A gravity survey of Northwest Lancashire

A survey involving 860 gravity measurements over the Fylde area of Lancashire is described. A line of movement in the pre-Permian basement rocks is shown to cross the area from north north east to south south west forming the eastern edge of a deep sedimentary basin. Displacement of the Carboniferous along this line has reached 1000 m in places and was probably an important influence on Permo-Triassic deposition in Northwest Lancashire.     

Fern species richness and abundance are indicators of climate change on high-elevation islands: evidence from an elevational gradient on Tahiti (French Polynesia)

Coastal sector impacts from sea level rise (SLR) are a key component of the projected economic damages of climate change, a major input to decision-making and design of climate policy. Moreover, the ultimate global costs to coastal resources will depend strongly on adaptation, society’s response to cope with the local impacts. This paper presents a new open-source optimization model to assess global coastal impacts from SLR from the perspective of economic efficiency. The Coastal Impact and Adaptation Model (CIAM) determines the optimal strategy for adaptation at the local level, evaluating over 12,000 coastal segments, as described in the DIVA database (Vafeidis et al. 2006), based on their socioeconomic characteristics and the potential impacts of relative sea level rise and uncertain sea level extremes. A deterministic application of CIAM demonstrates the model’s ability to assess local impacts and direct costs, choose the least-cost adaptation, and estimate global net damages for several climate scenarios that account for both global and local components of SLR (Kopp et al. 2014). CIAM finds that there is large potential for coastal adaptation to reduce the expected impacts of SLR compared to the alternative of no adaptation, lowering global net present costs through 2100 by a factor of seven to less than $1.7 trillion, although this does not include initial transition costs to overcome an under-adapted current state. In addition to producing aggregate estimates, CIAM results can also be interpreted at the local level, where retreat (e.g., relocate inland) is often a more cost-effective adaptation strategy than protect (e.g., construct physical defenses).     

The free surface response of a subsurface anomaly to an SH wave with source on the surface

The response of a shallow subsurface anomaly to an SH wave with source on the surface is computed by using a two dimensional finite element technique. It is demonstrated that the effects of the depth and relative material composition of the anomaly can be identified in the power spectra. The behaviour of the response agrees with principles established from investigating cases where the SH wave is incident from below.     

Calculated silica activities in carbonatite liquids

Carbonatite magmas precipitate silicates, in addition to the abundant carbonates, oxides, and phosphates. Calculated silica activities for equilibria involving silicates and a silica component in magmatic liquids predict specific assemblages for silicate and oxide phases in carbonatites. These assemblages provide tests of alternative sources (carbonatite magma, coeval silicate magma, or older rock) for silicate minerals in carbonatites. Quartz, feldspars, and orthopyroxene are unlikely to be primary magmatic phases in carbonatites, because the silica activity in carbonatite magmas is too low to stabilize these minerals. Zircon and titanite should be unstable relative to baddeleyite and perovskite, respectively, but they do occur in carbonatites. Liquids dominated by carbonate are strongly nonideal with respect to dissolved silica. Consequently, activity coefficients for a silica component in carbonatite liquids are >>1, so that small mole fractions of SiO₂ translate into silica activities sufficient to stabilize phlogopite, clinopyroxene, amphibole, monticellite, and forsterite, among other silicates. Examination of silicate mineral assemblages in carbonatites in the light of silica activity indicates that many carbonatites are contaminated by solid silicate phases from external sources but these xenocrysts can be discriminated from magmatic minerals.     

Rapid and highly reproducible analysis of rare earth elements by multiple collector inductively coupled plasma mass spectrometry

