Experimental evaluation of elemental behavior during LA-ICP-MS: influences of plasma conditions and limits of plasma robustness

Matrix effects are one of the frequently observed and discussed issues challenging the accuracy of LA-ICP-MS results. The specific role of the ICP as a source of elemental fractionation seems not fully understood. We report the results of an experiment using six internationally available reference materials (five silicates, one carbonate) measured under 11 different plasma conditions. The thermal/energetic state of the plasma was estimated based on the ratio of Ar-38(+) and Ar-40(2)+ ions. We show that element specific behavior (volatile vs. refractory) dominates at cool but vanishes under hot plasma conditions. For robust (hot) plasma conditions matrix-tolerance seems to be achieved. Additionally we address the problem of matrix-load with respect to the plasma conditions. We've estimated practical limits for the amount of matrix which can be introduced into the ICP without significantly changing the plasma condition

Halogenated trace compounds in fumarolic gases of the Nicaraguan subduction zone volcanoes: variation patterns, budgets and impact on the Earth's atmosphere

Halogenated compounds are important constituents of the Earth's atmosphere, because they may have strong influence on its physical and chemical properties. The most abundant halogenated hydrocarbons in volcanic gas samples from fumaroles of the quiescently degassing Nicaraguan subduction zone volcanoes Momotombo, Cerro Negro and Mombacho, collected during four field campaigns between July 2001 and July 2003, were CH3Cl, CH3Br, CH3I, CH2Cl2, CHCl3, CCl4, C2H5Cl, C2H5Br, C2H5I and C2H3Cl. The variability in concentrations at different time-scales (minutes to years) is less than two orders of magnitude for the single volcanoes. Estimations of gas fluxes at Momotombo indicate that the contribution of quiescently degassing volcanoes to the global halo(hydro)carbon inventory is very low. Assuming that the investigated volcanoes are representative, the question of a significant volcanic contribution of stable halogenated compounds (CFCs, HFCs, HCFCs, Halons, CF4, NF3 and SF6) to the atmospheric budget can be negated. Furthermore a new sampling setup for fumarolic trace gases is presented

A simplified procedure for the determination of stable chlorine isotope ratios (δ37Cl) using LA-MC-ICP-MS

Stable chlorine isotopes (37Cl, 35Cl) are considered as important tracers of geochemical processes, especially in subduction zone systems. However, high-quality chlorine isotope data are scarcely available. Here we present a comparatively simple procedure for the precise and accurate determination of stable chlorine isotope ratios (δ37Cl) using LA-MC-ICP-MS. Chlorine was extracted from solid samples by pyrohydrolysis. After quantitative precipitation as AgCl the dried precipitates where analysed in a sample-standard bracketing approach using a weak laser ablation (0.3 J/cm2) for sample evaporation. Atlantic Ocean sea salt and the sea water standard IAPSO were used as SMOC (standard mean ocean chloride) for normalisation (δ37Cl = 0 ‰). The precision and accuracy of the presented method was validated analysing the reference materials JB-1a and JB-2. The chlorine isotope ratios of these standards were determined as δ37ClJB-1a = (−0.99 ± 0.06) ‰ and δ37ClJB-2 = (−0.60 ± 0.03) ‰ (errors 2SE), respectively, in accordance with published data. Applying the presented method a total amount of less than 1 μg of chlorine was consumed during a typical measurement including 10 ablation periods on the sample

Geological, mineralogical and textural impacts on the distribution of environmentally toxic trace elements in seafloor massive sulfide occurrences

