Zircon M257 ‐ a Homogeneous Natural Reference Material for the Ion Microprobe U‐Pb Analysis of Zircon

We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for ²⁰⁶pb/²³⁸U and 0.7392 ± 0.0003 for ²⁰⁷pb/²³⁵U. Its ²⁰⁶pb/²³⁸U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ～ 840 μg g^?1 U (Th/U ～ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 10¹⁸ g^?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.     

The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ¹⁸O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ¹⁸O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# _opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ³⁴S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ¹⁸O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ¹⁸O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.     

The tritium and carbon-14 profiles at the Geosecs I (1969) and GOGO I (1971) North Pacific stations

Tritium and¹⁴C data of the “GOGO I” station at 28.5°N, 121.6°W in November 1971 are reported. The tritium decline between 150 and 350 m depth is as pronounced as was observed on a previous occupation of the same position, station “Geosecs I” in September 1969, and the tritium concentrations below 200 m are unchanged.¹⁴C data from the depth range of tritium decline are corrected for fallout¹⁴C contribution. The correcting procedure requires simultaneous measurements of¹⁴C, ΣCO₂, and tritium. It is concluded that the natural absolute¹⁴C concentration attains a maximum near 400 m depth, of 7.5% excess over that of surface water.     

Sulfur isotope measurement of sulfate and sulfide by high-resolution MC-ICP-MS

We have developed a technique for the accurate and precise determination of ³⁴S/³²S isotope ratios (δ³⁴S) in sulfur-bearing minerals using solution and laser ablation multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We have examined and determined rigorous corrections for analytical difficulties such as instrumental mass bias, unresolved isobaric interferences, blanks, and laser ablation- and matrix-induced isotopic fractionation. Use of high resolution sector-field mass spectrometry removes major isobaric interferences from O₂⁺. Standard-sample bracketing is used to correct for the instrumental mass bias of unknown samples. Background on sulfur masses arising from memory effects and residual oxygen-tailing are typically minor (< 0.2‰, within analytical error), and are mathematically removed by on-peak zero subtraction and by bracketing of samples with standards determined at the same signal intensity (within 20%). Matrix effects are significant (up to 0.7‰) for matrix compositions relevant to many natural sulfur-bearing minerals. For solution analysis, sulfur isotope compositions are best determined using purified (matrix-clean) sulfur standards and sample solutions using the chemical purification protocol we present. For in situ analysis, where the complex matrix cannot be removed prior to analysis, appropriately matrix-matching standards and samples removes matrix artifacts and yields sulfur isotope ratios consistent with conventional techniques using matrix-clean analytes. Our method enables solid samples to be calibrated against aqueous standards; a consideration that is important when certified, isotopically-homogeneous and appropriately matrix-matched solid standards do not exist. Further, bulk and in situ analyses can be performed interchangeably in a single analytical session because the instrumental setup is identical for both. We validated the robustness of our analytical method through multiple isotope analyses of a range of reference materials and have compared these with isotope ratios determined using independent techniques. Long-term reproducibility of S isotope compositions is typically 0.20‰ and 0.45‰ (2σ) for solution and laser analysis, respectively. Our method affords the opportunity to make accurate and relatively precise S isotope measurement for a wide range of sulfur-bearing materials, and is particularly appropriate for geologic samples with complex matrix and for which high-resolution in situ analysis is critical.     

Anomalously nucleogenic neon in North Chile Ridge basalt glasses suggesting a previously degassed mantle source

Fresh basalt glasses from the North Chile Ridge (NCR) in the southeastern Pacific have Ne isotopic compositions distinctly different from typical mid-ocean ridge basalts (MORB). In a three-isotope plot of ²⁰Ne/²²Ne vs. ²¹Ne/²²Ne, the NCR data define a correlation line with a slope smaller than that of the MORB correlation line, i.e. their Ne composition is more nucleogenic than that of MORB. ³He/⁴He ratios are slightly lower than the MORB average, whereas in a few stepwise heating fractions very high ⁴⁰Ar/³⁶Ar ratios up to 28,000 are found. One model to explain the data assumes contamination of the NCR mantle source by material from the continental or oceanic crust, but in addition to difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical characteristics of NCR basalts reported earlier [Bach et al., Earth Planet. Sci. Lett. 142 (1996) 223–240], such as depletions in highly incompatible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the southern East Pacific Rise that was transported to the NCR and melted again. The time during which such a depleted reservoir would have to be separated from the MORB mantle is estimated at 10–100 Ma based on U/Th–Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evolution of the southeast Pacific.     

Next-generation robotic planetary reconnaissance missions: A paradigm shift

A fundamentally new scientific mission concept for remote planetary surface and subsurface reconnaissance will soon replace the engineering and safety constrained mission designs of the past, allowing for optimal acquisition of geologic, paleohydrologic, paleoclimatic, and possible astrobiologic information of Mars and other extraterrestrial targets. Traditional missions have performed local ground-level reconnaissance through rovers and immobile landers, or global mapping performed by an orbiter. The former is safety and engineering constrained, affording limited detailed reconnaissance of a single site at the expense of a regional understanding, while the latter returns immense datasets, often overlooking detailed information of local and regional significance. A “tier-scalable” paradigm integrates multi-tier (orbitatmosphereground) and multi-agent (orbiterblimpsrovers/sensorwebs) hierarchical mission architectures, not only introducing mission redundancy and safety, but enabling and optimizing intelligent, unconstrained, and distributed science-driven exploration of prime locations on Mars and elsewhere, allowing for increased science return, and paving the way towards fully autonomous robotic missions.     

