Type ii is a porphyritic pyroxene-rich basaltic andesite. The porphyritic plagioclase-rich basaltic andesite (Type iii) is similar, in most respects, to Type ii lavas but contains plagioclase phenocrysts. The last, and least common lava is an aphyric to sparsely phyric andesite (Type iv). Cr-spinels, included either in the olivine pseudomorphs of Type i lavas or in the groundmass of Type ii lavas, are Cr-rich and Mg-rich. In contrast, Cr-spinels included in clinopyroxenes and orthopyroxenes (Types i and ii lavas) show lower Cr* and Mg* ratios and higher aluminium contents. Orthopyroxenes from all rock types are Mg-rich enstatites. Clinopyroxenes display endiopsidic to augitic compositions and are TiO2 and Al2O3 depleted. All the crystals exhibit strong zoning patterns, usually normal, although, reverse zoning patterns are not uncommon. The plagioclases show compositions within the range of An90-64. The Fe-Ti oxides of the groundmass are TiO2-poor (Usp16-17). The Hole 793B basaltic andesites show, like the Site 458 bronzites from the Mariana forearc, intermediate features between arc tholeiites and boninites: (1) Cr-spinel in olivine, (2) presence of Mg-rich bronzite, Ca-Mg-rich clinopyroxenes, and Ca-plagioclase phenocrysts, and (3) transitional trace element depletion and epsioln-Nd ratios between arc tholeiites and boninites. Thus, the forearc magmatism of the Izu-Bonin and Mariana arcs, linked to rifting and extension, is represented by a depleted tholeiitic suite that displays boninitic affinities.

Geochemistry of ODP Leg 135 basalts

Cr-spinels in cores drilled during Ocean Drilling Program Leg 135 exhibit wide variations in composition and morphology that reflect complex petrogenetic histories. These Cr-spinels are found within basaltic lava flows that erupted in north-trending sub-basins within the Lau Basin backarc. Cr-spinels from Sites 834 and 836 occur as euhedral groundmass grains and inclusions in plagioclase, and range up to 300 ?m in size. These Cr-spinels are similar in composition, morphology, and mode of occurrence to Cr-spinels found within depleted, N-type mid-ocean-ridge basalts (N-MORB), reflecting similar crystallization conditions and host lava composition to N-MORB. Their compositional range is relatively narrow, with Cr/(Cr + Al + Fe3+) (Cr#) and Mg/(Mg + Fe2+) (Mg#) varying from 0.38 to 0.48 and 0.56 to 0.72, respectively; like Cr-spinels from N-MORB, they contain low amounts of TiO2 (0.37%-1.05%) and Fe3+/(Cr + Al + Fe3+) (Fe3+#; <0.11). In contrast, Cr-spinels from Site 839 have much higher Cr# at a given Mg#, with Cr# varying from 0.52 to 0.76 and Mg# varying from 0.27 to 0.75. These Cr-spinels are similar in composition to those from primitive, boninitic or low-Al2O3 arc basalts, sharing their low TiO2 and Fe3+# (typically below 0.35% and 0.1, respectively for spinel grain interiors). Site 839 Cr-spinels occur as small (to 50 µm) euhedra within strongly zoned olivine or as unusually large (to 3 mm), euhedral to subhedral megacrysts. These megacrysts are strongly zoned in Mg#, but they display little zoning in Cr#, providing evidence of strong compositional disequilibria with the host melt. The magnesian cores of the megacrysts crystallized from primitive, near-primary melts derived from harzburgitic or highly depleted lherzolitic sources, and they provide evidence that the Site 839 spinel-bearing lavas were derived by the mixing of melt with a Mg# of 0.75-0.80 and evolved, Cr-spinel barren melt with a Mg# < 0.6 shortly before eruption.

Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.

