Thomas J. Shepherd

Electron microprobe and LA-ICP-MS study of monazite hydrothermal alteration:: Implications for U-Th-Pb geochronology and nuclear ceramics

This paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration, namely sericitization, chloritization, and greisenization. The samples were taken from three Paleozoic granites: the St. Nectaire granite (Massif Central, France), the Carnmenellis granite (Cornwall, England), and the Skiddaw granite (Lake District, England). Fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340°C and salinities from 3 to 18 wt% NaCl equivalent. Possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy (BSE-SEM), electron microprobe, laser Raman spectroscopy and laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) include cationic substitutions, monoclinic to hexagonal structure transition accompanied by chemical exchanges, selective Th removal, dissolution-reprecipitation, and dissolution with replacement by a different mineral. These results show that, despite the compositional and crystallographic simplicity of monazite, it exhibits varied chemical responses to different mineral-fluid interactions. Elemental and isotopic measurements by LA-ICP-MS for the unaltered monazites yield statistically significant 232Th-208Pb and 238U-206Pb magmatic crystallization ages. In some cases it was possible to give a reasonable estimate of the age of mineral-fluid interaction using the altered parts of the monazites or newly precipitated crystals. For other minerals, 232Th-208Pb and 238U-206Pb systematics were strongly disturbed by variable inputs and/or depletions of U, Th, and Pb. The data suggest that monazite-like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies, especially if the fluids are oxidizing or tend to dissolve monazite.

Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. (1) exoskarns 2. (2) high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias 3. (3) polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization 4. (4) late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40Ar39Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40Ar39Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210°C myr−1 to ~60°C myr−1 with a mean of 100°C myr−1. These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The mineralization history is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorite from stage 3 polymetallic mineralization give ages of 259 ± 7, 266 ± 3 and 267 ± 12 Ma, postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and cooling of the host granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

Recovery of 16S ribosomal RNA gene fragments from ancient halite

During the last decade, sensitive techniques for detecting DNA have been successfully applied to archaeological and other samples that were a few hundred to a few thousand years old. Nevertheless, there is still controversy and doubt over claims of exceptionally ancient DNA. Additional accounts stretching back nearly a century suggest that microorganisms may survive over geological time in evaporite deposits. There is, however, often doubt over the age relationship between evaporite formation and the incorporation of microorganisms. Here, we have used petrographic and geochemical techniques (laser ablation microprobe–inductively coupled plasma–mass spectrometry) to verify the estimated geological age of halite (NaCl) evaporite samples. Fragments of 16S ribosomal RNA genes were detected by polymerase chain reaction amplification of DNA extracted from halite samples ranging in age from 11 to 425 Myr (millions of years). Haloarchaeal 16S rDNA amplicons were present in one sample (11–16 Myr), whereas other samples (65–425 Myr) yielded only bacterial 16S rDNA amplicons. Terminal restriction fragment length polymorphism analyses indicate complex and different populations of microorganisms or their free DNA in ancient halites of different ages.

Validation of LA-ICP-MS fluid inclusion analysis with synthetic fluid inclusions

Abstract Laser ablation.inductively coupled plasma.mass spectrometry (LA-ICP-MS) has become recognized as a sensitive, efficient, and cost-effective approach to measuring the major-, minor-, and trace-solute compositions of individual fluid inclusions in minerals. As a prerequisite for the routine analysis of natural inclusions in our laboratory, the precision and accuracy of the technique was assessed using sets of multi-element synthetic fluid inclusions. Five multi-element standard solutions were prepared, and incorporated as fluid inclusions in quartz crystals at 750 °C and 7 kbar. Fluid inclusions were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole reaction cell for the removal of Ar-based interferences. The internal standard used in all cases was Na. Analytical precision for K, Rb, and Cs is typically better than 15% RSD, whereas Li, Mg, Ca, Sr, Ba, Mn, Fe, Cu, Zn, and Cl analyses are typically reproducible within 30% RSD. Measured concentrations approximate a Gaussian distribution, suggesting that analytical errors are random. Analyses for most elements are accurate within 15%. Limits of detection vary widely according to inclusion volume, but are 1 to 100 µg/g for most elements. These figures of merit are in excellent agreement with previous studies. We also demonstrate that, over the range investigated, precision and accuracy are insensitive to inclusion size and depth. Finally, the combination of our LAICP- MS analyses with microthermometric data shows that charge-balancing to NaCl-H2O equivalent chloride molality is the most valid approach to LA-ICP-MS data reduction, where chloride-dominated fluid inclusions are concerned.

Laser ablation ICP-MS elemental analysis of individual fluid inclusions: An evaluation study

Details are given of the elemental analysis of single fluid inclusions using a UV laser ablation microprobe interfaced to an inductively coupled plasma mass spectrometer. The UV laser, a frequency quadrupled Nd:YAG operating at 266 run, allows higher spatial resolution (<2 μm) than can be achieved using near-IR or visible wavelengths. Tests have been carried out on 10–100 μm diameter aqueous (liquid + vapour) inclusions in fluorite, quartz, and halite up to 60 μm beneath the surface. A key feature of the system is a novel high temperature ablation cell which substantially improves the efficiency and reproducibility of fluid release. Calibration was carried out using a dual gas flow system that allowed use of standard solutions and NIST glasses for tuning the instrument and for obtaining relative sensitivity factors. As an alternative to synthetic fluid inclusions, a new calibration approach is described involving the encapsulation of microdroplets of standard solutions in hydrophobic epoxy resins fluid inclusion analogues. To illustrate the scope and performance of the instrument, data are reported for Ba, Ca, Cs, Cu, K, Mg, Mn, Na, Pb, Rb, Sr, and Zn in saline aqueous inclusions associated with evaporite and low temperature base metal deposits. Element detection limits vary according to the mass of material released for analysis and are thus related to the volume and composition of each inclusion. Precision is estimated to be better than 30%.

