Igneous geochemistry of OP Leg 126 samples

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126 in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxene basaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of the geochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematics they are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggest a systematic geochemical gradation between volcanics related to trench distance and a variably depleted source. A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxene andesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents of Ti, Zr, and low-field-strength elements (LFSE). Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predicted during crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) are anomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive and pillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesite glass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basement lavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a result of this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related to andesitic or dacitic precursors by silica removal or addition.

Edipo saliendo de la caverna

El problema del conocimiento ha sido uno de los que mayor importancia ha tenido durante los siglos VI y IV a.C. en Grecia. Debido a esta importancia, dicho problema ha sido abordado desde diferentes perspectivas y ámbitos, como la poesía, la filosofía, la historia, etc. El objetivo del presente trabajo es hacer manifiestas las similitudes que existen entre el modo en que Sófocles concibe el conocimiento, y el modo en que lo concibe Platón. Para ello se analizaran pasajes del Edipo rey, de República y de Carta VII

La ironía en el Edipo Rey, (Prólogo y Episodio I). Hacia una redefinición y una propuesta tipológica de esta estrategia discursiva

En las últimas décadas los lingüistas dedicaron especial atención a la ironía, proponiendo diversas definiciones, mutuamente excluyentes que, quizás por prejuicios de escuela, priorizaban un solo aspecto de csta estrategia discursiva. Basándonos en la lectura de los primeros tramos del Edipo Rey, paradigmáticamente irónicos, intentaremos llegar a una definición comprensiva del recurso y propondremos una categorización de las ironías presentes en el texto, organizada sobre un doble eje enunciativo y retórico

La ironía en el Edipo Rey, (Prólogo y Episodio I). Hacia una redefinición y una propuesta tipológica de esta estrategia discursiva

En las últimas décadas los lingüistas dedicaron especial atención a la ironía, proponiendo diversas definiciones, mutuamente excluyentes que, quizás por prejuicios de escuela, priorizaban un solo aspecto de csta estrategia discursiva. Basándonos en la lectura de los primeros tramos del Edipo Rey, paradigmáticamente irónicos, intentaremos llegar a una definición comprensiva del recurso y propondremos una categorización de las ironías presentes en el texto, organizada sobre un doble eje enunciativo y retórico

Edipo saliendo de la caverna

El problema del conocimiento ha sido uno de los que mayor importancia ha tenido durante los siglos VI y IV a.C. en Grecia. Debido a esta importancia, dicho problema ha sido abordado desde diferentes perspectivas y ámbitos, como la poesía, la filosofía, la historia, etc. El objetivo del presente trabajo es hacer manifiestas las similitudes que existen entre el modo en que Sófocles concibe el conocimiento, y el modo en que lo concibe Platón. Para ello se analizaran pasajes del Edipo rey, de República y de Carta VII

Edipo saliendo de la caverna

El problema del conocimiento ha sido uno de los que mayor importancia ha tenido durante los siglos VI y IV a.C. en Grecia. Debido a esta importancia, dicho problema ha sido abordado desde diferentes perspectivas y ámbitos, como la poesía, la filosofía, la historia, etc. El objetivo del presente trabajo es hacer manifiestas las similitudes que existen entre el modo en que Sófocles concibe el conocimiento, y el modo en que lo concibe Platón. Para ello se analizaran pasajes del Edipo rey, de República y de Carta VII