With mining of seafloor massive sulfides (SMS) coming closer to reality, it is vital that we have a good understanding of the geochemistry of these occurrences and the potential toxicity impact associated with mining them. In this study, SMS samples from seven hydrothermal fields from various tectonic settings were investigated by in-situ microanalysis (electron microprobe (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) to highlight the distribution of potentially-toxic trace elements (Cu, Zn, Pb, Mn, Cd, As, Sb, Co, Ni, Bi, Ag and Hg) within the deposits, their minerals and textures. We demonstrate that a combination of mineralogy, trace element composition and texture characterisation of SMS from various geotectonic settings, when considered along with our current knowledge of oxidation rates and galvanic coupling, can be used to predict potential toxicity of deposit types and individual samples and highlight which may be of environmental concern. Although we cannot quantify toxicity, we observe that arc-related sulfide deposits have a high potential toxicity when compared with deposits from other tectonic settings based on their genetic association of a wide range of potentially toxic metals (As, Sb, Pb, Hg, Ag and Bi) that are incorporated into more reactive sulfosalts, galena and Fe-rich sphalerite. Thus, deposits such as these require special care when considered as mining targets. In contrast, the exclusive concern of ultra-mafic deposits is Cu, present in abundant, albeit less reactive chalcopyrite, but largely barren of other metals such as As, Pb, Sb, Cd and Hg. Whilst geological setting does dictate metal endowment, ultimately mineralogy is the largest control of trace element distribution and subsequent potential toxicity. Deposits containing abundant pyrrhotite (high-temperature deposits) and Fe-rich sphalerite (ubiquitous to all SMS deposits) as well as deposits with abundant colloform textures also pose a higher risk. This type of study can be combined with “bulk lethal toxicity” assessments and used throughout the stages of a mining project to help guide prospecting and legislation, focus exploitation and minimise environmental impact

Polymetallic Sulfide–Quartz Vein System in the Koudiat Aïcha Massive Sulfide Deposit, Jebilet Massif, Morocco: Microanalytical and Fluid Inclusion Approaches

The Koudiat Aïcha Zn-Cu-Pb deposit (3–Mt ore @ 3 wt.% Zn, 1 wt.% Pb, 0.6 wt.% Cu) in the Jebilet massif (Morocco) comprises stratabound lenticular orebodies and crosscutting sulfide-bearing quartz ± carbonate veins in the lower Carboniferous Sarhlef volcano sedimentary succession. The veins are characterized by abundant pyrrhotite, sphalerite, subordinate chalcopyrite and galena and rare Ag and Au minerals. The stratabound massive sulfide ores are attributed to a “VMS” type, whereas the origin of the sulfide–quartz ± carbonate veins remains poorly understood. New mineralogical and microanalytical data (SEM, EPMA and LA-ICP-MS) combined with fluid inclusion results point to two-stage vein formation. The early stage involved C–H–O–N Variscan metamorphic fluids which percolated through fractures and shear zones and deposited pyrite at >400 °C, followed by the formation of pyrrhotite and sphalerite (300 ± 20 °C) in quartz veins and in banded and breccia ores. The pyrrhotite–sphalerite mineralization was overprinted by aqueous brines (34 to 38 wt% eq. NaCl + CaCl2) that precipitated carbonate and Cu-Pb sulfides (±Ag-Au) at ~180–210 °C through mixing with low-salinity fluids during tectonic reworking of early-formed structures and in late extension fractures. The latter ore fluids were similar to widspread post-Variscan evaporitic brines that circulated in the Central Jebilet. Overlapping or successive pulses of different ore fluids, i.e., metamorphic fluids and basinal brines, led to metal enrichment in the quartz–carbonate veins compared to the massive sulfide ores. These results underscore that even a single deposit may record several distinct mineralizing styles, such that the ultimate metal endowment may be cumulative over multiple stages

Trace Metal Distribution in Sulfide Minerals from Ultramafic-Hosted Hydrothermal Systems: Examples from the Kairei Vent Field, Central Indian Ridge