A method has been developed for the rapid chemical separation and highly reproducible analysis of the rare earth elements (REE) by isotope dilution analysis by means of a multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS). This technique is superior in terms of the analytical reproducibility or rapidity of analysis compared with quadrupole ICP-MS or with thermal ionization mass spectrometric isotope dilution techniques. Samples are digested by standard hydrofluoric-nitric acid-based techniques and spiked with two mixed spikes. The bulk REE are separated from the sample on a cation exchange column, collecting the middle-heavy and light REE as two groups, which provides a middle-heavy REE cut with sufficient separation of the light from the heavier REE to render oxide interferences trivial, and a Ba-free light REE cut. The heavy (Er-Lu), middle (Eu-Gd), and light REE (La-Eu) concentrations are determined by three short (1 to 2 min) analyses with a CETAC Aridus desolvating nebulizer introduction system. Replicate digestions of international rock standards demonstrate that concentrations can be reproduced to <1%, which reflects weighing errors during digestion and aliquotting as inter-REE ratios reproduce to ≤0.2% (2 SD). Eu and Ce anomalies reproduce to <0.15%. In addition to determining the concentrations of polyisotopic REE by isotope dilution analysis, the concentration of monoisotopic Pr can be measured during the light REE isotope dilution run, by reference to Pr/Ce and Pr/Nd ratios measured in a REE standard solution. Pr concentrations determined in this way reproduce to <1%, and Pr/REE ratios reproduce to <0.4%. Ce anomalies calculated with La and Pr also reproduce to <0.15% (2 SD). The precise Ce (and Eu) anomaly measurements should allow greater use of these features in studying the recycling of materials with these anomalies into the mantle, or redox-induced effects on the REE during recycling and dehydration of oceanic lithosphere, partial melting, metamorphism, alteration, or sedimentation processes. Moreover, this technique consumes very small amounts (subnanograms) of the REE and will allow precise REE determinations to be made on much smaller samples than hitherto possible.     

Polysomatism,polytypism, defect microstructures,and reaction mechanisms in regularly and randomly interstratified serpentine and chlorite

High-resolution (HRTEM) and analytical electron (AEM) microscopic evidence for a polysomatic series based on regular interstratifications of serpentine (amesite) and chlorite (clinochlore) are reported from an altered skarn in Irian Jaya. The assemblage includes regular interstratifications of one clinochlore and two (2:1; three structural variants), three (3:1), and four (4:1) amesite composition 1:1 layers as well as randomly interstratified serpentine and chlorite. The order of abundance of regularly interstratified minerals is 1:1>2:1>4:1>3:1. Atomic-resolution images, image simulations, and comparison between calculated and observed diffracted intensities verify the proposed 1:1 and 2:1 structures and reveal details of their defect microstructures. AEM data show that compositions are linear combinations of the associated amesite and clinochlore. The 1:1, 2:1, 3:1, and 4:1 minerals occur both as discrete sub-micron crystals and as domains within serpentine or chlorite. Some crystals of the 2:1 phase were sufficiently large for study by X-ray precession and powder methods. Crystals of the regularly interstratified 2:1, 3:1, and 4:1 phases are usually bent. High-resolution images reveal that, within polygonal segments, the layers commonly exhibit a few degrees of curvature with segments separated by antigorite-type offsets. Deformed chlorite crystals are probably replaced by interstratified minerals during an aluminum metasomatic event. Al may have been deposited from sulfuric acid-rich solutions when they interacted with calcite and dolomite to form the anhydrite-rich corona around the phyllosilicate-rich region of the core. The interstratified chlorite (clinochlore composition) suggests aluminum addition by selective conversion of a sub-set of the chlorite layers to amesite. Defect microstructures suggest that crystals of regularly interstratified material grew by direct structural modification of preexisting chlorite. Regular interstratifications may form in response to thermally controlled limits on Al solubility in chlorite and heterogeneities in the distribution of Al-rich solutions during metasomatism. Regularly interstratified minerals coexist with randomly interstratified serpentine/chlorite, chrysotile, antigorite, lizardite, and several amesite and chlorite polytypes. Tentative chlorite and amesite identifications include one-layer (b=97°, probably IIbb), one-layer (b=90, possibly Ibb), two-, and three-layer chlorites, and 2H₁ (but possibly 1M or 1T), rhombohedral (3R or 6R), and twelve-layer (Tc; non standard) serpentine polytypes. The complex phyllosilicates attest to rampant chemical and structural disequilibrium.     

The Pikes Peak batholith,Colorado front range,and a model for the origin of the gabbro—anorthosite—syenite—potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith.The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts.Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends.Crystallization was largely at P_H2O < P_total; P_H2O approached P_total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f_O2 of 10^?17 or 10^?18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity.All types of syenite and granite contain 5–6% K₂O through a range of SiO₂ of 63–76%. Average Na₂O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe₂O₃ ratios are high. Fluorine ranges from 0.3 to 0.6%.The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence.A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K₂O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na₂O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.