Geochemistry of ODP Leg 127 igneous rocks

Legs 127 and 128 of the Ocean Drilling Program cored basement samples from two sites in the Yamato Basin (Sites 794 and 797) and one site in the Japan Basin (Site 795) of the Japan Sea. These samples represent sills and lava flows erupted or shallowly intruded in a marine environment during backarc extension and spreading in the middle Miocene. In this paper, we describe the geochemical characteristics of these igneous units using 52 new instrumental neutron activation analyses (INAA), 8 new X-ray fluorescence (XRF) analyses, and previous shipboard XRF analyses. The sills intruded into soft sediment at Sites 794 and 797 were subject to extensive hydrothermal activity, estimated at <230° C under subgreenschist facies conditions, which heavily to totally altered the fine-grained unit margins and moderately to heavily altered the coarse-grained unit interiors. Diagenesis further altered the composition of these igneous bodies and lava flows at Sites 794, 795, and 797, most intensely at unit margins. Our study of two well-sampled units shows that Mg, Ca, Sr, and the large-ion lithophile elements (LILE) mobilized during alteration, and that the concentrations of Y, Yb, and Lu decreased and Ce increased in the most severely altered samples. Nevertheless, our study shows that the rare-earth elements (REE) were relatively immobile in the majority of the samples, even where secondary mixed-layer clays comprised the great majority of the rock. Fresher Yamato Basin samples are compositionally heterogenous tholeiitic basalts and dolerites. At Site 794 in the north-central portion of the basin, Units 1 to 5 (upper basement) comprise mildly light rare-earth element (LREE) enriched basalts and dolerites (chondrite-normalized La/Sm of 1.4-1.8), while the stratigraphically lower Units 6 to 9 are less enriched dolerites with (La/Sm)N of 0.7-1.3. All Site 794 samples lack Nb and Ta depletions and LILE enrichments, lacking a strong subduction-related incompatible element geochemical signature. At Site 797 in the western margin of the basin, two stratigraphically-definable unit groups also occur. The upper nine units are incompatible-element depleted tholeiitic sills and flows with strong depletions of Nb and Ta relative to normal mid-ocean ridge basalt (N-MORB). The lower twelve sills represent LREE-enriched tholeiites (normalized La/Sm ranges from 1.1 to 1.8), with distinctly higher LILE and high field-strength element (HFSE) contents. At Site 795 at the northern margin of the Japan Sea, three eruptive units consist of basaltic andesite to calc-alkaline basalt (normalized La/Sm of 1.1 to 1.5) containing moderate depletions of the HFSE relative to N-MORB. The LILE-depleted nature of these samples precludes their origin in a continental arc, indicating that they more likely erupted within a rifting oceanic arc system. The heterogenous nature of the Japan Sea rocks indicate that they were derived at each site from multiple parental magmas generated from a compositionally heterogenous mantle source. Their chemistry is intermediate in character between arc basalts, MORB, and intraplate basalts, and implies little involvement of continental crust at any point in their genesis. Their flat chondrite-normalized, medium-to-heavy rare earth patterns indicate that the primary magmas which produced them last equilibrated with and segregated from spinel lherzolite at shallow depths (<30 kbar). In strong contrast to their isotopic compositional arrays, subduction-related geochemical signatures are usually poorly defined. No basin-wide temporal or geographic systematics of rock chemistry may be confidently detailed; instead, the data show both intimate (site-specific) and widespread backarc mantle heterogeneity over a narrow (2 Ma or so) range in time, with mantle heterogeneity most closely resembling a "plum-pudding" model.

Geochemistry of the sheeted dike complex at Pito Deep

Alteration of sheeted dikes exposed along submarine escarpments at the Pito Deep Rift (NE edge of the Easter microplate) provides constraints on the crustal component of axial hydrothermal systems at fast spreading mid-ocean ridges. Samples from vertical transects through the upper crust constrain the temporal and spatial scales of hydrothermal fluid flow and fluid-rock reaction. The dikes are relatively fresh (average extent of alteration is 27%), with the extent of alteration ranging from 0 to >80%. Alteration is heterogeneous on scales of tens to hundreds of meters and displays few systematic spatial trends. Background alteration is amphibole-dominated, with chlorite-rich dikes sporadically distributed throughout the dike complex, indicating that peak temperatures ranged from <300°C to >450°C and did not vary systematically with depth. Dikes locally show substantial metal mobility, with Zn and Cu depletion and Mn enrichment. Amphibole and chlorite fill fractures throughout the dike complex, whereas quartz-filled fractures and faults are only locally present. Regional variability in alteration characteristics is found on a scale of <1-2 km, illustrating the diversity of fluid-rock interaction that can be expected in fast spreading crust. We propose that much of the alteration in sheeted dike complexes develops within broad, hot upwelling zones, as the inferred conditions of alteration cannot be achieved in downwelling zones, particularly in the shallow dikes. Migration of circulating cells along rides axes and local evolution of fluid compositions produce sections of the upper crust with a distinctive character of alteration, on a scale of <1-2 km and <5-20 ka.

