Composition of Fe-Mn minerals from island arcs of the West Pacific

This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kuril, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxyhydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxyhydroxides are todorokite and "Ca-birnessite". The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxyhydroxides.

Clay mineralogy of the Cenozoic Pagodroma Group, East Antarctica

The Cenozoic Pagodroma Group in the northern Prince Charles Mountains, East Antarctica, is a glaciomarine succession of fjordal character, comprising four uplifted formations of different ages. The composition of the <2 µm fraction of sediments of the Pagodroma Group was analysed in order to help identify source areas, past weathering conditions and glacial regimes. Both clay and non-clay minerals have been quantified. The assemblage of the upper Oligocene to lower Miocene Mount Johnston Formation is characterised by the dominance of illite and intermediate concentrations of chlorite. Similar to that assemblage is the clay mineral suite of the middle Miocene Fisher Bench Formation, where illite and chlorite together account for 95% of the clay minerals. The middle to upper Miocene Battye Glacier Formation is the only formation with significant and persistent smectite concentrations, although illite is still dominant. The kaolinite concentration is also high and is even higher than that of chlorite. The clay fraction of the upper Pliocene to lower Pleistocene Bardin Bluffs Formation is characterised by maximum kaolinite concentrations and relatively low illite and chlorite concentrations. The bulk of the clay fraction in each formation can be explained by the physical weathering and erosion of a nearby source under glacial conditions. In the case of Mount Johnston Formation and Fisher Bench Formation this source may be situated in the metavolcanic and gneissic rocks of Fisher Massif. The sediments of the Bardin Bluffs Formation indicate a local source within the Amery Oasis, where Proterozoic granitoid rocks and gneisses, and Permo-Triassic fluvial rocks of the Amery Group are exposed. These results suggest a strong local imprint on the glacial sediments as northwards flowing ice eroded the bedrock in these areas. The origin of the clay fraction of the Battye Glacier Formation is a matter of debate. The smectite and kaolinite content most easily can be explained by erosion of sources largely hidden beneath the ice upstream. Less likely, these clay minerals reflect climatic conditions that were much warmer and wetter than today, facilitating chemical weathering.

Major elements oxides and mineralogy of primary minerals in ODP Hole 195-1201D basalts

The basement cored at Site 1201 (west Philippine Basin) during Ocean Drilling Program Leg 195 consists of a 91-m-thick sequence of basalts, mostly pillow lavas and perhaps one sheet lava flow, with a few intercalations of hyaloclastite and interpillow sedimentary material. Hydrothermal alteration pervasively affected the basalt sequence, giving rise to a variety of secondary minerals such as K-Fe-Mg-clay minerals, oxyhydroxides and clay minerals mixtures, natrolite group zeolites, analcite, alkali feldspar, and carbonate. The primary minerals of pillow and sheet basalts that survived the intense hydrothermal alteration were investigated by electron microprobe with the aim of characterizing their chemical composition and variability. The primary minerals are mostly plagioclase, ranging in composition from bytownite through labradorite to andesine, chromian-magnesian-diopside, and spinels, both Ti magnetite (partially maghemitized) and chromian spinel. Overall, the chemical features of the primary minerals of Site 1201 basalts correspond to the primitive character of the bulk rocks, suggesting that the parent magma of these basalts was a mafic tholeiitic magma that most likely only suffered limited fractional crystallization and crystallized at high temperatures (slightly below 1200°C) and under increasing fO2 conditions. The major element composition of clinopyroxene suggests a backarc affinity of the mantle source of Site 1201 basement.