Nd and Sr isotope constraints on the origin of the Cornubian batholith, SW England

Combined Nd and Sr isotope data for the Hercynian granites of SW England provide important new evidence for the origin of the Cornubian batholith. The Nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration. ENd values for the main plutons range from −4.7 to −7.1 whereas values for the Palaeozoic country rocks are significantly lower, typically –8 to –11. Depleted mantle Nd model ages (1.3–1.8 Ga) indicate a heterogeneous source region for the granite magmas and imply that the age of the basement beneath southern England is significantly older than previously estimated. Calculated ENd values for the Palaeozoic country rocks at the time of granite intrusion suggest that they were unlikely to have been the main source material for the magmas. END-ESr data for the granites do not define a simple binary mixing array between a mafic, depleted mantle melt and Proterozoic crust. The best fit model is consistent with partial melting of a composite lower crustal source comprising immature metasedimentary and mafic meta-volcanic rocks, and mixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source. Such hybrid magmas are thought to have been generated during an Ivrea-type underplating of the lower crust during continental collision; their isotope signatures being determined by the relative proportions of crust and mantle material.

Using biochemical and isotope geochemistry to understand the environmental and public health implications of lead pollution in the lower Guadiana River, Iberia: A freshwater bivalve study

Lead is a natural component of aquatic ecosystems with no known biological role and is highly toxic. Its toxicity stems from its ability to mimic biologically important metals and to produce membrane damage through lipid peroxidation (LPO). Most lead poisoning symptoms are thought to occur by interfering with an essential enzyme, delta-aminolevulinic acid dehydratase (ALAD), the activity of which is markedly inhibited by lead. The purpose of this work was to study the levels and effects of lead pollution (responses of ALAD and oxidative stress biomarker LPO) in the freshwater bivalve Corbicula fluminea along the lower Guadiana River (Portugal and Spain); a major river system impacted by historic mining pollution and more recent anthropogenic inputs. The results show that the enzymatic activity of ALAD is negatively correlated with the total Pb concentration of the whole tissue suggesting that ALAD has considerable potential as a biomarker of lead exposure in C. fluminea. To identify the sources of lead to which bivalves have been exposed, high precision (206)Pb/(204)Pb, (207)Pb/(204)Pb, (208)Pb/(204)/Pb ratios for C. fluminea confirm that historical mining activities in the Iberian Pyrite Belt are the dominant source of lead pollution in the lower Guadiana River. The isotope patterns however exhibit marked seasonal and geographic variation in response to rainfall and river water management. Locally, other anthropogenic sources of lead have been detected in C. fluminea close to population centres, thus adding to its versatility as a freshwater bio-indicator. Overall, the study highlights the value of natural ecosystems as monitors of water quality and their importance for public health assessment and surveillance.

Timing and significance of crosscourse mineralization in SW England

Rb-Sr isotope analyses of inclusion fluids from quartz have demonstrated a Triassic age (236 ± 3 Ma) for N–S-trendmg Pb–Zn–F vein mineralization in the Tamar Valley district of the Cornubian orefield. Consideration of the stratigaphy and structure of the Permo-Triassic basins both onshore and in the English Channel suggest that this mineralization results from the formation of fractures during regional extension-driven subsidence, and the subsequent ingress of basinal brines to the Variscan basement.

Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the Castromil gold deposit, northern Portugal

Abstract Castromil (northern Portugal) is one of several important orogenic gold deposits located within the “Central Iberian” geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways, and the reconstruction of the P–T–X conditions during mineralisation were obtained by combining the geometric characteristics of veins and microstructures together with a detailed study of the inclusion fluids. Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone, characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic fluids dominated by CO2 and CH4 that were probably generated by high-temperature fluid–rock interaction (400–500 °C) with graphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the deposit. At temperatures between 400 and 500 °C, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of 10±1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and represents the main phase of quartz–arsenopyrite–pyrite deposition. The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275 °C). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system. The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments. To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of economic grade (10–15 g/t) gold ores.

Comparison of long-term geochemical interactions at two natural CO2-analogues: Montmiral (Southeast Basin, France) and Messokampos (Florina Basin, Greece) case studies

Publisher Summary This chapter assesses the impact of long-term CO2 accumulations in two sandy reservoirs of different origin: the high-temperature and high-pressure reservoir at great depth at Montmira and the shallow, low-temperature and low-pressure reservoir at Messokampos. Petrographic characterization of the reservoirs enabled the identification of both the effects of CO2-induced geochemical interactions as well as their impact on reservoir lithologies. Subsequently, geochemical modeling was applied to reproduce the observed effects, identify their driving parameters and to assess their impact in terms of potential mineral trapping and porosity changes. It indicates that the porosity increase attributed to this reactivity requires that the sediment is flushed intensively with CO2-rich pore waters and that a flow regime in the reservoir must have been in place at a certain point in the reservoirs geological history. The impact of these reactions is minor and does not seem to influence the porosity of the sediment. Comparison of the geochemical interactions at the two sites shows that a reservoirs temperature and pressure conditions determine the impact of CO2 interactions, with elevated temperatures significantly increasing the reaction rates of mineral-trapping reactions.