The ultramafic-hosted Kairei vent field is located at 25°19′ S, 70°02′ E, towards the Northern end of segment 1 of the Central Indian Ridge (CIR-S1) at a water depth of ~2450 m. This study aims to investigate the distribution of trace elements among sulfide minerals of differing textures and to examine the possible factors controlling the trace element distribution in those minerals using LA-ICP-MS spot and line scan analyses. Our results show that there are distinct systematic differences in trace element distributions throughout the different minerals, as follows: (1) pyrite is divided into three types at Kairei, including early-stage euhedral pyrite (py-I), sub-euhedral pyrite (py-II), and colloform pyrite (py-III). Pyrite is generally enriched with Mo, Au, As, Tl, Mn, and U. Pyrite-I has high contents of Se, Te, Bi, and Ni when compared to the other types; py-II is enriched in Au relative to py-I and py-III, but poor in Ni; py-III is enriched in Mo, Pb, and U but is poor in Se, Te, Bi, and Au relative to py-I and py-II. Variations in the concentrations of Se, Te, and Bi in pyrite are most likely governed by the strong temperature gradient. There is generally a lower concentration of nickel than Co in pyrite, indicating that our samples precipitated at high temperatures, whereas the extreme Co enrichment is likely from a magmatic heat source combined with an influence of serpentinization reactions. (2) Chalcopyrite is characterized by high concentrations of Co, Se, and Te. The abundance of Se and Te in chalcopyrite over the other minerals is interpreted to have been caused by the high solubilities of Se and Te in the chalcopyrite lattice at high temperatures. The concentrations of Sb, As, and Au are relatively low in chalcopyrite from the Kairei vent field. (3) Sphalerite from Zn-rich chimneys is characterized by high concentrations of Sn, Co, Ga, Ge, Ag, Pb, Sb, As, and Cd, but is depleted in Se, Te, Bi, Mo, Au, Ni, Tl, Mn, Ba, V, and U in comparison with the other minerals. The high concentrations of Cd and Co are likely caused by the substitution of Cd2+ and Co2+ for Zn2+ in sphalerite. A high concentration of Pb accompanied by a high Ag concentration in sphalerite indicates that Ag occurs as Pb–Ag sulfosalts. Gold is generally low in sphalerite and strongly correlates with Pb, suggesting its presence in microinclusions of galena. The strong correlation of As with Ge in sphalerite from Kairei suggests that they might precipitate at medium temperatures and under moderately reduced conditions. (4) Bornite–digenite has very low concentrations of most trace elements, except for Co, Se, and Bi. Serpentinization in ultramafic-hosted hydrothermal systems might play an important role in Au enrichment in pyrite with low As contents. Compared to felsic-hosted seafloor massive sulfide deposits, sulfide minerals from ultramafic-hosted deposits show higher concentrations of Se and Te, but lower As, Sb, and Au concentrations, the latter often attributed to the contribution of magmatic volatiles. As with typical ultramafic-hosted seafloor massive sulfide deposits, Se enrichment in chalcopyrite from Kairei indicates that the primary factor that controls the Se enrichment is temperature-controlled mobility in vent fluids

Miocene to Holocene marine tephrostratigraphy offshore northern Central America and southern Mexico: Pulsed activity of known volcanic complexes

We studied the tephra inventory of fourteen deep sea drill sites of three DSDP and ODP legs drilled offshore Guatemala and El Salvador (Legs 67, 84, 138), and one leg offshore Mexico (Leg 66). Marine tephra layers reach back from the Miocene to the Holocene. We identified 223 primary ash beds and correlated these between the drill sites, with regions along the volcanic arcs, and to specific eruptions known from land. In total, 24 correlations were established between marine tephra layers and to well‐known Quaternary eruptions from El Salvador and Guatemala. Additional 25 tephra layers were correlated between marine sites. Another 108 single ash layers have been assigned to source areas on land resulting in a total of 157 single eruptive events. Tephra layer correlations to independently dated terrestrial deposits provide new time markers and help to improve or confirm age models of the respective drill sites. Applying the respective sedimentation rates derived from the age models, we calculated ages for all marine ash beds. Hence, we also obtained new age estimates for eight known, but so far undated large terrestrial eruptions. Furthermore, this enables us to study the temporal evolution of explosive eruptions along the arc and we discovered five pulses of increased activity: 1) a pulse during the Quaternary, 2) a Pliocene pulse between 6 and 3 Ma, 3) a Late Miocene pulse between 10 and 7 Ma, 4) a Middle Miocene pulse between 17–11 Ma, and 5) an Early Miocene pulse (~>21 Ma)