Spatially controlled Fe and Si isotope variations: an alternative view on the formation of the Torres del Paine pluton

We present new Fe and Si isotope ratio data for the Torres del Paine igneous complex in southern Chile. The multi-composition pluton consists of an approximately 1 km vertical exposure of homogenous granite overlying a contemporaneous 250-m-thick mafic gabbro suite. This first-of-its-kind spatially dependent Fe and Si isotope investigation of a convergent margin-related pluton aims to understand the nature of granite and silicic igneous rock formation. Results collected by MC-ICP-MS show a trend of increasing δ⁵⁶Fe and δ³⁰Si with increasing silica content as well as a systematic increase in δ⁵⁶Fe away from the mafic base of the pluton. The marginal Torres del Paine granites have heavier Fe isotope signatures (δ⁵⁶Fe = +0.25 ± 0.02 2se) compared to granites found in the interior pluton (δ⁵⁶Fe = +0.17 ± 0.02 2se). Cerro Toro country rock values are isotopically light in both Fe and Si isotopic systems (δ⁵⁶Fe = +0.05 ± 0.02 ‰; δ³⁰Si = ?0.38 ± 0.07 ‰). The variations in the Fe and Si isotopic data cannot be accounted for by local assimilation of the wall rocks, in situ fractional crystallization, late-stage fluid exsolution or some combination of these processes. Instead, we conclude that thermal diffusion or source magma variation is the most likely process producing Fe isotope ratio variations in the Torres del Paine pluton.     

Complex zoning between super-calcic pigeonite and augite from the Graveyard Point sill, Oregon: a record of the interplay between bulk and interstitial liquid fractionation

The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO₂ 3.5 wt% K₂O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo₇₉-Fo₂), augite (X _Fe = 0.18−0.95), feldspars (An₈₀Or₁-An₁Or₆₂), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo_18–28 Fs_41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO₂ (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures. Received: 25 November 1998 / Accepted: 14 June 1999     

Potential effect of climate change on observed fire regimes in the Cordilleran forests of South-Central Interior, British Columbia

Climate change is predicted to result in a warmer and drier climate in many parts of the world, including south-central British Columbia. With a shift in climate, a change in fire regimes is likely to occur. In this study, a statistically significant increase in mean fire size was predicted to occur along with an increase maximum fire size and decrease in the mean fire interval. A change in these fire regime characteristics suggests a climate-change driven shift in fire regimes may occur by the 2020s. The shift in fire regime suggests the proportion of the landscape burning every 50 years or less will increase from 34 % to 93 % by the 2080s. Change in fire regimes will have direct implications for ecosystem management as the combination of large, flammable fuel types and fire-prone climatic conditions will increase the risk of larger more frequent fires and increase the costs and dangers involved in managing fire-prone forests in the Cordilleran region of south-central British Columbia. The climate change-driven shift in fire regime questions the use of historic fire regime characteristics for determining landscape-level conservation targets within the study area.     

The Little Ice Age climate of New Zealand reconstructed from Southern Alps cirque glaciers: a synoptic type approach

Little Ice Age (LIA) austral summer temperature anomalies were derived from palaeoequilibrium line altitudes at 22 cirque glacier sites across the Southern Alps of New Zealand. Modern analog seasons with temperature anomalies akin to the LIA reconstructions were selected, and then applied in a sampling of high-resolution gridded New Zealand climate data and global reanalysis data to generate LIA climate composites at local, regional and hemispheric scales. The composite anomaly patterns assist in improving our understanding of atmospheric circulation contributions to the LIA climate state, allow an interrogation of synoptic type frequency changes for the LIA relative to present, and provide a hemispheric context of the past conditions in New Zealand. An LIA summer temperature anomaly of ?0.56 °C (±0.29 °C) for the Southern Alps based on palaeo-equilibrium lines compares well with local tree-ring reconstructions of austral summer temperature. Reconstructed geopotential height at 1,000 hPa (z1000) suggests enhanced southwesterly flow across New Zealand occurred during the LIA to generate the terrestrial temperature anomalies. The mean atmospheric circulation pattern for summer resulted from a crucial reduction of the ‘HSE’-blocking synoptic type (highs over and to the west of NZ; largely settled conditions) and increases in both the ‘T’- and ‘SW’-trough synoptic types (lows passing over NZ; enhanced southerly and southwesterly flow) relative to normal. Associated land-based temperature and precipitation anomalies suggest both colder- and wetter-than-normal conditions were a pervasive component of the base climate state across New Zealand during the LIA, as were colder-than-normal Tasman Sea surface temperatures. Proxy temperature and circulation evidence were used to corroborate the spatially heterogeneous Southern Hemisphere composite z1000 and sea surface temperature patterns generated in this study. A comparison of the composites to climate mode archetypes suggests LIA summer climate and atmospheric circulation over New Zealand was driven by increased frequency of weak El Niño-Modoki in the tropical Pacific and negative Southern Annular Mode activity.     

