Holocene sedimentation in the Strait of Otranto, Mediterranean Sea

An extensive radiograph study of 24 undisturbed, up to 206-cm long box and gravity cores from the western part of the Strait of Otranto revealed a great variety of primary bedding structures and secondary burrowing features. The regional distribution of the sediments according to their structural, textural, and compositional properties reflects the major morphologic subdivisions of the strait into shelf, slope, and trough bottom (e.g., the bottom of the northern end of the Corfu-Kephallinia Trough, which extends from the northeastern Ionian Sea into the Strait of Otranto): (1) The Apulian shelf (0 to -170m) is only partly covered by very poorly sorted, muddy sands without layering. These relict(?) sands are rich in organic carbonate debris and contain glauconite and reworked (?Pleistocene) ooids. (2) The slope sediments (-170 to -1,000 m) are poorly sorted, sandy muds with a high degree of burrowing. One core (OT 5) is laminated and shows slump structures. An origin of these slumped sediment masses from older deposits higher on the slope was inferred from their abnormal compaction, color, texture, organic content, and mineral composition. (3) Cores from the northern end of the Corfu-Kephallinia Trough (-980 to -1,060 m) display a few graded sand layers, 2-5 cm (maximum 30 cm) thick with parallel and ripple-cross-laminations, deposited by oceanic bottom or small-scale turbidity currents. They are intercalated with homogeneous lutite. (4) Hemipelagic sediments prevail in the more southerly part of the Corfu-Kephallinia Trough and on the "Apulian-Ionian Ridge", the southern submarine extension of the Apulian Peninsula. Below a core depth of 160 cm, these cores have a laminated ("varved") zone, representing an Early Holocene (Boreal-Atlanticum) "stagnation layer" (14C age approximately 9,000 years). The terrigenous components of the surface sediments as well as those of the deeper sand layers can be derived from the Apulian shelf and the Italian mainland (Cretaceous Apulian Plateau and Gargano Mountains, southern Apennines, volcanic province of the Monte Vulture). Indicated by the heavy mineral glaucophane, a minor proportion of the sedimentary material is probably of Alpine origin. If this portion is considered to be first-cycle clastic material it reaches the Strait of Otranto after a longitudinal transport of 700 km via the Adriatic Sea. The lack of phyllosilicates in the coarse- to medium-grained shelf samples might be explained by the activity of the "Apulian Current" (surface velocities up to 4 knots) which in the past possibly has affected the bottom almost down to depths of the shelf edge. The percentage of planktonic organisms, and also the plankton: benthos ratio in the sediments is a useful indicator for bathymetry (depth zonation).

Chemical and isotopic compositions of zircons from an eclogitized gabbronorite dike at the Gridino village, Northern Karelia

A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geochronological) of genesis of the zircons in their various populations and, correspondingly, ages of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). Three populations of zircons separated from two rock samples comprised xenogenic, magmatic (gabbroic), and metamorphic zircons, with the latter found exclusively in the sample of retrograded eclogitized gabbroids. Group I zircons are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and geochemistry of these zircons is very diverse. Group II zircons contain mineral inclusions of ortho- and clinopyroxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons formed during the late magmatic crystallization of the gabbroids at temperatures of 1150-1160°C, and their U-Pb age 2389±25 Ma corresponds to this process. Eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that marginal portions of prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end-member. Group III zircons contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have U-Pb age 1911±9.5 Ma, which corresponds to age of overprinted amphibolite-facies metamorphism.

Mineral composition and major element oxides of gabbros from ODP Hole 179-1105A, southwest Indian Ridge

