The mineral exploration industry requires new methods and tools to address the challenges of decl... more The mineral exploration industry requires new methods and tools to address the challenges of declining mineral reserves and increasing discovery costs. Laser-induced breakdown spectroscopy (LIBS) represents an emerging geochemical tool for mineral exploration that can provide rapid, in situ, compositional analysis and high-resolution imaging in both laboratory and field and settings. We demonstrate through a review of previously published research and our new results how LIBS can be applied to qualitative element detection for geochemical fingerprinting, sample classification, and discrimination, as well as quantitative geochemical analysis, rock characterization by grain size analysis, and in situ geochemical imaging. LIBS can detect elements with low atomic number (i.e., light elements), some of which are important pathfinder elements for mineral exploration and/or are classified as critical commodities for emerging green technologies. LIBS data can be acquired in situ, facilitati...
A broad suite of geological materials was studied a using a handheld laser‐induced breakdown spec... more A broad suite of geological materials was studied a using a handheld laser‐induced breakdown spectroscopy (LIBS) instrument. Because LIBS is simultaneously sensitive to all elements, the full broadband emission spectrum recorded from a single laser shot provides a ‘chemical fingerprint’ of any material – solid, liquid or gas. The distinguishing chemical characteristics of the samples analysed were identified through principal component analysis (PCA), which demonstrates how this technique for statistical analysis can be used to identify spectral differences between similar sample types based on minor and trace constituents. Partial least squares discriminant analysis (PLSDA) was used to distinguish and classify the materials, with excellent discrimination achieved for all sample types. This study illustrates through four examples (carbonate minerals and rocks, the oxide mineral pair columbite–tantalite, the silicate mineral garnet and native gold) how portable, handheld LIBS analyse...
The availability of both global and regional elevation datasets acquired by modern remote sensing... more The availability of both global and regional elevation datasets acquired by modern remote sensing technologies provides an opportunity to significantly improve the accuracy of stream mapping, especially in remote, hard to reach regions. Stream extraction from digital elevation models (DEMs) is based on computation of flow accumulation, a summary parameter that poses performance and accuracy challenges when applied to large, noisy DEMs generated by remote sensing technologies. Robust handling of DEM depressions is essential for reliable extraction of connected drainage networks from this type of data. The least-cost flow routing method implemented in GRASS GIS as the module r.watershed was redesigned to significantly improve its speed, functionality, and memory requirements and make it an efficient tool for stream mapping and watershed analysis from large DEMs. To evaluate its handling of large depressions, typical for remote sensing derived DEMs, three different methods were compared: traditional sink filling, impact reduction approach, and least-cost path search. The comparison was performed using the Shuttle Radar Topographic Mission (SRTM) and Interferometric Synthetic Aperture Radar for Elevation (IFSARE) datasets covering central Panama at 90 m and 10 m resolutions, respectively. The accuracy assessment was based on ground control points acquired by GPS and reference points digitized from Landsat imagery along segments of selected Panamanian rivers. The results demonstrate that the new implementation of the least-cost path method is significantly faster than the original version, can cope with massive datasets, and provides the most accurate results in terms of stream locations validated against reference points.
... (1988) apply garnet-biotite thermometry in sillimanite-grade sam-ples from the Garhwal Himala... more ... (1988) apply garnet-biotite thermometry in sillimanite-grade sam-ples from the Garhwal Himalaya to ... Altema-tively, fluids may have existed, for example resulting in syn-metamorphic quartz lenses ... grade rocks are also shown, indicating a likely range for the un-melted protolith ...
A combination of hydrogen and oxygen isotope analyses and fluid inclusion studies has defined the... more A combination of hydrogen and oxygen isotope analyses and fluid inclusion studies has defined the composition of fluids involved in the metamorphism of Lower Palaeozoic rocks in the English Lake District. Three fluid fields have been defined from secondary phases: 1, syn-burial metamorphic D-enriched fluids from epidote and chlorite at a temperature between 250 and 350°C; D-depleted fluid measured from groundmass and quartz inclusions; 3, a mixed magmatic-meteoric fluid with an intermediate H-isotopic composition estimated from W/R granite data and calculated from illite.