39Ar/40Ar ratios of whole rock samples from ODP Hole 107-655B and 107-651A (Table 1)

Six whole rocks from the basaltic lava series drilled in the Vavilov basin have been analyzed by 39Ar-40Ar stepwise heating method. One sample from the upper part of the Hole 655B basement gave a plateau-age at 4.3 ± 0.3 Ma whereas the other ones showed disturbed age spectra caused by alteration processes. The weighted averages of ages measured at low and intermediate temperatures on these five samples are concordant (1) one to each other and (2) with independent estimates deduced from paleontological and paleomagnetical arguments. Ages of 4.3 ± 0.3 Ma and from 3 to 2.6 Ma may represent reasonable estimates of the crystallization ages of the basaltic lava series of the Holes 655B and 651A, respectively. These ages must be taken with caution because they correspond to argon released from secondary phases characterized by low argon retention.

Geochemistry of basalts from Serocki Volcano

Basalts collected during drilling and diving programs on Serocki Volcano mostly fall within a limited compositional range, and are moderately evolved, normal MORBs with distinctive high MgO contents (averaging 7.60 wt%) and high A1203 concentrations (averaging 16.14 wt% in whole rock samples). However, samples recovered from within the central crater have lower Ti02 and FeO*/MgO, and higher MgO and Al2O3 concentrations, and are most similar to glasses recovered at Site 649 about 45 km to the north. Comparison of the observed geochemical variations with low-pressure experimental work and other samples from the region suggests that the Serocki Volcano and Site 649 data are compatible with crystal-liquid fractionation involving both olivine and early-stage clinopyroxene, as well as plagioclase, and that the sources may be similar even though Sites 648 and 649 are located in different, but adjacent, spreading cells. Consideration of the stratigraphy and morphology of Serocki Volcano suggests that this feature is more properly described as a megatumulus or lava delta, associated with a steeper, conical peak to the southwest. The evolution of Serocki Volcano involved early construction of a marginal rampart of pillows, followed by doming of this feature and the formation of a perched lava pond. Draining of this pond resulted in collapse and the formation of the central crater.

Chemical composition of volcanic glass of some tephra layers in Northeastern Pacific Ocean

Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei)], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Geochemistry of selected Ferrar samples from Northern Victoria Land and George V Land during GANOVEX VIII

Newly sampled basaltic andesites and andesites from the tholeiitic Ferrar Supergroup of northern Victoria Land and George V Land, Antarctica, are attributed to the known low-Ti and high-Ti series. Aside from known sparsely distributed high-Ti extrusives, a high-Ti sill was found in the Alamein Range outside the Rennick Graben. Low-Ti lavas, sills and dikes display wide petrographical, mineral and geochemical variations, reflecting extensive in-situ differentiation. High-Ti rocks from Litell Rocks are homogeneous with respect to mineralogy and geochemistry, minor deviations are shown by the sampled sill. Chilled margins of low-Ti sills, dikes and lava flows exhibit nearly constant bulk-rock chemistry (mg# ~60) within the studied area. Compared to chilled margins from Tasmanian sills, the striking uniformity of the pre-emplacement chemistry of Ferrar magmas over large distances supports the magma transport model of Elliot et al. (1999, doi:10.1016/S0012-821X(99)00023-0). In the area investigated, compositional variations within the low-Ti series, caused by in-situ differentiation, increase towards the Wilson-Bowers Terrane boundary, possibly displaying the asymmetrical distribution of outcrops over this area. Absence of Ferrar occurrences east of the Bowers Terrane remains a matter of palaeo-geodynamic discussion. Besides, the secondary mineralogy of extrusives from Litell Rocks and Monument Nunataks exhibits noticeable differences, which indicates an elevated thermal gradient in the vicinity of Litell Rocks compared to Monument Nunataks during the Cretaceous.