Hole 1105A penetrated 158 m of gabbros at a site offset 1.3 km east-northeast from Hole 735B on the Atlantis Bank near the Atlantis II Fracture Zone. A total of 118 m of dominantly medium- to coarse-grained intercalated Fe-Ti oxide gabbro and olivine gabbro was recovered from Hole 1105A that shows many petrographic features similar to those recovered from the upper part of Hole 735B. The main rock types are distinguished based on the constituent cumulus phases, with the most primitive gabbros consisting of olivine, plagioclase, and clinopyroxene. The inferred crystallization order is subsequently Fe-Ti oxides (ilmenite and titanomagnetite), followed by orthopyroxene, then apatite, and finally biotite. Orthopyroxene appears to replace olivine in a narrow middle interval. The magmatic evolution is likewise reflected in the mineral compositions. Plagioclase varies from An66 to An28. Olivine varies from Fo78 to Fo35. The gap in olivine crystallization occurs between Fo46 and Fo40 and coincides approximately with the appearance of orthopyroxene (~En50). The clinopyroxenes show large compositional variation in Mg/(Mg + Fe total) from 0.84 to 0.51. The nonquadrilateral cations of clinopyroxene similarly show large variations with Ti increasing for the olivine gabbros and decreasing for the Fe-Ti oxide gabbros with the decrease in Mg/(Mg + Fe total). The apatites are mainly flourapatites. The compositional variation in the gabbros is interpreted as a comagmatic suite resulting from fractional crystallization. Pyroxene geothermometry suggests equilibration temperatures from 1100°C and below. The coexisting Fe-Ti oxide minerals indicate subsolidus equilibration temperatures from 900°C for olivine gabbros to 700°C for the most evolved apatite-bearing gabbros. The cryptic variation in the olivine gabbros defines two or three lenses, 40 to 60 m thick, each characterized by a distinct convex zoning with a lower segment indicating upward reverse fractionation, a central maximum, and an upper segment showing normal fractionation. The Fe-Ti oxide gabbros show cryptic variations independent of the host olivine gabbros and reveal a systematic upward normal fractionation trend transgressing host olivine gabbro boundaries. Forward fractional crystallization modeling, using a likely parental magma composition from the Atlantis II Fracture Zone (MgO = 7.2 wt%; Mg/[Mg + Fe2+] = 0.62), closely matches the compositions of coexisting olivine, plagioclase, and clinopyroxene. This modeling suggests cosaturation of olivine, plagioclase, and clinopyroxene from 1155°C and the addition of Fe-Ti oxides from 1100°C. The liquid line of descent initially shows increasing FeO with moderately increasing SiO2. After saturation of Fe-Ti oxides, the liquid strongly decreases in FeO and TiO2 and increases in SiO2, reaching dacitic compositions at ~10% liquid remaining. The calculations indicate that formation of olivine gabbros can be accounted for by <65% fractionation and that only the residual 35% liquid was saturated in Fe-Ti oxides. The modeling of the solid fractionation products shows that both the olivine gabbro and the Fe-Ti oxide gabbros contain very small amounts of trapped liquid (<5%). The implications are that the gabbros represent crystal mush that originated in a recharging and tapping subaxial chamber. Compaction and upward melt migration in the crystal mush appear to have been terminated with relatively large amounts of interstitial liquid remaining in the upper parts of the cumulate mush. This termination may have been caused by tectonic disturbances, uplift, and associated withdrawal of magma into the subaxial dike and sill system. Prolonged compaction and cooling of the trapped melt in the mush formed small differentiated bodies and lenses by pressure release migration and crystallization along syntectonic channels. This resulted in differentiation products along lateral and vertical channelways in the host gabbro that vary from olivine gabbro, to Fe-Ti oxide gabbro, gabbronorite, and apatite gabbros and show large compositional variations independent of the host olivine gabbros.

Mineralogy and geochemistry of ODP Hole 176-735B minerals

We present a synthesis of some 20,504 mineral analyses of ~500 Hole 735B gabbros, including 10,236 new analyses conducted for this paper. These are used to construct a mineral stratigraphy for 1.5-km-deep Hole 735B, the only long section of the lower crust drilled in situ in the oceans. At long wavelengths, generally >200 m, there is a good chemical correlation among the principal silicate phases, consistent with the in situ crystallization of three or four distinct olivine gabbro bodies, representing at least two major cycles of intrusion. Initial cooling and crystallization of these bodies must have been fairly rapid to form a crystal mush, followed by subsequent compaction and migration of late iron-titanium-rich liquids into shear zones and fractures through which they were emplaced to higher levels in the lower crust where they crystallized and reacted with the olivine gabbro host rock to form a wide variety of ferrogabbros. At the wave lengths of the individual intrusions, as represented by the several olivine gabbro sequences, there is a general upward trend of iron and sodium enrichment but a poor correlation between the compositions of the major silicate phases. This, together with a wide range in minor incompatible and compatible element concentrations in olivine and pyroxene at a given Mg#, is consistent with widespread permeable flow of late melt through these intrusions, in contrast to what has been documented for a 600-m section of reputedly fast-spreading ocean crust in the Oman Ophiolite. This unexpected finding could be related to enhanced compaction and deformation-controlled late-stage melt migration at the scale of intrusion at a slow-spreading ocean ridge, compared to the relatively static environment in the lower crust at fast-spreading ridges.