Spectrochimica Acta Part B: Atomic Spectroscopy, 2001
The Army Research Office of the Army Research Laboratory recently sponsored the development of a ... more The Army Research Office of the Army Research Laboratory recently sponsored the development of a commercial Ž. laser-induced breakdown spectroscopy LIBS chemical sensor that is sufficiently compact and robust for use in the Ž. field. This portable unit was developed primarily for the rapid, non-destructive detection of lead Pb in soils and in paint. In order to better characterize the portable system, a comparative study was undertaken in which the performance of the portable system was compared with a laboratory LIBS system at the Army Research Laboratory that employs a much more sophisticated laser and detector. The particular focus of this study was to determine the effects on the performance of the field sensor's lower spectral resolution, lack of detector gating, and the multiple laser pulsing that occurs when using a passively Q-switched laser. Surprisingly, both the laboratory and portable LIBS systems exhibited similar performance with regards to detection of Pb in both soils and in paint over the 0.05᎐1% concentration levels. This implies that for samples similar to those studied here, high-temporal resolution time gating of the detector is not necessary for quantitative analysis by LIBS. It was also observed that the multiple pulsing of the laser did not have a significant positive or negative effect on the measurement of Pb concentrations. The alternative of using other Pb lines besides the strong 406-nm line was also investigated. No other Pb line was superior in strength to the 406-nm line for the latex paint and the type of soils used in the study, although the emission line at 220 nm in the UV portion of the spectrum holds potential for avoiding elemental interferences. These results are
Laser-induced breakdown spectroscopy (LIBS) is a simple and straightforward technique of atomic e... more Laser-induced breakdown spectroscopy (LIBS) is a simple and straightforward technique of atomic emission spectroscopy that can provide multi-element detection and quantification in any material, in-situ and in real time because all elements emit in the 200–900 nm spectral range of the LIBS optical emission. This study evaluated two practical applications of LIBS—validation of labels assigned to garnets in museum collections and discrimination of LCT (lithium-cesium-tantalum) and NYF (niobium, yttrium and fluorine) pegmatites based on garnet geochemical fingerprinting, both of which could be implemented on site in a museum or field setting with a handheld LIBS analyzer. Major element compositions were determined using electron microprobe analysis for a suite of 208 garnets from 24 countries to determine garnet type. Both commercial laboratory and handheld analyzers were then used to acquire LIBS broadband spectra that were chemometrically processed by partial least squares discrimina...
The ability to rapidly conduct in-situ chemical analysis of multiple samples of soil and other ge... more The ability to rapidly conduct in-situ chemical analysis of multiple samples of soil and other geological materials in the field offers many advantages over a traditional approach that involves collecting samples for subsequent examination in the laboratory. This study explores the application of complementary spectroscopic analyzers and a data fusion methodology for the classification/discrimination of >100 soil samples from sites across the United States. Commercially available, handheld analyzers for X-ray fluorescence spectroscopy (XRFS), Raman spectroscopy (RS), and laser-induced breakdown spectroscopy (LIBS) were used to collect data both in the laboratory and in the field. Following a common data pre-processing protocol, principal component analysis (PCA) and partial least squares discriminant analysis (PLSDA) were used to build classification models. The features generated by PLSDA were then used in a hierarchical classification approach to assess the relative advantage o...
Numerous ephemeral streams flow within the McMurdo Dry Valley Region of Antarctica that transport... more Numerous ephemeral streams flow within the McMurdo Dry Valley Region of Antarctica that transport glacial meltwater to perennially ice-covered, closed-basin lakes during the austral summer. The diurnal behavior for two Taylor Valley streams of different character was examined during the summer of 2010-11. Andersen Creek is a short, 1st-order proglacial stream, whereas Von Guerard Stream is a long, high-order stream with an extensive hyporheic zone that has a substantial cyanobacterial algal mat community in its middle reaches. Both streams display strong daily cycles for temperature, electrical conductivity, dissolved oxygen, and pH. Conductivity varies in concert with flow, with solute dilution occurring during the daily high-flow pulse. Dissolved oxygen co-varies strongly with pH at Andersen Creek but not for Von Guerard Stream. Each stream has a distinct geochemical character that for Andersen Creek is a direct reflection of its glacial source, unmodified by secondary effects, wh...
Geochemical evolution of igneous rocks and changing magma sources during the formation and closure of the Central American land bridge of Panama
Geological Society of America Memoirs
ABSTRACT The geological development of Panama's isthmus resulted from intermittent magmat... more ABSTRACT The geological development of Panama's isthmus resulted from intermittent magmatism and oceanic plate interactions over approximately the past 100 m.y. Geochemical data from similar to 300 volcanic and intrusive rocks sampled along the Cordillera de Panama document this evolution and are used to place it in a tectonic framework. Three distinct trace-element signatures are recognized in the oldest basement rocks: (1) oceanic basement of the Caribbean large igneous province (CLIP basement) displays flat trace-element patterns, (2) CLIP terranes show enriched ocean-island basalt (OIB) signatures, and (3) CLIP rocks exhibit arc signatures. The Chagres igneous complex represents the oldest evidence of arc magmatism in Panama. These rocks are tholeiitic, and they have enriched but highly variable fluid-mobile element (Cs, Ba, Rb, K, Sr) abundances. Ratios of these large ion lithophile elements LILEs) to immobile trace elements (e. g., Nb, Ta, middle and heavy rare earth elements) have a typical, but variably depleted, arc-type character that was produced by subduction below the CLIP oceanic plateau. These early arc rocks likely comprise much of the upper crust of the Cordillera de Panama and indicate that by 66 Ma, the mantle wedge beneath Panama was chemically distinct (i.e., more depleted) and highly variable in composition compared to the Galapagos mantle material, from which earlier CLIP magmas were derived. Younger Miocene andesites were erupted across the Cordillera de Panama from 20 to 5 Ma, and these display relatively uniform trace-element patterns. High field strength elements (HFSEs) increase from tholeiitic to medium-K arc compositions. The change in mantle sources from CLIP basement to arc magmas indicates that enriched sub-CLIP (i.e., plume) mantle material was no longer present in the mantle wedge by the time that subduction magmatism commenced in the area. Instead, a large spectrum of mantle compositions was present at the onset of arc magmatism, onto which the arc fluid signature was imprinted. Arc maturation led to a more homogeneous mantle wedge, which became progressively less depleted due to mixing or entrainment of less-depleted backarc mantle through time. Normal arc magmatism in the Cordillera de Panama terminated around 5 Ma due to the collision of a series of aseismic ridges with the developing and emergent Panama landmass. Younger heavy rare earth element-depleted magmas (younger than 2 Ma), which still carry a strong arc geochemical signature, were probably produced by ocean-ridge melting after their collision.