Comparison and integration of spatial data using covariance matrices

A differential approach to map comparison and integration is presented utilizing the variance-covariance matrices of polynomial trend surfaces. The approach is based on the statistics presented in the trend-surface computer program, CORSURF. Examples in south-central Kansas and the Maracaibo Basin in Venezuela are presented to show how the technique could be used in basinanalysis studies.     

Near-K/T age of clastic deposits from Texas to Brazil: impact,volcanism and/or sea-level lowstand?

Near-K/T boundary clastic deposits from Texas, Mexico, Haiti, Guatemala and Brazil, often described as impact-generated tsunami deposits, are stratigraphically below well-defined K/T boundary horizons and appear not to be causally related to the K/T boundary event. Stratigraphic evidence indicates that their deposition began during the last 170–200 kyr of the Maastrichtian, coincident with a major eustatic sea-level lowstand that lowered sea level by as much as 70–100 m. Clastic deposition ended a few tens of thousands of years before the K/T boundary during a rapidly rising sea level. The presence of glass in clastic deposits in Haiti, northeastern Mexico and Yucatan suggests that the sea-level lowstand coincided with a time of major volcanism or pre-K/T boundary bolide impact.     

Examples of structural control of hydrothermal mineralization: Fault zones in epicontinental sedimentary basins — A review

By compiling data available from progressive geoscientific research and by complementary studies it seems possible for an increasing number of mineral districts to find indications of the regional structural control of hydrothermal ore deposits. The relationship between hydrothermal mineralization, faulting or fault reactivation and related processes can, of course, best be examined at the less deformed sedimentary record of ancient epicontinental sedimentary basins. There is increasing evidence that the sediment-hosted submarine-hydrothermal Zn-Pb-Ba mineralizations such as Meggen, Rammelsberg, Howards Pass and McArthur River are spatially and genetically related to deep synsedimentarily active fault systems of intracontinental basins or stretched passive continental margins. For long periods of basin development the faults provided favorable conduits for episodically released basin-dewatering brines, which precipitated the leached metals into third-order basins situated next to these faults. There are many similarities to epigenetic mineralizations, which formed either pre-, syn- or postorogenetically from brines leaching the (meta)sediments and associated magmatites and migrating along fault zones to their near-surface trap structures.

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Zusammenfassung Für viele Erzprovinzen bietet sich durch Kompilation der in den letzten Jahrzehnten rasch angewachsenen Zahl geowissenschaftlicher Daten und durch gezielte ergänzende Untersuchungen die Möglichkeit, Anhaltspunkte oder Belege für die regionale Strukturbindung von hydrothermalen Vererzungen zu finden. Naturgemäß lassen sich die Zusammenhänge zwischen hydrothermaler Mineralisation, Störungstektonik und begleitenden Prozessen am besten an nur gering orogen überprägten ehemaligen Sedimentbecken untersuchen. So gibt es vermehrt Belege dafür, daß die sediment-gebundenen, submarin-hydrothermalen Zn-Pb-Ba Vererzungen wie Meggen, Rammeisberg, Howards Pass, McArthur River etc. räumlich und genetisch an tiefreichende Störungssysteme gebunden sind, die während der Sedimentation in intrakontinentalen Riftbecken oder auf ausgedünnten passiven Kontinentalrändern aktiv waren. Diese Störungssysteme bündelten die salinaren Beckenentwässerungsströme und ermöglichten über lange Zeiträume ihren episodischen Aufstieg zum Meeresboden, wo die aus den Sedimenten aufgenommenen Metalle als Sulfide in Spezialbecken abgesetzt wurden. Es bestehen prinzipielle Ähnlichkeiten mit vielen epigenetischen prä-, syn- oder postorogenen Mineralisationen; auch hier migrierten die salinaren Lösungen, die ihren Metallgehalt aus den (Meta)Sedimenten und eingelagerten Magmatiten bezogen, entlang von Störungssystemen bis zu den oberflächennahen Fallenstrukturen.

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Résumé La compilation des données géologiques qui se sont largement multipliées au cours des dernières décennies, complétée par des rechercùhes complémentaires particulières, nous offre pour beaucoup de régions la possibilité de trouver des indices ou même des preuves d'un contrôle structural des minéralisations hydrothermales. Il va de soi que les relations entre la minéralisation hydrothermale, la formation et la réactivation des failles ainsi que les processus associés sont les plus faciles à étudier dans les couches sédimentaires les moins touchées par une orogenèse postérieure. Ainsi, il existe des preuves que les gisements stratiformes et sédimentaires de Zn-Pb-Ba du type Howards Pass, McArthur River et Meggen sont liés à des systèmes de failles synsédimentaires profondes, en relation avec des bassins intracontinentaux du genre rift et avec des marges continentales passives en extension. Ces systèmes de failles synsédimentaires ont drainé des eaux salines intersticielles et ont rendu possible leur mouvement ascendant vers les fonds marins où leur contenu métallifère s'est déposé dans des bassins locaux. Des nombreuses recherches récentes mettent en évidence la ressemblance avec les minéralisations épigénétiques formées avant, pendant et après l'orogenèse; là aussi, les eaux interstitielles, après avoir reçu leur contenu métallifère des (meta)sédiments et des magmatites associées ont migré le long de systèmes de failles jusqu'aux pièges structuraux proches de la surface.

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