Petrology and geochemistry of basalts from the Vavilov Basin

During ODP Leg 107, two holes were drilled in the basement of Vavilov Basin, a central oceanic area of the Tyrrhenian sea. Hole 655B is located near the Gortani ridge in off-axis position at the western rim of the basin; Hole 651A is located on a basement swell at the axis of the basin. This paper deals with mineral chemistry, major and trace element geochemistry, and petrogenesis of the basalts recovered in the two holes. The mineralogy of the basalts is broadly homogeneous, but all of them have suffered important seawater alteration. Their major-element compositions are similar to both normal-mid-ocean-ridge-basalts (N-MORB) and back-arc-basalts (BAB) except for Na2O contents (BAB-like), and K2O which is somewhat enriched in upper unit of Hole 651 A. Their affinity with N-MORB and BAB is confirmed by using immobile trace elements such as Zr, Y, and Nb. However, basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements. Hole 655B basalts are homogeneous enriched tholeiites, similar to those from DSDP Hole 373 (located on the opposite side of the basin near the eastern rim), and show affinities with enriched MORB (E-MORB). At Hole 651 A, the two basalt units are chemically distinct. One sample recovered in lower unit is rather similar to those from Hole 655B, but basalts from upper unit display calc-alkaline characteristic evidenced by the increase of light-ion-lithophile-element (LILE)/high-field-strength-element (HFSE) ratio, and appearance of a negative Nb-anomaly, making them comparable with orogenic lavas from the adjacent Eolian arc. The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N-MORB type source progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active subduction retreats southeastward.

Chemical composition of ferromanganese manifestations from the Sea of Okhotsk

New data on microstructures and mineral and chemical compositions of ferromanganese crusts sampled from the western slope of the Kuril Island Arc in the Sea of Okhotsk during cruises of R/V Vulkanolog are discussed. The study of the crusts using analytical electron microscopy methods revealed that their manganese phase is represented by vernadite, Fe-vernadite, todorokite, asbolane, and asbolane-buserite, while iron phase consists of hematite, hydrohematite, ferroxyhite, and magnetite. Lithic mineral assemblage includes apatite, quartz, epidote, and montmorillonite. According to chemical analysis most of the crusts contain significant part of volcanogenic and hydrothermal material. It is evident from elevated values of Mn/Fe and (Mn+Fe)/Ti ratios, low concentrations of some trace elements, and positive Eu anomaly.

Sediments and tephra layers from the southern Tyrrhenian abyssal plain near Marsili Seamount

From the south-eastern Tyrrhenian deep-sea floor, four sediment cores of "Meteor" cruise 22 (1971) are described. These cores were taken in the basin between the Aeolian Islands and the Marsili Seamount, an elevation of more tha 3000 m above the sea floor. The sedimentation of the deep-sea basin is distinguished by a sequence of turbidites with a high sedimentation rate. The composition of the clastic material and the position of the cores in the mouth area of the morphologically very pronounced Stromboli Canyon suggest an interpretation of the turbidite sequence as fan of this canyon onto the deep-sea floor. A white rhyolitic pumice-tephra at the base of the 4 m thick sequence of turbidites in core M22-102 has been correlated with the Pelato eruption of the island of Liparo in the 6th century A.D. At the foot of the Marsili Seamount - apparently in morphologically elevated positions - the influence of the turbidite sedimentation increases, the rate of sedimentation is lower and stratigraphic omissions are probable. Here, rather compacted globigerina marls have been found in only 15 -25 cm depth. In addition, volcanic material in the form of lapilli layers, palagonitized ashes and detrital volcanic sands of the Marsili Seamount have been encountered in this area. An up to 3 cm thick layer of completely palagonitized basaltic ash intercalates with the marls at the base of two cores. Layers of very fresh olivine basaltic lapilli in core 103 and palagonitized lapilli of latitic composition in core 104 testify to an explosive submarine volcanism of the Marsili Seamount. According to the stratigraphy of core 103, the latest manifestations of this basaltic volcanism belong to the late Pleistocene (Emiliana huxleyi-zone of Nannoplankton stratigraphy) The basaltic lapilli are glassy to perhyaline with phenocrysts or microphenocrysts predominantely of olivine. The petrological character of the basaltic volcanites with high MgO, Ni, Cr and high MgO/FeO- and Ni/Co-ratios exhibits primitive basaltic features. These basalts clearly differ from basalts of the ocean floors, mid-ocean ridges and marginal basins. Prominent features are a missing iron-enrichment trend and low TiO2. Al2O3 tends to be high, as well as K2O and related trace elements (Ba, Sr). In spite of silica undrsaturation and high color index, the Marsili basalt exhibit some analogies with the calcalkaline basalts of the Aeolian arc, as well as the undersaturated basalts of some other circumoceanic areas.