(Table 1) Percentage of non-opaque heavy minerals, heavy residue, magnetite, and opaque heavy minerals, DSDP Sites 58-445 and 58-446

Heavy-mineral analyses were made for 39 samples, 27 from DSDP Site 445 and 12 from Site 446. About one-fourth of the samples were so loose that they were easily disaggregated in water. The amount of heavy residue and the magnetite content of the heavy fraction were very high, 0.2 to 44 per cent and (on the average) more than 20 per cent, respectively. Among the non-opaque heavy minerals, common hornblende (0 to 80%) and augite (0 to 98%) are most abundant. Pale-green and bluish-green amphiboles (around 10%) and the epidote group (a few to 48%) are next in abundance. Euhedral apatite and biotite and irregularly shaped chromite are not abundant, but are present throughout the sequence. Hacksaw structure is developed in pale-green amphibole and augite. At Site 445, a fair amount of chlorite and a few glauconite(?) grains are present from Core 445-81 downward. The content of common hornblende and opaque minerals also changes from Core 445-81 downward. A geological boundary may exist between Cores 445-77 and 445-81. Source rocks of the sediments at both sites were basaltic volcanic rocks (possibly alkali suite), schists, and ultramafic rocks. The degree of lithification and amount of heavy residue, and the content of magnetite, non-opaque heavy minerals (excluding mafic minerals), and mafic minerals in the cores were compared with Eocene, Oligocene, and Miocene sandstones of southwest Japan. In many respects, the sediments at Sites 445 and 446 are quite different from those of southwest Japan. From the early Eocene to the early Miocene, the area of these sites belonged to a different geologic province than southwest Japan.

Geochemsitry of minerals and glass shards at DSDP Leg 67 Holes

During Leg 67, the Middle-America Trench transect off Guatemala was drilled across the convergent margin of southern Mexico and Central America south of the Tehuantepec Ridge. The data of Leg 66, north of the Tehuantepec Ridge, and that of Leg 67 provided the opportunity to establish a continuous chronology of airborne volcanic ashes intercalated within the sediments (Aubouin et al., 1979; von Huene et al., 1980). Sites of both expeditions are favorably located for obtaining a good record of the explosive volcanicity of these areas, given the proximity of the volcanic sources and the position of the sites under the prevailing winds.

Platinum-group, major, and trace element abundances in ODP Leg 183 basalts

Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ~0.1 (Ir, Ru) to ~5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (~30%) and fractional crystallization (~45%). First order approximations of the fractionation required to produce the KP basalts from an ~30% partial melt of a spinel peridotite were determined using the PELE program. The model was adapted to better fit the physical and chemical observations from the KP basalts, and requires an initial crystal fractionation stage of at least 30% olivine plus Cr-spinel (49:1), followed by magma replenishment and fractional crystallization (RFC) that included clinopyroxene, plagioclase, and titanomagnetite (15:9:1). The low Pd values ([Pd/Pt]_pm < 1.7) for these samples are not predicted by currently available Kd values. These Pd values are lowest in samples with relatively higher degrees of alteration as indicated by petrographic observations. Positive anomalies are a function of the behavior of the PGEs; they can be reproduced by Cr-spinel, and titanomagnetite crystallization, followed by titanomagnetite resorption during the final stages of crystallization. Our modeling shows that it is difficult to reproduce the PGE abundances by either depleted upper or even primitive mantle sources. Crustal contamination, while indicated at certain sites by the isotopic compositions of the basalts, appears to have had a minimal affect on the PGEs. The PGE abundances measured in the Kerguelen Plateau basalts are best modeled by melting a primitive mantle source to which was added up to 1% of outer core material, followed by fractional crystallization of the melt produced. This reproduces both the abundances and patterns of the PGEs in the Kerguelen Plateau basalts. An alternative model for outer core PGE abundances requires only 0.3% of outer core material to be mixed into the primitive mantle source. While our results are clearly model dependent, they indicate that an outer core component may be present in the Kerguelen plume source.

Geochemical compositions of minerals from chilled dike margins of ODP Hole 140-504B

Mineral and whole-rock geochemical data are presented for chilled dike margins from the lower sheeted dike complex of Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B. Compositions of phenocrystic plagioclase (An80-89); olivine (Fo82-86); clinopyroxene (Wo52En40Fs8, with Cr2O3 up to 1.2%); and rare chromian spinel (Cr# 43) are consistent with those from the lavas and the upper dike complex recovered previously (DSDP Legs 69, 70, 83, and ODP Leg 111). Major and trace element compositions fall in group D of Autio and Rhodes (1983) and have high CaO/Na2O, and low TiO2, K2O, and (La/Sm)N values consistent with previous analyses from this site.

