Lewis C. Calk

A king-sized theropod coprolite

Fossil faeces (coprolites) provide unique trophic perspectives on ancient ecosystems. Yet, although thousands of coprolites have been discovered, specimens that can be unequivocally attributed to carnivorous dinosaurs are almost unknown. A few fossil faeces have been ascribed to herbivorous dinosaurs, but it is more difficult to identify coprolites produced by theropods because other carnivorous taxa coexisted with dinosaurs and most faeces are taxonomically ambiguous. Thus sizeable (up to 20 cm long and 10 cm wide) phosphatic coprolites from Belgium and India, that have been attributed to dinosaurs might have been produced by contemporaneous crocodylians or fish. But there is no ambiguity about the theropod origin of the Cretaceous coprolite we report here. This specimen is more than twice as large as any previously reported carnivore coprolite, and its great size and temporal and geographic context indicate that it was produced by a tyrannosaur, most likely Tyrannosaurus rex. The specimen contains a high proportion (30–50%) of bone fragments, and is rare tangible evidence of theropod diet and digestive processes.

Degassing and differentiation in subglacial volcanoes, Iceland

Abstract Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60°C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice-impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset-bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset-bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H2O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H2O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice-impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile-rich alkalic melts, causing them to rise to the top of the chamber and erupt.

Jackstraw-textured talc-olivine rocks, Preston Peak area, Klamath Mountains, California

An unusual ultramafic rock characterized by elongate olivine crystals forms several small masses in alpine-type, tectonitic peridotite of the Preston Peak ophiolite, Klamath Mountains, California. The largest mass of this distinctive ultramafic rock, which has been mapped in detail, has contacts with the surrounding peridotite tectonite that vary from indistinct to sharp. In several areas, the country-rock tectonitic peridotite is impregnated by a network of talc-olivine veins locally containing elongate olivine crystals. Within the mass, the elongate olivine crystals, many of them pseudomorphosed by serpentine, are typically arranged in a crisscrossed pattern (jackstraw texture), but radial and parallel patterns also occur. The olivine crystals are bladelike but lack the skeletal or dendritic textures characteristic of elongate igneous olivines. Microprobe analyses of the olivine blades indicate that they are magnesium-rich (Fo 89.4 ) and unzoned. The coexisting mineral assemblage includes talc, tremolite, magnesite, chlorite, and pentlandite, minerals that are not compatible with a magmatic origin. Furthermore, at several localities within the mass, relict structures and minerals of the peridotite tectonite are preserved in the Jackstraw-textured rocks, indicating a replacement origin. Major- and minor-element chemical data support a replacement origin, in that the Jackstraw-textured talc-olivine rocks are similar in chemical composition to the surrounding tectonitic peridotite of the ophiolite. The most obvious difference is that the jackstraw-textured rocks contain more sulfur. Experimental data on the coexisting mineral assemblages indicate a low-temperature (500 to 600 °C) origin, and field relations suggest the importance of a volatile-rich phase.