Chemical and mineral composition of rocks and sediments from the Amirante Arc

New geological and geophysical data on the Amirante Arc, which locates to the south of the Seychelles Islands, are presented. These data were obtained by Pacific Oceanological Institute during the 33-rd cruise of R/V Professor Bogorov in 1990. The Amirante Arc represents a seamount chain, which has submeridional strike and total length about 400 km. To the west of the Amirante Arc there are a deep sea trench and a back-arc basin, i.e. this area is characterized by structural elements associated with the subduction zone of Western Pacific type. According to our data the Amirante Arc is composed by tholeiites of ocean plateau type. This facts are evidences that the Amirante Arc differs from typical Pacific island arcs. This gives an opportunity to distinguish a special type of oceanic structures, i.e. non-volcanic (amagmatic) ridges. The Amirante Ridge has been probably formed as a result of oceanic crust heaping due to horizontal displacements of its blocks in the process of spreding ridge formation in the Indian Ocean during Cretaceous-Paleogene.

Grain characteristics and XRD raw data of ODP Sites 162-981 and 162-984

Multisensor track data, including magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE) wet bulk density, and natural gamma emission, were collected on all cores recovered during Ocean Drilling Program Leg 162. Data from the upper Pliocene and lower Pleistocene of Sites 981 and 984 are here compared to results from analyses of a limited set of discrete samples, including benthic foraminiferal isotopic composition, grain size, carbonate content, abundance of foraminifers and lithic particles, and clay mineralogy. Natural gamma emission most closely monitors the input of felsic terrigenous material to these two sites. Magnetic susceptibility also tracks felsic terrigenous input at Site 981 but appears to reflect a separate, more mafic, terrigenous component at Site 984. The GRAPE record does not correlate well with any discretely measured variable at Sites 981 or 984.

Major element oxides and 40Ar/39Ar plateau and isochron ages of ODP Hole 165-1001A

40Ar-39Ar incremental heating experiments and electron microprobe analyses were performed on basaltic rocks recovered from Site 1001 during Ocean Drilling Program Leg 165. The lower Nicaraguan Rise, on which Site 1001 lies, appears to be part of a larger Caribbean oceanic plateau that makes up the core of the Caribbean plate. Our results indicate an eruption age of 81 ± 1 Ma. A single flow-rim glass is tholeiitic and almost identical to the shipboard X-ray fluorescence analyses of the whole rock. The slightly porphyritic basalts have at least two populations of plagioclase, groundmass, and glomerocrystic plagioclase laths that appear to be in equilibrium with the surrounding melt and corroded tabular phenocrysts that have a higher An content (An84-86).

(Table 1) Geochemistry of upper Paleocene and upper Eocene glass particles at DSDP Hole 80-549

Two ash horizons have been identified in Hole 549, one in the upper Paleocene (basal NP9), the other in the upper Eocene (NP18); both are mixed lithic crystal tuffs of rhyolitic composition. These tuffs are absent in Hole 550 owing to unconformities, but the basal Eocene (NP10) of Hole 550 includes a series of over 50 thin bentonite layers. Intermediate plagioclase associated with these bentonites indicates that the original ash was of basaltic to andesitic composition. The bentonites are absent in Hole 549, probably because of an unconformity, but they have been identified in Hole 401 (Leg 48, Bay of Biscay). Two of the pyroclastic phases can be matched with phases previously reported for the North Sea Basin. The bentonites of Site 550 are probably equivalent to the widespread "ash series" of northwestern Europe, which may therefore be regarded as being lower Eocene in terms of Martini's calcareous nannoplankton zonation.

Geochemical investigations of volcanic ash layers from ODP Leg 119

Geochemical investigations were carried out on 19 discrete ash layers and on 42 dispersed ash accumulations in Oligocene to Pleistocene sediments from Sites 736, 737, 745, and 746 of ODP Leg 119 (Kerguelen Plateau in the southern Indian Ocean). The chemical data obtained from more than 500 single-grain glass analyses allow the characterization of two dominant petrographic rock series. The first consists of transitional- to alkali-basalts, the second mainly of trachytes with subordinated alkali-rhyolites and rhyolites. Chemical correlation with possible source areas indicates that the tephra layers from the northern Kerguelen Plateau Sites 736 and 737 were probably erupted from the nearby Kerguelen Islands. The investigated ash layers clearly reflect the Oligocene to recent changes in the composition of the volcanic material recorded from the Kerguelen Islands. The dispersed ashes from Sites 745 and 746 in the Australian-Antarctic Basin display almost the same range in chemical compositions as those from the north. Heard Island and other sources may have contributed to their formation, in addition to the Kerguelen Islands. Dispersed ash of calc-alkaline composition is most probably derived from the South Sandwich island arc, indicating sea-ice rafting as an important mechanism of transport.

