Discovery of halloysite books in altered silicic Quaternary tephras, northern New Zealand

Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3= 5.2 wt.%) than tubes (Fe2O3= 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10–20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite.

Secondary mineral formation in the White River tephra in grassland and forest soils in central Yukon Territory

Selected surface horizons of grassland and forest soils formed under a cold, semi-arid climate were investigated to evaluate the formation of secondary minerals within the White River tephra, a Late Holocene rhyolitic tephra (~115014C yr BP) veneer that overlies the soil landscapes of central Yukon. Concentrations of extractable Fe (< 0.48%), Al (< 0.26%) and Si (< 0.082%) concentrations in surface tephra-contaning horizons of grassland and forest pedons are low. The high amount of exchangeable calcium in grassland soils is likely due to cycling by vegetation and perhaps, aeolian inputs of Ca and Mg carbonates. Al is incorporated into Al-humus complexes in forest pedons and allophane in grassland pedons. Allophane content is low (< 0.56%) in all soils as is ferrihydrite (< 0.34%). Mineral composition of the sand fraction from tephra horizons is dominated by volcanic glass, plagioclase feldspars, amphiboles, epidote, pyroxenes and very limited quantities of quartz and primary Fe oxides. Chlorite and an expanding phyllosilicate were also detected and are assumed to be of detrital origin. Clay mineralogy is dominated by volcanic glass, quartz, feldspars and minimal quantities of kaolinite and dehydrated halloysite in surficial horizons. Kaolinite is assumed to be of detrital origin while dehydrated halloysite is a product of a low leaching and dry environment where limited resilication occurs. Scanning electronmicroscopy (SEM) investigation indicates the presence of opaline silica in surface horizons from forest pedons which has likely formed due to freezing of the soil solution in combination with dehydration and resilication. Overall, the soil horizons formed within the veneer of White River tephra have experienced minimal weathering and very little silicate clay mineral development. Key words: Tephra, glass, Yukon, minerals (secondary), weathering

Biotite composition as a tool for the identification of Quaternary tephra beds

Stratigraphically important Quaternary rhyolitic tephra deposits that erupted from the Okataina and Taupo volcanic centers in New Zealand can be geochemically identified using the FeO and MgO contents of their biotite phenocrysts. The FeO/MgO ratio in biotite does not correlate with FeO/MgO in the coexisting glass phase so that tephra beds with similar glass compositions can be discriminated by their different biotite compositions. Some individual tephra deposits display sequential changes in biotite composition that allow separate phases of the eruption to be identified, greatly increasing the potential precision for correlation. In addition, devitrified lavas that are unsuitable for glass analysis can be correlated to coeval tephra deposits by their biotite compositions. Biotite is common in high-K2O (>4 wt%) tephra beds and is widely dispersed in ash plumes because of its platy form, thus making it important in correlation studies.

Miocene tephra beds in the Cypress Hills of Saskatchewan, Canada

Six rhyolitic tephra layers from ancient loess and related detritus in the Cypress Hills, Saskatchewan, represent separate volcanic eruptions from the Snake River Plain. Idaho, U.S.A. The weighted mean age and uncertainty of the youngest tephra bed is 8.3 ± 0.2 Ma, using the isothermal plateau fissiontrack technique on its hydrated glass shards. The loess that hosts five of these tephra beds extends across the Cypress Plain, which is the oldest (Middle Miocene) and highest depositional surface in the Interior Plains, and also occurs on four juxtaposed erosion surfaces. It appears that the first and maybe the second erosion surface began forming before 10 Ma, and that formation of the second, third, and fourth erosion surfaces was completed between 10 and 8.3 Ma.