Radiometric dating of Tasmanian speleothems—evidence of cave evolution and climatic change
Journal of the Geological Society of Australia
Th/U dates on Tasmanian speleothems enable preliminary estimates to be made of ages and rates of ... more Th/U dates on Tasmanian speleothems enable preliminary estimates to be made of ages and rates of evolution of Tasmanian karst caves. Occurrence of speleothem deposition has varied markedly in time, the highest abundance being associated with marine isotope stages 1 and 5. Rates of growth of from 21 to 79 mm/ka have been determined for four equilibrium diameter stalagmites, some
A mineralogical survey at Castleguard Cave has revealed complex suites of carbonate and sulfate m... more A mineralogical survey at Castleguard Cave has revealed complex suites of carbonate and sulfate minerals, plus seasonal and permanent ice. Carbonate minerals observed (and number of occurrences in 55 samples) are as follows: calcite (43), huntite (15), hydromagnesite (6), aragonite (4), monohydrocalcite (I). Sulfate minerals observed (number of occurrences in 10 samples) are: gypsum (9), thenardite/mirabilite (2), epsomite (I). Ancient massive dripstone and flowstone deposits consist only of calcite. Smaller dripstones, chalky coatings on dripstone, thin flowstone crusts, erratics, and pasty wall deposits ("moonmilk") were commonly combinations of calcite plus huntite. Aragonite occurs mainly as crystalline wall concretions associated with moonmilk deposits and as small anthodites. Hydromagnesite and monohydrocalcite are only found as dry, crusty, "moonrnilk" wall deposits. Distribution of carbonate and sulfate minerals is related to the thermal regime and presence of discrete seepage inputs. Small calcite speleothems occur to some extent throughout the cave. Massive dripstones and abundant decoration are restricted to sites of present or past seepage concentration. Sulfate mineralization and basic and hydrated carbonates are confined to warm, drier portions of the central cave. Chemical and stable isotope data indicate that calcite is precipitated at constant CO 2 partial pressure. Evaporation is the primary process responsible for the presence of the diverse sulfate and "exotic" carbonate minerals in the cave.
The Columbia Icefield rests upon limestones containing natural caves that drain waters from the g... more The Columbia Icefield rests upon limestones containing natural caves that drain waters from the glacier sole. The principal cave is sealed at one end by an extrusion of glacier ice 300 m below the icefield surface. The hydrologic regime of the cave indicates that the modern icefield is temperate in character and that water is present at the glacier sole throughout the year. An interpretation of the air temperature pattern in the cave suggests that the geothermal flux to the glacier is only 10-40% of the expected value because heat is abstracted by melt water circulating through the rock. U, Th and O isotopic analyses of calcite speleothems further indicate that the base of the icefield has probably been temperate throughout the past 150000 years. The cave was inundated when glaciers expanded during the classical Wisconsinan main Würm period. The inundation implies maintenance of a permanent water table at some hundreds of meters above the base in a valley glacier 750—800 m in depth.
Evaluation of controls on silicate weathering in tropical mountainous rivers: Insights from the Isthmus of Panama
Geology
The Isthmus of Panama comprises a lithologically diverse andesitic oceanic arc of Late Cretaceous... more The Isthmus of Panama comprises a lithologically diverse andesitic oceanic arc of Late Cretaceous to Holocene age; it has large spatial variation in rainfall, displays a large range of physical erosion rates, and, therefore, is an ideal location to examine silicate weathering in the tropics. We use a multiyear data set of river chemistry for a 450 km transect across the Cordillera Central of west-central Panama to investigate controls on chemical weathering in tropical small mountainous rivers. Sea-salt corrected cation weathering yields (Casil + Mgsil + Na + K) range over more than an order in magnitude from 3.1 to 31.7 t/km2/yr, while silicate weathering yields (Casil + Mgsil + Na + K + Si) range from 6.9 to 69.5 t/km2/yr. Watershed lithology is the primary control on riverine chemistry, but landscape topographic character and land cover and/or land use also influence solute delivery potential. Strong statistical links of small mountainous river chemical weathering fluxes with rainfall and physical weathering rates attest to the importance of runoff and erosion in maintaining elevated bedrock weathering rates. CO2 consumption ranges from 155 × 103 mol/km2/yr to 1566 × 103 mol/km2/yr, in the upper range of global rates, leading us to suggest that andesite terrains should be considered separately when calculating removal of CO2 from the atmosphere via silicate weathering.
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Papers by R. S Harmon