Geochemistry of clinopyroxene megacrysts, xenocrysts, and phenocrysts in diabase dikes from ODP Hole 140-504B

We report the major, rare earth, and other trace element compositions of clinopyroxenes from two Leg 140, Hole 504B diabase dikes. These pyroxenes reflect a complex history of crystal growth and magma evolution. The large ranges of composition found reflect incorporation of exotic phenocrysts into the melt, the early formation of crystal clots before dike intrusion during an undercooling event, and in-situ fractionation of melt during and following dike emplacement. Some of the pyroxenes occur in coarse two- and three-phase glomerocrysts, which may be ôprotogabbrosö representing early stages of melt crystallization in the lower crust. Large variations in trace element composition are found. These likely reflect heterogeneous nucleation and growth of plagioclase and pyroxene in the melt, as well as complex interface kinetics that may affect partition coefficients during rapid crystal growth expected during undercooling. This can explain the formation of irregular chemical sector zoning in some equant anhedral phenocrysts. Undercooling of magmas in the lower crust most likely reflects input of fresh hot melt into a stagnating melt-storage zone. Dikes intruded upward from an inflated melt-storage zone during such a cycle are likely to be larger than those intruded from the storage zone between such cycles, when it would be deflated, consistent with the greater overall thickness of the phyric dikes in the Leg 140 section of Hole 504B.

Chemical and mineral compositions of bottom sediments from the Sea of Japan

Physical properties (water content, bulk density, magnetic susceptibility, natural remanent magnetization, nature of magnetization, and composition of ferromagnetic fraction), chemical, and (optionally) mineral composition of bottom sediments from the north-west Sea of Japan have been studied. Their stratigraphic subdivision based on composition of diatoms has been carried out. Obtained data have allowed to find out some aspects of influence of paleogeographic conditions and diagenetic processes on change of physical properties of the sediments, as well as on their composition in Holocene and Late Pleistocene.

Petrology and geochemistry of basalt at DSDP Hole 66-487

Major oxide and trace element determinations of the composition basalts from the bottom of Hole 487, together with microprobe analyses of their minerals (olivine, magnesiochromite, salite, and plagioclase), prove that they are depleted oceanic tholeiites.

Geochemistry of marin ash layers and epiclastic rocks from the Kerguelen Plateau

Epiclastic volcanogenic rocks recovered from the Kerguelen Plateau during Ocean Drilling Program Legs 119 and 120 comprise (pre-)Cenomanian(?) claystones (52 m thick, Site 750); a Turonian(?) basaltic pebble conglomerate (1.2 m thick, Site 748; Danian mass flows (45 m thick, Site 747); and volcanogenic debris flows of Quaternary age at Site 736 (clastic apron of Kerguelen Island). Pyroclastic rocks comprise numerous Oligocene to Quaternary marine ash layers. The epiclastic sediments with transitional mid-ocean-ridge basalt (T-MORB) origin indicate weathering (Site 750) and erosion (Site 747) of Early Cretaceous T-MORB from a then-emergent Kerguelen Plateau, connected to Late Cretaceous tectonic events. The basal pebble conglomerate of Site 748 has an oceanic-island basalt (OIB) composition and denotes erosion and reworking of seamount to oceanic-island-type volcanic sources. The vitric- to crystal-rich marine ash layers are a few centimeters thick, have rather uniform grain sizes around 60 ± 40 µm, and are a result of Plinian eruptions. Crystal-poor silicic vitric ashes may also represent co-ignimbrite ashes. The ash layers have bimodal, basaltic, and silicic compositions with a few intermediate shards. The basaltic ashes are evolved high-titanium T-MORB; a few grains in a silicic pumice lapilli layer have a low-titanium basaltic composition. The silicic ashes comprise trachytic and rhyolitic glass shards belonging to a high-K series, except for a few low-K glasses admixed to a basaltic ash layer. Feldspar and clinopyroxene compositions fit the glass chemistry: high-Ti tholeiite-basaltic glasses have Plagioclase of An40-80 and pigeonite to augite clinopyroxene compositions. Silicic ashes have K-rich anorthoclase and minor Plagioclase around An20 and ferriaugitic to hedenbergitic clinopyroxene compositions. The line of magmatic evolution for the glass shards is not compatible with simple two-end member (high-Ti T-MORB and high-K rhyolite) mixing, but favors successive Ca-Mg-Fe pyroxene, Ti magnetite, and apatite fractionation, and K-rich alkali feldspar fractionation in trachytic magmas to yield rhyolitic compositions. Plagioclase fractionation occurs throughout. This qualitative model is in basic accordance with the observed mineral assemblage. However, as the time span for explosive volcanism spans >30 m.y., this basic model cannot comply with fractional crystallization in a single magma reservoir. The ash layers resulted from highly explosive eruptions on Kerguelen and, with less probability, Heard islands since the Oligocene. The explosive history starts with widespread Oligocene basaltic ash layers that indicate sea-level or subaerial volcanism on the Northern Kerguelen Plateau. After a hiatus of 24 m.y.(?), explosive magmatic activity was vigorously renewed in the late Miocene with more silicic eruptions. A peak in explosive activity is inferred for the Pliocene-Pleistocene. The composition and evolution of Kerguelen Plateau ash layers resemble those from other hotspot-induced, oceanic-island realms such as Iceland and Jan Mayen in the North Atlantic, and the Canary Islands archipelago in the Central Atlantic.

