Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy

Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ratio on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.     

Diagenetic alterations in temperate shelf carbonates from southeastern Australia

Temperate shelf carbonates form in cool marine waters and have skeletal and mineralogical compositions which are different from their tropical counterparts. They commonly lack non-skeletal grains and are often composed of low- and high-magnesium calcite with subordinate aragonite. Many of the aragonitic components found in tropical carbonates, such as corals, ooids, blue-green algae and lime mud, are absent.

Temperate shelf carbonates undergo diagenesis in marine waters with lower carbonate saturation than do tropical carbonates, and are exposed to cool climates with moderate to low rainfall. Marine cementation is rare because of low carbonate saturations in the surrounding waters. However, aragonite and high-magnesium calcite cements have been reported forming under specialized conditions associated with biogenic precipitation, submarine methane and sulphate-reducing bacteria, and more commonly in the intertidal environment where evaporation has increased carbonate concentrations.

In Pleistocene and Tertiary temperate shelf carbonates from southeastern Australia, evidence of marine diagenesis is rare to absent. Diagenetic stabilization of aragonite and high-magnesium calcite has taken from 80,000 y to 1 My, or longer, during subaerial exposure. This is slower than rates reported from tropical climates. A general lack of aragonite in some facies within these temperate carbonates leads to a lack of secondary porosity and only sparse low-magnesium calcite cement, even after prolonged fresh-water diagenesis. However, with lengthy exposure and under the right climatic conditions, karstic solution and calcrete precipitation can occur.

In sequences containing siliciclastic clays, pyrite and glauconite, abundant iron is present in interstitial waters leading to the precipitation of ferroan calcite cements in the phreatic and shallow burial environments, and to the substitution of iron for magnesium in stabilizing high-magnesium calcite skeletal material.

A unique void-filling, micritic internal sediment occurs in discrete layers in many of the Tertiary temperate shelf carbonate sequences in southeastern Australia. This internal sediment is localized as a pore-filling material above permeability barriers such as fine-grained sediments or volcanics, and above paleo-water tables which formed during periods of subaerial exposure. It is a feature of the vadose zone and lithifies to form a dense micritic low-magnesium calcite cement with characteristic pink/brown coloration, often associated with erosion surfaces and nodule beds.

Dolomite is uncommon in the southeastern Australian temperate shelf carbonates. It forms associated with preferential fluid pathways or mixing zones. Ferroan dolomite forms in siliciclastic clay-rich carbonates in the shallow burial environment. The ubiquitous fine, evaporite-related dolomite so common in tropical carbonates is absent.     

Magma mixing in the San Francisco Volcanic Field,AZ

A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO₂) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO₂). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO₂, Al₂O₃, MnO, Na₂O, K₂O, and Zr and poorer in MgO, CaO, P₂O₅, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas.     

Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field,California

 Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins, and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred during the episode of CVF magmatism between ∼7.5 Ma and present. Variable compositions of quenched melts resulted from mixing of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization, and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts ( >∼60% SiO₂) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P, and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents. Received: 15 June 1995 / Accepted: 13 December 1995     

Preliminary study on the preservation of giant clam (Tridacnidae) shells from the Balobok Rockshelter archaeological site,south Philippines

Tridacnidae shells, a valuable archive of past environments, are common in the Balobok Rockshelter archaeological site on Sanga‐Sanga Island in the south Philippines. This site was occupied during the mid‐Holocene (ca. 5000–8800 ¹⁴C yr B.P.), a period of Neolithic cultural expansion in the Philippines. This paper focuses on the preservation of two shell specimens, Hippopus hippopus and Tridacna maxima, unearthed from two mid‐Holocene layers within the rockshelter. The shells' mineralogy and microstructure (prismatic and crossed‐lamellar) were studied using micro‐Fourier transform infrared spectroscopy and scanning electron microscopy to determine if the samples were suitable as paleoenvironmental records. Both shells are still aragonitic but aragonite crystals of both microstructure types are partly dissolved. This dissolution, characteristic of meteoric water alteration, precludes their utility in paleoenvironmental geochemical studies. Nevertheless, these shells are abundant in archaeological sites in the region and may be better preserved in other depositional contexts; more studies on Philippine Tridacnidae shell diagenesis are needed. © 2011 Wiley Periodicals, Inc.