Mud, sand, heavy and light minerals composition in lower Cretaceous at DSDP Hole 93-603B

During the drilling of Hole 603B on Deep Sea Drilling Project Leg 93, an unexpected series of sand-, silt-, and claystone turbidites was encountered from Cores 603B-45 through -76 (1224-1512 m sub-bottom depth). Complete and truncated Bouma sequences were observed, some indicating deposition by debris flows. Sand emplacement culminated with the deposition of a 30-m-thick, unconsolidated sand unit (Cores 603B-48 through -45). The purpose of this preliminary study is to determine the nature of the heavy mineral suites of this sediment in order to make tentative correlations with onshore equivalents. The heavy mineralogy of Lower Cretaceous North American mid-Atlantic coastal plain sediment has been extensively studied. This sediment is classified as the Potomac Group, which has a varied heavy mineral suite in its lower part (Patuxent Formation), and a limited suite in its upper part (Patapsco Formation). The results of this study reveal a similar trend in the heavy mineral suites of sediment in Hole 603B. Hauterivian through lower Barremian sediment has a heavy mineral suite that is dominated by zircon, apatite, and garnet, with minor amounts of staurolite and kyanite. Beginning in the mid-Barremian, a new source of sediment becomes dominant, one which supplies an epidote-rich heavy mineral suite. The results of the textural analyses show that average grain size of the light mineral fraction increases upsection, whereas sorting decreases. The epidote-rich source may have delivered sediment with a slightly coarser mean grain size. This sediment may represent a more direct continental input at times of maximum turbidite activity (mid-Barremian) and during deposition of the upper, unconsolidated sand unit.

Chemical and isotopic compositions of Archean magmatic and metasedimentary rocks and minerals from the Podolia domain, Ukrainian Shield

Zircons from the oldest magmatic and metasedimentary rocks in the Podolia domain of the Ukrainian shield were studied and dated by the U-Pb method on a NORDSIM secondary-ion mass spectrometer. Age of zircon cores in enderbite gneisses sampled in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of terrigenous zircons in quartzites from the Odessa quarry as well as in garnet gneisses from the Zaval'e graphite quarry have age within 3650-3750 Ma. Zircon rims record two metamorphic events around 2750-2850 Ma and 1900-2000 Ma. Extremely low U content in zircons of the second age group indicates conditions of the granulite facies metamorphism in Paleoproterozoic within the Podolia domain. Measured data on orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest existence of the ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of formation and geochemical features of the primordial crust.

Third-generation synchrotron x-ray diffraction of 6-μm crystal of raite, ≈Na3Mn3Ti0.25Si8O20(OH)2⋅10H2O, opens up new chemistry and physics of low-temperature minerals

The crystal structure of raite was solved and refined from data collected at Beamline Insertion Device 13 at the European Synchrotron Radiation Facility, using a 3 × 3 × 65 μm single crystal. The refined lattice constants of the monoclinic unit cell are a = 15.1(1) Å; b = 17.6(1) Å; c = 5.290(4) Å; β = 100.5(2)°; space group C2/m. The structure, including all reflections, refined to a final R = 0.07. Raite occurs in hyperalkaline rocks from the Kola peninsula, Russia. The structure consists of alternating layers of a hexagonal chicken-wire pattern of 6-membered SiO4 rings. Tetrahedral apices of a chain of Si six-rings, parallel to the c-axis, alternate in pointing up and down. Two six-ring Si layers are connected by edge-sharing octahedral bands of Na+ and Mn3+ also parallel to c. The band consists of the alternation of finite Mn–Mn and Na–Mn–Na chains. As a consequence of the misfit between octahedral and tetrahedral elements, regions of the Si–O layers are arched and form one-dimensional channels bounded by 12 Si tetrahedra and 2 Na octahedra. The channels along the short c-axis in raite are filled by isolated Na(OH,H2O)6 octahedra. The distorted octahedrally coordinated Ti4+ also resides in the channel and provides the weak linkage of these isolated Na octahedra and the mixed octahedral tetrahedral framework. Raite is structurally related to intersilite, palygorskite, sepiolite, and amphibole.