Geochemistry of ODP Hole 135-839B lavas

Four petrographic lava types occur, ranging from aphyric to moderately phyric clinopyroxene-olivine tholeiitic basalts (Unit 1); olivine-clinopyroxene picritic basalts, sparsely to strongly olivine-phyric (Unit 3-type); olivine-clinopyroxene basalts (clinopyroxene dominant) (Unit 4); and moderately to strongly phyric two-pyroxene-plagioclase basaltic andesites (Unit 9-type). The olivine phyric lavas contain forsteritic olivines (extending to Fo92), and very magnesian Cr-rich spinels similar to those occurring in boninitic lavas. The basaltic andesites are mineralogically and petrographically indistinguishable from the modern Tofua Arc basaltic andesites, one notable feature being the highly calcic cores in plagioclase phenocrysts (up to An95). The forsteritic olivines, the Cr-spinels, and the calcic plagioclases are unlikely to have been precipitated in the lava compositions in which they occur, and are thought to have been incorporated from highly primitive melts by way of mixing processes (as advocated by Allan, this volume). Notwithstanding the evidence for mixing, the major element chemistries of the Unit 1- and Unit 9-type lavas are shown to be consistent with the derivation of the Unit 9-type basaltic andesites by means of fractional crystallization, through magmas of similar chemistry to Unit 1. Some trace element discrepancies in the modeling, and the relative volcanic stratigraphy of Site 839, however, preclude a direct liquid line of descent between the actual recovered units. Trace element data as well as TiO2 and Na2O data clearly illustrate the arc-like affinities of the magmas, with strong highfield-strength element depletion and large-ion-lithophile element enrichment. The abundance patterns are very close to those of the Tofua and Kermadec arc magmas, and also Valu Fa. Pb-, Sr-, and Nd-isotopic compositions indicate closest affinities with a "Pacific" MORB source, apparently characteristic of the western, older part of the Lau Basin. A subduction-related isotopic contribution is, however, inferred. The sources of the Site 839 magmas are thus inferred to be similar to, but less depleted geochemically, than those of the modern Tofua Arc magmas. The Site 839 sequence is interpreted as an older remnant of a volcanic construct of the "proto-Tofua arc", originally developed adjacent to the Tonga Ridge. Opening of the eastern Lau Basin, because of southward migrating propagators, has split and isolated the sequence, leaving it stranded within the modern Lau Basin.

Geochemistry of ODP Hole 135-841B

Two igneous rock units were recovered at Site 841. More than 200 m of island-arc rhyolites, rhyolitic tuffs, lapilli tuffs, and pumice breccias, divided into five units, compose the basement at the site. These rhyolitic volcanics are late middle Eocene or older and formed part of a subaerial rhyolitic volcano. These low-K rhyolites were produced by fractional crystallization of a more mafic arc-tholeiitic lava or by dehydration melting of lower crustal arc tholeiites. The Site 841 basement rocks are similar in composition to high-SiO2 lavas in the Eocene basement on 'Eua and crystallized from depleted island-arc-tholeiitic basalts like those exposed on 'Eua. No evidence is present in the rhyolites, or in the clasts enclosed within them, for boninite series magmas at Site 841. The Site 841 rhyolitic complex bears no resemblance to Cretaceous rhyolites from the Lord Howe Rise, which are enriched in K and incompatible elements. The volcanic rocks at Site 841 are part of a widely distributed Eocene volcanic episode that marked the earliest phases of subduction in the Tonga region; they are not part of an older crustal fragment. The second igneous sequence is a series of basaltic dikes and sills that intruded Miocene sediments. These basalts have trace element abundances and ratios identical to upper Miocene lavas from the Lau Ridge. The Site 841 basalts do not have any geochemical characteristics that suggest they were generated by unusual thermal conditions in the shallow sub-forearc mantle. They are most reasonably interpreted as intrusions fed by basement dikes propagated from the associated active arc. No evidence for local serpentinite exposures, like those that are common in the Mariana forearc, was found at Site 841. The results from Site 841 provide strong support for hypotheses of forearc evolution that have been advanced for the Izu-Bonin-Mariana system.

