Ti-rich Minerals Research Articles

Geochemical discrimination of Silurian mudstones according to depositional process and provenance within the Southern Welsh Basin

AbstractA preliminary geochemical investigation of Silurian (Llandovery) basinal mudstones (turbidites and hemipelagites) from the Southern Welsh Basin is described. Turbidite mudstones show higher concentrations of Fe2O3, MgO, TiO2, MnO, LOI, Zn and Zr than laminated hemipelagites. This is consistent with the observed higher concentrations of chlorite and Ti-bearing minerals in turbidite mudstones. Laminated hemipelagites show higher values of REEs (Ce and La), concentrated within authigenic monazites, and Ni, As, Cu and Pb within sulphide minerals (pyrite and galena) reflecting the influence of primary organic carbon levels and anoxic bottom waters on early diagenesis. Deposition of hemipelagites under oxidizing conditions is reflected in lower concentrations of authigenic sulphide mineral hosted elements compared with laminated hemipelagic lithologies. There is a distinct geochemical difference between mudstones of easterly and southerly provenance in the Southern Welsh Basin. This is shown for both turbidites and hemipelagites. The differences are due to the increased input of illite and the chemical elements associated with this mineral (K2O, A12O3, Rb and Ba). Turbidite mudstones sourced from the south show increased levels of heavy minerals, especially those associated with Ti-rich minerals. There is also an increase in elements associated with detrital monazites: Th and Y. The hemipelagites show higher values of REE and chalcophile elements consistent with their more reduced nature.

Petrogenesis and tectonic setting of the Roman Volcanic Province, Italy

The volcanism in the Roman Province of Italy can be modelled by the partial melting of heterogeneously enriched mantle sources. The heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central Apennines. New petrographical and chemical data are presented for the high-K calc-alkaline and the shoshonitic volcanics of the Campania region. Primary magmas are present only locally. The existence of spatial zonation in the volcanism of Campania is documented for the first time. The shoshonitic, leucite-basanitic and leucititic volcanics of the Phlegraean Fields-Procida-Ischia and the Somma-Vesuvius areas are, at similar degrees of evolution, about two times richer in Nb and Ba than those of northwestern Campania and the Latium part of the Roman Province. Accordingly, distinct north-western and south-eastern subprovinces can be defined. The evaluation of enrichment factors, that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low K- and the high K suites, shows that the mantle sources prior to K-metasomatism were different in the two sub-provinces of the Roman Province. In the north-western one, they resembled the sources of ocean-island tholeiites and moderately enriched MOR-basalts. In the south-east they were similar to those of ocean island alkaline lavas and enriched MORB's. Modelling based on K, P, Ce, Sr, Rb, Ba, Th, Sm, Eu, Gd, Y, Nb and 87 Sr 86 Sr was carried out. It indicates that the range of mantle sources of the volcanics in northwestern Campania and Latium can be modelled by the addition of 3 to 20% of materials derived by partial melting of carbonaceous pelites to a Sr-enriched mantle wedge comparable to the Honolulu mantle source least enriched in Nb. The production of Sr-enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust, or a small amount of metasomatism caused by the presence of carbonatite melts. The near absence of Ti, Ta, Nb, Yb and the highly fractionated REE in the metasomatizing component requires the presence of residual garnet and accessory Ti-rich minerals during the partial melting of the subducted sediments. The writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust, probably sediments. This process played a dominant role in the generation of the mantle sources from which the high-K calc-alkaline, shoshonitic, leucite-basanitic and leucititic magmas of the Roman Province were derived.

An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature

Experimental determination of partition coefficients ( D) for Nb and Ta between Ti-rich minerals (sphene, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic andesite, andesite and trachyte composition, indicate that these elements partition strongly into sphene and rutile, and moderately into ilmenite and Ti-magnetite. The D values for sphene increase with decreasing temperature and with increasing SiO 2 content of the coexisting liquid, but appear unchanged by pressure between 7.5 and 16 kb, and oxygen fugacity between magnetite-wustite and haematite-magnetite buffers. Also, D Nb D Ta . varies from 0.3–0.4 for sphene, increasing to 0.6–0.7 for rutile, 0.7–0.8 for ilmenite and 0.8 for Ti-magnetite. Thus fractionation involving these minerals may cause an increase in the Nb Ta ratio of any derivative liquid; this will be most noticeable where sphene is involved.

Analysis of sub-micron mineral matter in coal via scanning transmission electron microscopy

The fine-scale mineral matter in three US coals has been analysed via scanning transmission electron microscopy (STEM). The samples observed were a North Dakota lignite, a Kentucky bituminous, and a Pennsylvania semi-anthracite. Specific mineral types, differing among the three coals examined, appear to predominate at this fine size scale (particles ⩽ 200 nm in diameter). Fe-rich and Ba-rich minerals in the lignite, a Ti-rich mineral in the bituminous and Ca-rich and Ti-rich minerals in the semianthracite were the predominant species found. The inherent mineral content in the observed organic background also differed from coal to coal. The distributions of mineral species in the size range ⩽ 200 nm reported herein do not reflect the distributions in the larger size ranges obtained by more macroscopic techniques.

Origin and metamorphism of peperite and associated rocks in the Devonian Elwell Formation, northern Sierra Nevada, California

The Late Devonian Elwell Formation is part of a late Paleozoic island-arc assemblage exposed in the northern Sierra Nevada, California. In the 10-km interval from Mount Elwell (Plumas County) to Packer Lake (Sierra County), it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity-current and debris-flow deposits. Peperite, formed by interaction of intrusive andesitic magma and still-moist, incompletely lithified “chert” not far beneath the sea floor, occurs sparingly. The whole assemblage was thoroughly recrystallized (greenschist-facies regional metamorphism) during the Late Jurassic Nevadan orogeny. The phosphatic chert is composed of white, fluorapatite-rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material (probably graphite). Radiolaria (family Entactiniidae) are so abundant that the chert is thought to have originated as radiolarian ooze. The phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment-water interface. The andesitic sills are generally aphyric and aphanitic; original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins. The andesite was charged with plagioclase (now albite) microlites, and quench morphologies occur where sills intruded the wettest “chert.” Relatively thick sills commonly display columnar structure, whereas a few originally more viscous sills are flow laminated. The top of a sill emplaced essentially at the sediment-water interface is pillowed. Many sills bear xenoliths of phosphatic chert, and scarce quartz megacrysts (evidently derived from the underlying, dacitic Sierra Buttes Formation) appear nearly everywhere. Concentrations of major-element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs; rare-earth–abundance patterns are particularly diagnostic of this paleotectonic setting. Most peperite has poorly sorted (coarse ash to coarse block) andesitic fragments uniformly distributed through a voluminous black chert matrix. Where invaded “chert” retained the most pore water, quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior. Rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the “chert” by steam explosions. Steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert, and shattered already-formed phosphatic nodules. Where the invaded “chert” was better lithified, freezing sill margins simply granulated quietly in situ by thermal-contraction cracking of microlitic glass; fluid “chert” injected widely spaced contraction joints deep within the sills. In all cases, there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments. Greenschist-facies metamorphism produced two mineral assemblages in the interiors of andesitic sills, the second perhaps reflecting locally higher Pco 2 /Ph 2 o during metamorphism: (1) albite + quartz + epidote + actinolite + chlorite + Ti-rich mineral ± calcite ± stilpnomelane ± white mica, and (2) albite + quartz + chlorite + Ti-rich mineral + calcite ± stilpnomelane ± white mica. Epidote-rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres; such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite. Additional examples of local meta-somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill. Cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills.