Igneous origin of K-feldspar megacrysts in deformed granite of the Papoose Flat Pluton, California, USA

Many points of evidence, especially igneous microstructures and structures resulting from solid-state deformation, indicate that K-feldspar megacrysts in deformed granites of the Papoose Flat pluton are residual phenocrysts, not porphyroblasts. Evidence of an igneous origin includes features such as crystal shapes, simple twinning, zonally arranged euhedral plagioclase inclusions, oscillatory compositional zoning, and local occurrence in microgranitoid enclaves. Evidence of solid-state deformation of the megacrysts (which is consistent with their existence prior to the mylonitic deformation) includes marginal recrystallization and neocrystallization, microcline twinning, marginal replacement by myrmekite, and recrystallized/neocrystallized “tails”. Evidence of porphyroblastic growth, such as overgrown inclusion trails, is absent. This appears to be the situation in most felsic augen gneisses and mylonites.     

Weathering, dust, and biocycling effects on soil silicon isotope ratios

Silicon isotope ratios (δ³⁰Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ³⁰Si values from a climate gradient of Hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ³⁰Si. In upper (<1 m deep) soil horizons along the climate gradient, Si depletion correlates with decreases of residual δ³⁰Si values in low rainfall soils and increases in high rainfall soils. Strong positive correlation between soil δ³⁰Si and dust-derived quartz and mica content show that both trends are largely controlled by the abundance of these weathering-resistant minerals. The data also lend support to the idea that fractionation of Si isotopes in secondary phases is controlled by partitioning of silicon between dissolved and precipitated products during the initial weathering of primary basalt. Secondary mineral δ³⁰Si values from lower (>1 m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential ²⁸Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ³⁰Si values in shallow (<1 m deep) soil horizons showed evidence of biocycling induced Si redistribution and substantially lower δ³⁰Si values than predicted. Low δ³⁰Si values in shallow soil horizons compared to predictions can be attributed to repeated fractionation as secondary minerals undergo additional cycles of dissolution and precipitation. Primary mineral weathering, secondary mineral weathering, dust accumulation, and biocycling are major processes in terrestrial Si cycling and these results demonstrate that each can be traced by δ³⁰Si values interpreted in conjunction with mineralogy and measures of Si depletion.     

The role of geoarchaeology in the interpretation of fragmented buildings and occupation surfaces: The case of coastal settlements in northeast Scotland

Around the world, poorly preserved buildings and occupation deposits often represent the primary evidence for archaeological structures and settlements. Integrated geoarchaeological methods, such as soil chemistry and micromorphology, can be used to maximise the information obtained from such deposits regarding site preservation and the use of space. However, archaeologists are often reluctant to apply these methods if they suspect that preservation is poor or stratigraphy is not visible in the field. To assess the role that geoarchaeology can play in the interpretation of fragmented and poorly preserved structures, this paper presents the results of two case studies in which multiple geoarchaeological methods (microrefuse analysis, pH, electrical conductivity, magnetic susceptibility, loss-on-ignition, portable XRF and micromorphology) were applied to poorly preserved occupation deposits and fragmented buildings in early medieval coastal settlements in northeast Scotland. Micromorphology proved to be fundamental for recognising and understanding the composition of occupation deposits that had formerly been floor surfaces. It also aided interpretations for the use of space and maintenance practices and improved an understanding of the post-depositional processes that had affected stratigraphic visibility at the macroscale. When subjected to principal component analysis, the geochemical, magnetic and microrefuse data were able to provide new details about activity areas, and successfully identified and filtered out the effects of post-medieval contamination. Most significantly, the integrated approach demonstrates that fragmented buildings and poorly preserved occupation surfaces can retain surviving characteristics of the use of space, even if the floor surfaces were not preserved well enough to be clearly defined in the field or in thin section.