Geochemistry of Central American Volcanic Arc basement samples

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low d18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high d18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

Spherules and basement analyses from DSDP Hole 5-32

Lower Miocene basaltic glass spherules from DSDP Site 32 pelagic sediments in the eastern Pacific are compositionally diverse, and new analyses and interpretations have been added to those of earlier workers. The spherules are of titanian ferrobasalt which is compositionally similar to highly evolved abyssal basalts and to some oceanic island eruptives, and they were most likely shaped during intense lava fountaining during a number of separate eruptions. These eruptions tapped distinct but related magma batches in terms, for example, of distinctively high TiO2 and FeO* contents. Their age overlaps that of some of the eruptions of the Columbia River Plateau Basalts, but they are compositionally distinct from most of the latter basalts. Although about 15 m.y. old, they show little alteration. The low chlorine and sulfur contents compared to those of abyssal ferrobasalts are consistent with degassing prior to quenching during subaerial eruptions, and rule out production of the spherules by submarine fountaining. Lava fountaining alone is insufficient to account for the distance of about 100 km from even the closest possible seamount source. Instead, large phreatomagmatic eruption columns reaching at least 15 km and including lava fountaining immediately after the initial explosion are required. Alternatively, and deemed less likely, is their deposition by turbidites derived from Pioneer Seamount.

40Ar-39Ar dating and isotopic composition of groundmass and felspar samples from core AND-2A

The AND-2A drillcore (Antarctic Drilling Program-ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar-39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar-39Ar ages, indicating that the AND-2A drillcore recovered <230 m of Middle Miocene (~128-358 m below sea floor, ~11.5-16.0 Ma) and >780 m of Early Miocene (~358-1093 m below sea floor, ~16.0-20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ~19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ~17.5-18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the 'proto-Mount Morning' as the main source.

Geochemistry of Pliocene-Pleistocene volcanic sands of ODP Site 136-842

Several distinct, thin (2-7 cm), volcanic sand layers ("ashes") were recovered in the upper portions of Holes 842A and 842B. These holes were drilled 320 km west of the island of Hawaii on the outer side of the arch that surrounds the southern end of the Hawaiian chain. These layers are Pliocene to Pleistocene in age, graded, and contain fresh glass and mineral fragments (mainly olivine, plagioclase, and clinopyroxene) and tests of Pleistocene to Eocene radiolarians. The glass fragments are weakly vesicular and blocky to platy in shape. The glass and olivine fragments from individual layers have large ranges in composition (i.e, larger than expected for a single eruption). These features are inconsistent with an explosive eruption origin for the sands. The only other viable mechanism for transporting these sands hundreds of kilometers from their probable source, the Hawaiian Islands, is turbidity currents. These currents were probably related to several of the giant debris slides that were identified from Gloria sidescan images around the islands. These currents would have run over the ~500-m-high Hawaiian Arch on their way to Site 842. This indicates that the turbidity currents were at least 325 m thick. Paleomagnetic and biostratigraphic data allow the ages of the sands to be constrained and, thus, related to particular Hawaiian debris flows. These correlations were checked by comparing the compositions of the glasses from the sands with those of glasses and rocks from islands with debris flows directed toward Site 842. Good correlations were found for the 110-ka slide from Mauna Loa and the ~1.4-Ma slide from Lanai. The correlation with Kauai is poor, probably because the data base for that volcano is small. The low to moderate sulfur content of the sand glasses indicates that they were derived from moderately to strongly degassed lavas (shallow marine or subaerially erupted), which correlates well with the location of the landslide scars on the flanks of the Hawaiian volcanoes. The glass sands may have been formed by brecciation during the landslide events or spallation and granulation as lava erupted into shallow water.