The Development of Laser Ablation ICP-MS and Calibration Strategies: Examples from the Analysis of Trace Elements in Volcanic Glass Shards and Sulfide Minerals

This contribution presents a review of the recent developments in laser ablation inductively coupled plasma-mass spectrometry. We describe the important developments which have occurred in the laser systems used, leading to a spatial resolution of around 20 (im, and give an overview of the major instrument developments which have affected the geological applications of laser ablation ICP-MS. We describe the calibration of laser ablation for the analysis of trace elements in two different matrices: volcanic glass shards and sulfide minerals. We show how single glass shards can be analysed using the National Institute of Standards and Technology (NIST) glass certified reference materials for calibration and demonstrate the effect of using single spot analyses compared to rastering of the calibration sample. We show the importance of inter-shard variation and demonstrate that averaged single shard analyses produce data which compare well with bulk analyses. The calibration of the laser system for sulfide mineral analysis is discussed and two different strategies are proposed, one using spiked pressed powder pellets of sulfides and the other metal reference materials. We present conclusions and recommendations for the calibration of laser ablation ICP-MS instruments.     

Depositional history and stratigraphical evolution of the Sakoa Group (Lower Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Sakoa Group is the lowermost stratigraphical succession of the Karoo Supergroup and the oldest sedimentary unit in Madagascar, spanning the Late Carboniferous through Early Permian epochs. The Sakoa Group is exposed in the southern Morondava Basin. It is predominantly a siliciclastic sequence comprising seven lithofacies associations: (1) diamictites; (2) conglomeratic sandstones; (3) sandstones; (4) interbedded thin sandstones and mudstones; (5) mudstones; (6) coals; and (7) limestones. These facies represent deposition in the early extensional stages of continental rift development. The sediments were deposited predominantly on alluvial fans, and in braided to meandering stream and overbank environments. Locally lacustrine and coal swamp environments formed in low areas of the basin floor during rift initiation. Subsidence rates remained fairly constant throughout the Early Permian and were accompanied by a gradual reduction in relief of the basin margins and an increased geomorphic maturity of the fluvial systems flowing across the basin floor. Near the end of the Early Permian the southern Morondava Basin was inundated by a marine transgression , which resulted in deposition of the Vohitolia Limestone. Subsequent tectonic uplift and erosion resulted in a regional unconformity between the Sakoa Group and the overlying Sakamena Group.     

The Effects of Climate Change on Water Resources in the West: Introduction and Overview

The results of an experimental `end to end' assessment of the effects of climate change on water resources in the western United States are described. The assessment focuses on the potential effects of climate change over the first half of the 21st century on the Columbia, Sacramento/San Joaquin, and Colorado river basins. The paper describes the methodology used for the assessment, and it summarizes the principal findings of the study. The strengths and weaknesses of this study are discussed, and suggestions are made for improving future climate change assessments.     

Genetic relationship of rift-stage crustal structure, terrane accretion, and foreland tectonics along the southern Appalachian-Ouachita orogen

Abrupt along-strike variations in tectonostratigraphic composition, internal structural style, and detachment level in the southern Appalachian and Ouachita foreland thrust belts are defined at a large-scale bend in strike and a truncation of Ouachita structures by the frontal Appalachian thrust fault. The along-strike variations correspond to differences in the pre-orogenic rifted Laurentian margin, in the history and nature of terrane accretion, and in the response of the foreland to these differences. Within the Ouachita embayment of the Laurentian margin, diachronous arc-continent collision migrated northwestward along a rift-stage transform margin from the Black Warrior foreland basin on the southeast in Late Mississippian time to a short-wavelength, high-amplitude foreland basin (Arkoma basin) on the northwest in front of the Ouachita thrust-belt salient in Early-Middle Pennsylvanian time. Off-shelf, deep-water strata of both passive-margin and synorogenic facies comprise an accretionary prism and subduction complex, and the Ouachita allochthon consists of mud-dominated thrust sheets that are internally disharmonic and folded. The allochthon of off-shelf strata was thrust over the passive-margin carbonate shelf, which remains in the Ouachita footwall. Along the southeast side of the Alabama promontory of the Laurentian margin, passive-margin shelf carbonates are imbricated in the Appalachian thrust belt, which is characterized by internally coherent thrust sheets and high-amplitude frontal ramps. The palinspastic extent of shelf-carbonate rocks corresponds to the extent of structurally shallow basement rocks on the upper-plate rift-stage margin of the Alabama promontory of Laurentian crust. Terranes accreted to the Laurentian margin during the Taconic and Acadian orogenies were driven over the shallow basement by continent-continent collision of Laurentia with Africa (Gondwana). Emplacement of the thrust-translated terranes tectonically stripped and replaced the shelf carbonate. The frontal thrust fault of the Appalachian thrust belt truncates the southeastern end of the slightly older frontal Ouachita thrust belt, as well as the southeastern part of the greater Black Warrior basin in the Ouachita foreland. Shallow basement beneath the Appalachian thrust belt extends cratonward beneath the low-amplitude Appalachian foreland basin.