Diatom biostratigraphy of sediments from the Kerguelen Plateau, Southern Ocean

The biostratigraphic distribution and abundance of lower Oligocene to Pleistocene diatoms is documented from Holes 747A, 747B, 748B, 749B, and 751A drilled during Ocean Drilling Program Leg 120 on the Kerguelen Plateau in the southeast Indian Ocean. The occurrence of middle and upper Eocene diatoms is also documented, but these are rare and occur in discrete intervals. The recovery of several Oligocene to Pleistocene sections with minimal coring gaps, relatively good magnetostratigraphic signatures, and mixed assemblages of both calcareous and siliceous microfossils makes the above four Leg 120 sites important biostratigraphic reference sections for the Southern Ocean and Antarctic continent. A high-resolution diatom zonation divides the last 36 m.y. into 45 zones and subzones. This zonation is built upon an existing biostratigraphic framework developed over the past 20 yr of Southern Ocean/Antarctic deep-sea coring and drilling. After the recent advances from diatom biostratigraphic studies on sediments from Legs 113, 114, 119, and 120, a zonal framework for the Southern Ocean is beginning to stabilize. The potential age resolution afforded by the high-diversity diatom assemblages in this region ranks among the highest of all fossil groups. In addition to the 46 datum levels that define the diatom zones and subzones, the approximate stratigraphic level, age, and magnetic anomaly correlative of more than 150 other diatom datums are determined or estimated. These total 73 datum levels for the Pliocene-Pleistocene, 67 for the Miocene, and 45 for the Oligocene. Greater stratigraphic resolution is possible as the less common and poorly documented species become better known. This high-resolution diatom stratigraphy, combined with good to moderately good magnetostratigraphic control, led to the recognition of more than 10 intervals where hiatuses dissect the Oligocene-Pleistocene section on the Kerguelen Plateau. We propose 12 new diatom taxa and 6 new combination

La ironía en el Edipo Rey, (Prólogo y Episodio I). Hacia una redefinición y una propuesta tipológica de esta estrategia discursiva

En las últimas décadas los lingüistas dedicaron especial atención a la ironía, proponiendo diversas definiciones, mutuamente excluyentes que, quizás por prejuicios de escuela, priorizaban un solo aspecto de csta estrategia discursiva. Basándonos en la lectura de los primeros tramos del Edipo Rey, paradigmáticamente irónicos, intentaremos llegar a una definición comprensiva del recurso y propondremos una categorización de las ironías presentes en el texto, organizada sobre un doble eje enunciativo y retórico

Age determination and stable isotope record of sediment cores at 9°S in the Indian Ocean

A continuous 10-m-long section consisting of roughly two thirds Ethmodiscus rex (a diatom) and one third mixed planktonic foraminifera was identified in a core from 3800 m depth at 9°S on the Indian Ocean's 90°E Ridge. Radiocarbon dates place the onset of deposition of this layer at >30,000 years B.P. and its termination at close to 11,000 years B.P. However, precise dating of the foraminifera from the Ethmodiscus layer itself proved to be impossible owing to the presence of secondary calcite presumably precipitated from the pore waters. During the Holocene, high calcium carbonate content ooze free of diatoms was deposited at this locale. As the site currently lies beneath the pathway taken by upper ocean waters entering the Indian Ocean from the Pacific (via the Indonesian Straits), it appears that during glacial time, thermocline waters moving along this same path provided the silica and other nutrients required by these diatoms.

Middle Miocene to Quaternary diatom biostratigraphy of DSDP Site 90-594

A diatom biostratigraphy is presented for middle Miocene through Quaternary sediments recovered from the Chatham Rise east of New Zealand's South Island. The upper 590 m of the 639.5-m composite-section Site 594 represents approximately 16 m.y. and is characterized by moderately to very poorly preserved diatoms of antarctic to temperate affinity. Pliocene through Quaternary assemblages are poorly preserved and dominated by antarctic-subantarctic species which provide detailed biostratigraphic control. Recognized are 11 of 14 zones of the middle upper Miocene to Quaternary Neogene Southern Ocean diatom zonation (NSD 7-NSD 20) of Ciesielski (1983; this chapter). Four Neogene Southern Ocean diatom zones (NSD 3-NSD 6) are recognized in the lower middle Miocene to middle upper Miocene of Site 594. Assemblages of this interval have a mixed high-latitude and temperate affinity; however, poor preservation limits correlation to high- and temperate-latitude zonal schemes. Neogene North Pacific diatom zones and subzones of NNPD 3 through NNPD 5 (Barron, in press, b) are correlated to Neogene Southern Ocean diatom zones NSD 3 through NSD 7: the upper portions of the Actinocyclus ingens Zone (NNPD 3) is correlative to the upper Nitzschia maleinterpretaria Zone (NSD 3); the Denticulopsis lauta Zone (NNPD 4) and Subzones a and b are correlative to the lower Coscinodiscus lewisianus Zone (NSD 4); and the D. hustedtü-D. lauta Zone (NNPD 5) and its Subzones a through d encompass the upper C. lewisianus Zone (NSD 4), N. grossepunctata Zone (NSD 5), N. denticuloides Zone (NSD 6), and the lower D. hustedtii-D. lauta Zone (NSD 7). A major disconformity spans the late Gilbert to early Gauss Chron (3.9-2.8 Ma). A second disconformity brackets the Miocene/Pliocene boundary; the section missing covers late Chron 5 and the early Gilbert chron (5.5-4.6 Ma). The remainder of the siliceous-fossil-bearing Miocene sediments at Site 594 appear to be correlative to lower paleomagnetic Chronozone 5 through upper Chronozone 16. Uppermost lower Miocene or lowermost middle Miocene sediments in the basal 50 m of Hole 594A are barren of diatoms.