Coordinate mapping of analytical contaminant transport solutions to non-uniform flow fields

Existing analytical solutions to 2D and 3D contaminant transport problems are limited by the mathematically convenient assumption of uniform flow. An approximate method is developed herein for coordinate mapping of 2D (vertically-averaged) transport solutions to non-uniform steady-state irrotational and divergence-free flow fields in single-layer aquifers. The method enables existing analytical transport solutions to be applied to aquifer systems with wells, non-uniform saturated thickness, surface water features, and (to a limited degree) heterogeneous hydraulic conductivity and recharge. This mass-conservative coordinate mapping approach is inexact in its approximation of the dispersion process but is still sufficiently accurate for many simple flow systems. The degree of model error is directly proportional to the variation of velocity magnitude within the domain. These mapped analytical solutions are compared to numerical simulation results and the coordinate mapping errors are investigated. The methods described herein may be used in the traditional capacity of analytical transport models, i.e., screening and preliminary site assessment, without sacrificing accuracy by assuming locally uniform flow conditions or applying an ad-hoc coordinate transformation. The solutions benefit from the traditional advantages of analytical methods, particularly the removal of artifacts due to spatial and temporal discretization: no time-stepping or numerical discretization is required.     

Introduction

We have recently measured the concentrations of W and Mo in a large number of terrestrial samples using a new neutron activation analysis method and from these data we have estimated the abundance of these elements in the mantle. The new Mo mantle abundance of 59 ppb is much lower, the W mantle abundance of 10 ppb is somewhat lower than previous estimates. The concentrations of W in some ocean floor basalts are much lower than previously reported. The good correlation of W with U confirms the highly incompatible behavior of W and the good correlation of Mo with Nd indicates a moderately incompatible nature for Mo.The new data on W and Mo provide important constraints regarding the possible mechanisms of core formation and accretion because W and Mo are refractory elements under reducing conditions which would have accreted in the Earth in chondritic proportions, unaffected by volatility. The Mo/W ratio of 5.9 in the mantle is less than a factor of two lower than the chondritic ratio of 9.8. The ratio of Mo to W is a sensitive indicator for metal or sulfide fractionation, because Mo is more siderophile and more chalcophile than W. This tightly limits the amount of metal or sulfide segregation from the mantle to less than 0.1% since the end of accretion. The data for the moderately siderophile elements Mo, W, Co and Ni suggest that core formation in the Earth was essentially complete after 85–95% of the Earth had accreted.     

Disconnected Surface Water and Groundwater: From Theory to Practice

When describing the hydraulic relationship between rivers and aquifers, the term disconnected is frequently misunderstood or used in an incorrect way. The problem is compounded by the fact that there is no definitive literature on the topic of disconnected surface water and groundwater. We aim at closing this gap and begin the discussion with a short introduction to the historical background of the terminology. Even though a conceptual illustration of a disconnected system was published by Meinzer (1923) , it is only within the last few years that the underlying physics of the disconnection process has been described. The importance of disconnected systems, however, is not widely appreciated. Although rarely explicitly stated, many approaches for predicting the impacts of groundwater development on surface water resources assume full connection. Furthermore, management policies often suggest that surface water and groundwater should only be managed jointly if they are connected. However, although lowering the water table beneath a disconnected section of a river will not change the infiltration rate at that point, it can increase the length of stream that is disconnected. Because knowing the state of connection is of fundamental importance for sustainable water management, robust field methods that allow the identification of the state of connection are required. Currently, disconnection is identified by showing that the infiltration rate from a stream to an underlying aquifer is independent of the water table position or by identifying an unsaturated zone under the stream. More field studies are required to develop better methods for the identification of disconnection and to quantify the implications of heterogeneity and clogging processes in the streambed on disconnection.