Late Cretaceous volcaniclastic rocks at DSDP Holes 74-525A and 74-528

Volcaniclastic rocks of Late Cretaceous age occur in four out of five sites (525, 527, 528, 529) drilled on the crest and the northwest flank of the Walvis Ridge during Leg 74. They are mostly interlayered with and overlie basement in the lowermost 10-100 m of the sedimentary section. Rocks from Holes 525A and 528 were studied megascopically and microscopically, by XRD, and XRF chemical analyses of whole-rock major and trace elements were undertaken. The dominant rock of Hole 528 volcaniclastics is a fine-grained (silt to fine sand), mostly matrix-bearing (partly matrix-rich) vitric "tuff," occurring as 5-110 cm thick, partly graded layers, some of which are distinctly bedded. Volcaniclastics of Hole 525A are generally richer in sanidine crystals. Most rocks contain some nonvolcanic clasts, chiefly foraminifers and lesser amounts of shallow-water fossil debris. Scoria shards, clasts of tachylite, and fine-grained basalts as well as chemical analyses suggest a basaltic to intermediate composition for most rocks of Hole 528, whereas volcaniclastics of Hole 525A are more silicic. The occurrence of tachylite and epiclastic, coarse-grained, basaltic clasts throughout the volcaniclastic sequence at Site 528 indicates shallow-water eruptions and perhaps even ocean island volcanism. The minor occurrence in Hole 528 of trachytic? pumice shards with phenocrysts of K-feldspar and the abundance of such shards in rocks from Hole 525A indicate Plinian eruptions characteristic of more mature stages of ocean island evolution. The sedimentary structures of volcaniclastic layers and their occurrence within deep sea calcareous oozes indicate a mass flow origin. Diagenetic alteration of the volcaniclastic rocks is pronounced, and four major stages of glass shard alteration are distinguished. Despite the effects of alteration and small-scale redistribution of elements and the admixture of nonvolcanic components, there were no drastic changes in the chemical composition of the rocks, except for pronounced increases in K and Rb and decreases in Ca and Fe. The basaltic volcaniclastic rocks very much resemble basement basalts in that they are moderately evolved tholeiites derived from an LIL-enriched mantle source with Zr/Nb ratios (Hole 528) of 5 to 6. This, in conjunction with the interbedding of volcaniclastic rocks and basement lavas, indicates contemporaneous seamount or island and basement volcanic activity involving magmas derived from similar sources.