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.

Occurrence and abundance of diatoms at DSDP Holes 72-515B and 72-516F

Samples from the upper Oligocene and lower Miocene of Holes 515B (Brazil Basin) and 516F (Rio Grande Rise) were examined for fossil marine diatom content. The preservation of the diatoms was poor and the species diversity low in both holes. However, it was possible to zone portions of the intervals studied using the zonation proposed by Gombos and Ciesielski (1983), which is based, as far as possible, on common and robust species. Thus, the interval in Hole 515B represented by Cores 515B-15 and 515B-16 is assigned to the Coscinodiscus rhombicus Zone and the interval represented by Cores 515B-17 through 515B-44 is assigned to the Rocella gelida Zone. The C. rhombicus Zone is early Miocene in age and the R. gelida Zone is late Oligocene to early Miocene in age. In Hole 516F the interval represented by Cores 516F-6 through 516F-10 is assigned to the R. gelida Zone Gate Oligocene to early Miocene), and the interval represented by Cores 516F-11 through 516F-15 is assigned to the Triceratium groningensis Zone (late Oligocene). Two new fossil diatom taxa are defined herein: Coscinodiscus lewisianus Greville f. concavus n. f. and Rocella semigelida n. sp.

Diatom biostratigraphy and datum events of ODP Hole 183-1140A

Ocean Drilling Program (ODP) Leg 183 Site 1140 provided a lower Oligocene to middle Miocene record of diatom assemblages from the northern Kerguelen Plateau. Samples were examined to improve the resolution of shipboard diatom biostratigraphy. The material is complementary to that recovered during ODP Legs 119 and 120, and the diatom zonation of Harwood and Maruyama could be readily applied. A standard succession of biostratigraphic zones from the middle Miocene and lower Oligocene was delineated, although some zones were unrecognizable because of poor core recovery. The detailed diatom biostratigraphy presented here agrees well with shipboard calcareous nannofossil biostratigraphy. Sediment accumulation rates based on diatom bioevents average 1.26 cm/k.y.

Composition of palygorskite-rich and montmorillonite-rich zeolite-containing sediments from DSDP Sites 164 and 196

Cretaceous, Tertiary, and Quaternary sediments from Deep Sea Drilling Project Sites 164 and 196 (13°12' N, 161°31' W and 30°07' N, 148°34' E, respectively) were analyzed for major chemical elements and mineralogy. Sediments from these sites contain large proportions of authigenic minerals: mainly palygorskite, clinoptilolite and chert in the Cretaceous, and montmorillonite, phillipsite and chert in the Tertiary. The montmorillonite-phillipsite assemblage is thought to be derived from volcanic ash or glass, and the palygorskite-clinoptilolite assemblage is thought to be derived by reaction of biogenic silica with volcanic ash or glass or with montmorillonite and phillipsite. Both assemblages have generally moderate Ti/Al ratios, ranging from 0.026 to 0.047, so most of the palygorskite, clinoptilolite, montmorillonite and phillipsite could not be derived in situ from alteration of basaltic material. Plagioclase compositions suggest that the volcanic precursors were silicic or intermediate, but it is also possible that the sediments have been extensively fractionated by redistribution from nearby seamounts. Available data on other Late Cretaceous sediments in the Pacific were analyzed. Clinoptilolite and chert are present nearly everywhere where palygorskite is abundant; phillipsite is rare where palygorskite is abundant. It is suggested that increased water temperatures during the Cretaceous increased reaction rates and determined the alteration products.