Chemical composition of Ninetyeast Ridge basalts

On Leg 121 of the Ocean Drilling Program, we recovered basaltic rocks from a total of three basement sites in the southern, central, and northern regions of Ninetyeast Ridge. These new sites complement the previous four basement holes drilled during Legs 22 and 26 of the Deep Sea Drilling Project, and confirm the predominantly tholeiitic, light rare earth element-enriched character of the basalts that cap the ridge. The basalts show marked iron enrichment; ferrobasalts occur at Sites 214 and 216 and oceanic andesites at Site 253. All of the basalts recovered during Leg 121 are altered, and range from aphyric olivine tholeiites (Site 756), to strongly plagioclase-phyric basalts (Site 757). Basalts from Site 758, which were clearly erupted in a submarine environment (pillow basalts are present in the section), are sparsely to strongly plagioclase-phyric. The basalts recovered at any one hole are isotopically homogeneous (except for the basalts from Site 758, which show a range of Pb isotopes), and it is possible to relate the magmas at any one site by high-level fractionation processes. However, there are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. Such variation, in view of the large volume of magmatic products that form the ridge system, is not surprising. There is not, however, a systematic variation in basalt composition along the ridge. We agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge (e.g., Iceland, Galapagos Islands, and St. Paul/Amsterdam islands). The absence of recovered alkalic suites is inconsistent with an intraplate setting, such as the Hawaiian Islands or Kerguelen Island. Thus, the major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelen plume and mid-ocean Ninetyeast Ridge basalt mantle.

Heavy mineral abundance of ODP Leg 174A sites

The purpose of this paper is to report the heavy mineral content of Miocene to Pleistocene sequences drilled during Ocean Drilling Program Leg 174A on the New Jersey Shelf. Sandy intervals recovered from Holes 1071A, 1071F, 1072A, and 1073A were sampled for heavy mineral analysis. Because of the low core recovery of the sandy parts of the succession, sampling has been incomplete. In spite of the resulting restriction and because of major variations in heavy mineral assemblages, eight distinct heavy mineral associations could be defined. The data presented thus considerably extend the present knowledge on the lithology of the stratigraphic record as described by Austin, Christie-Blick, Malone, et al. (1998). In this chapter the heavy mineral associations and their assignment to particular sequences are described.

Geochemistry, trace elements and minerals at DSDP Holes 80-550B and 80-551

The Leg 80 basalts drilled on the Porcupine Abyssal Plain 10 km southwest of Goban Spur (Hole 550B) and on the western edge of Goban Spur (Hole 551), respectively, are typical light-rare-earth-element- (LREE-) depleted oceanic tholeiites. The basalts from the two holes are almost identical; most of their primary geochemical and mineralogical characteristics have been preserved, but they have undergone some low-temperature alteration by seawater, such as enrichment in K, Rb, and Cs and development of secondary potassic minerals of the "brownstone facies." K/Ar dating fail to give realistic emplacement ages; the apparent ages obtained become younger with alteration (causing an increase in K2O). Hole 551 basalts are clearly different from the continental tholeiites emplaced on the margins of oceanizing domains during the prerift and synrift stages.