Chemical composition of basalts and basaltic glasses from the Sierra Leone Fracture Zone region

Major and trace element (including REE) geochemistry of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5-7°10'N) has been studied. Associations of basalts of various compositions with particular ocean-floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts (Variety 1) that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwestern flank of the MAR crest zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370°C) and enriched tholeiites (45-78 km, 1330-1450°C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in melts parental to rift-related basalts occurring near the plume. T- and P-MORBs are also developed near areas where mantle plumes are localized. High-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov Fracture Zone) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. Likely causes of temporal and spatial instability of the mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In sulfide-bearing rift morphostructures (so-called "Ore area'' and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. Functioning of these chambers is a combined effect of singular geodynamic regime and plume activity. In these chambers melts undergo deep differentiation leading to progressively increasing concentration of sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

Mineral composition and geochemistry of sediments from ODP Leg 127 sites

The lithostratigraphy of Neogene hemipelagic sediments recovered from the Japan Sea during Leg 127 was revised to improve intersite consistency and to remove confusion stemming from diagenetic modification of the lithology through the opal-A to opal-CT transformation. Special emphasis was put on the presence and nature of dark-light cycles in revising the lithostratigraphy. Mineral composition analysis was conducted for samples from Sites 794, 795, and 797. In addition, major element chemical composition analysis was conducted for these same sample sets from Site 794. The result of mineral composition analysis suggests that the detrital component, which consists of such minerals as quartz, plagioclase, illite, and kaolinite plus chlorite, is diluted to various degrees by biogenic silica (opal-A) and its diagenetic equivalents (opal-CT and quartz). Smectite, on the other hand, may be a diagenetic or hydrothermal alteration product of volcanic material, although more study is necessary to confirm its origin. As a whole, vertical variation in the sediment composition is consistent with the revised lithostratigraphy and helps to characterize the redefined lithologic units quantitatively.

Chemical composition of rocks and minerals from the Karymsky volcanic center, Eastern Kamchatka

The powerful eruption in the Akademii Nauk caldera on January 2, 1996 marked a new activity phase of the Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82-72), plagioclase (An92-73), and clinopyroxene (Mg# 83-70) in basalts of the 1996 eruption. The data were used to estimate composition of the parental melt and physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesium, high aluminum basalt (SiO2 = 50.2%, MgO = 5.6%, Al2O3 = 17%) of the mildly potassium type (K2O = 0.56%) and contained much dissolved volatile components (H2O = 2.8%, S = 0.17%, and Cl = 0.11%). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at pressure ~1.5 kbar, proceeded within a narrow temperature range of 1040+/-20°C, and continued until near-surface pressure ~100 bar was reached. Degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under pressure <1 kbar. Magma degassing in an open system resulted in escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. Release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated as 1.7x10**6 t H2O, 1.4x10**5 t S, and 1.5x10**4 t Cl. Concentrations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in plagioclase were found to be selectively enriched in Li. The Li-enriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Li-rich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption.

Heavy and light minerals in turbidites from DSDP Hole 19-183, the Aleutian abyssal plain

The Aleutian abyssal plain is a fossil abyssal plain of Paleogene age in the western Gulf of Alaska. The plain is a large, southward-thinning turbidite apron now cut off from sediment sources by the Aleutian Trench. Turbidite sedimentation ceased about 30 m.y. ago, and the apron is now buried under a thick blanket of pelagic deposits. Turbidites of the plain were recovered at site 183 of the Deep Sea Drilling Project on the northern edge of the apron. The heavy-mineral fraction of sand-sized samples is mostly amphibole and epidote with minor pyroxene, garnet, and sphene. The light-mineral fraction is mostly quartzose debris and feldspars. Subordinate lithic fragments consist of roughly equal amounts of metamorphic, plutonic, sedimentary, and volcanic grains. The sand compositions are arkoses in many sandstone classifications, although if fine silt is included with clay as matrix, the sand deposits are feldspathic or lithofeldspathic graywacke. The sands are apparently first-cycle products of deep dissection into a plutonic terrane, and they contrast sharply with arc-derived volcanic sandstones of similar age common on the adjacent North American continental margin. The turbidite sands are stratigraphically remarkably constant in composition, which indicates derivation from virtually the same terrane through a time span approaching 20 m.y. Comparison of Aleutian plain data with the compositions of coeval sedimentary rocks from the northeast Pacific margin shows that the Kodiak shelf area includes possible proximal equivalents of the more distal turbidites. Derivation from the volcaniclastic Mesozoic flysch of the Shumagin-Kodiak shelf is unlikely; more probably the sediments were derived from primary plutonic sources. The turbidites also resemble deposits in the Chugach Mountains and the younger turbidites of the Alaskan abyssal plain and could conceivably have been derived from the coast ranges of southeastern Alaska or western British Columbia. The Aleutian plain sediment most likely was not derived from as far south as the Oregon-Washington continental margin, where coeval sedimentary deposits are dominantly volcaniclastic.