Late Glacial environmental changes indicated by a coleopteran succession from North Wales

This investigation was designed to compare the environmental inferences that can be made from fossil Coleoptera with those that can be made from palynological data. In the Glanllynnau kettle hole, sedimentation began at 14,000 BP and continued until about 10,000 BP. The Coleoptera from these deposits show that during this period there was only one climatic oscillation and that this differed in both timing and intensity from oscillations inferred from palynological evidence. Thus, at about 13,000 BP, an intensely cold continental climate suddenly gave place to a period with summer temperatures at least as warm as those at the present day, but during this episode the landscape was entirely without trees. From 12,000 BP to about 10,000 BP there was a progressive deterioration in the temperature, and the period of Betula forest, broadly equivalent to the Allerød, is shown to have a less thermophilous fauna than that of the previous pollen Zone I. An attempt is made to explain this anomaly in terms of the differential rate of response of the plants and animals to rapid climatic changes.     

On the Origin of High-Alumina Arc Basalt and the Mechanics of Melt Extraction

Most models of high-alumina arc basalt petrogenesis rely heavilyon the supposition that the abundances of certain trace elements,in particular the relatively unfractionated Rare Earth Element(REE) patterns and the unusually high concentrations of K, Rb,Sr, and Ba are incompatible with a garnet-bearing subductedoceanic crustal (quartz eclogite) source rock. We have carefullyexamined this apparently unequivocal evidence in light of recentprogress on the physics of melt extraction and the heat transferand mechanics of magma ascent. The weakest element of all traceelement models involving a quartz eclogite source is the assumptionthat the element concentrations are fixed at the source andonly later modified in the near-surface environment. We expandon such models by monitoring the concentrations of REE and majorand trace elements during initial melting, ascent, and extractionof magma. This is done by combining calculated cooling curvesfor ascending magmatic bodies with high pressure phase equilibria.The amount that each phase contributes to the melt is monitoredalong with the composition of the melt and residual solids.With quartz eclogite, initial melting initiates gravitationalinstability of the entire source material (melt plus solids)before melt extraction can occur. During ascent of this mush,melting increases until the solids can be repacked to free themelt. This extraction takes place some 15–20 km abovethe slab, after about 50 per cent melting, at which point themelt has a pattern of REE and other trace element concentrationsalmost identical to those observed in high-alumina arc basalts,assuming an initial composition equivalent to altered oceaniccrust plus 5 per cent pelagic sediment. Sr abundances are theonly ones which are not well-matched by this process. The majorelement concentrations of the extracted melt also closely matchthose of high-alumina arc basalt. A similar, but less detailedevaluation of both fertile and depleted peridotite source rocksyields good agreement for the REE and other trace element concentrationsassuming a LREE-enriched source rock strongly enriched in K,Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied onlythrough substantial low pressure fractionation of mafic phases,in particular olivine. Though not rigorously tested, such aprocess may be compatible with the observed major element concentrationsof high-alumina basalt. However, the experimentally verifiedfact that high-alumina basalts could never have been in equilibriumwith either an olivine-bearing magma or source rock eliminatesthis possibility altogether. Thus, the simultaneous considerationof the mechanics of ascent and melt extraction along with phaseequilibria clearly shows that partial melting of quartz eclogitebest satisfies the chemical constraints of major, trace, andREE characteristics of high-alumina arc basalts.     

Variable Impact of the Subducted Slab on Aleutian Island Arc Magma Sources: Evidence from Sr, Nd, Pb, and Hf Isotopes and Trace Element Abundances

Major and trace element compositions and Sr, Nd, Pb, and Hfisotope ratios of Aleutian island arc lavas from Kanaga, Roundhead,Seguam, and Shishaldin volcanoes provide constraints on thecomposition and origin of the material transferred from thesubducted slab to the mantle wedge. ⁴⁰Ar/³⁹Ar dating indicatesthat the lavas erupted mainly during the last