Local Hydrothermal Activity Research Articles

Lithium sources in oilfield waters from the Qaidam Basin, Tibetan Plateau: Geochemical and Li isotopic evidence

Oilfield waters represent an important source of lithium resources to meet increasing global demand. However, the distribution of Li resources in the western Qaidam Basin is still unclear, and the enrichment mechanism of Li remains to be explained. In this study, we present new hydrochemical data for all the oil-bearing anticlines in the Qaidam Basin. Oilfield waters in the Nanyishan and Dafengshan oilfields are characterized by Li concentrations ranging from 110.1 to 237.5 mg/L, and Mg concentrations ranging from 0.70 to 1.52 g/L. These concentrations are readily recovered and represent a key target for oilfield water exploitation in the future. In oilfield water samples from the Qaidam Basin, δ7Li values show large variation, possibly implying multiple sources of Li, including residual water from ancient lakes, alpine snow water infiltrating along deep faults, springs, and deep hydrothermal fluids. δ7Li values of the oilfield waters from Nanyishan and Dafengshan are lower than those of the river and spring water samples, but close to those of volcanic rocks around the world. These results indicate that the oilfield waters with significant Li-enrichment are associated with local hydrothermal activity. This is the first time that the δ7Li data of oilfield waters in the Qaidam Basin have been reported. It appears that δ7Li is a good tracer for determining the origin and the key controlling factors of Li accumulation processes in oilfield waters.

The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates

Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (3HeET) and excess thorium-230 (230ThXS), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic Ridge suggest 50–80% deficits in sedimentary 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (typically within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.

Correlating hydrothermal system dynamics and eruptive activity – A case-study of Piton de la Fournaise volcano, La Réunion

Piton de la Fournaise volcano, La Réunion Island, is a basaltic shield volcano, which underwent an intense cycle of eruptive activity between 1998 and 2008. Self-potential and other geophysical investigations of the volcano have shown the existence of a well-established hydrothermal system within the summit cone. The present study investigates the relationship between changes in the hydrothermal system and eruptive activity at the summit cone of Piton de la Fournaise. Here, we consider the depth of the hydrothermal activity section to be the area where the hydrothermal flow is the most intense along its path. Ten complete-loop self-potential surveys have been analyzed through multi-scale wavelet tomography (MWT) to characterize depth variations of the hydrothermal system between 1993 and 2008. Our MWT models strongly support the existence of six main hydrothermal flow pathways associated with the main edifice structure. Each of these pathways is part of the main hydrothermal system and is connected to the main hydrothermal reservoir at depth. In both 2006 and 2008, around Dolomieu crater, based on our results, the hydrothermal activity sections are located between 2300 and 2500 m a.s.l., which correlate well with the elevation of the observed fumarole belt within the post-2007-collapse crater wall. Our results show that the depths of the local hydrothermal activity sections change substantially over the investigated period. Vertical displacement of the main potential generation area, associated with these hydrothermal activity sections, is observed on the order of several hundred meters at the transition between the period of quiescence (1993–1997) and the resumption of eruptive activity in 1998 and 2007, respectively. From 1999 to March 2008, the hydrothermal system was consistently located at relatively shallow depths. By quantitatively determining the vertical displacement of hydrothermal fluids over 16 years, we identify a significant link between hydrothermal system and magmatic activity. Hydrothermal system depth below the surface is an indicator of the activity level (pressurization/depressurization of the volcano) within the shallow magmatic systems. Thus, when used in conjunction with long term volcano monitoring, this approach can contribute substantially to detection of the precursory signals of changes in volcanic activity.

Raman geothermometry of carbonaceous material in the basal Ediacaran Doushantuo cap dolostone: The thermal history of extremely negative δ13C signatures in the aftermath of the terminal Cryogenian snowball Earth glaciation

Extremely negative δ13C values (as low as −48‰) of blocky calcite cements filling cavities, sheet-cracks, and fractures in the basal Ediacaran cap dolostone of the Doushantuo Formation in South China have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal Cryogenian snowball Earth glaciation. Alternatively, these extremely negative δ13C values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the Doushantuo cap dolostone. The latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13C-depleted calcite occurred at temperatures up to 476°C (mean=378°C, n=4) whereas carbonate fabrics with moderately negative δ13C values (generally>−10‰) were formed at much lower temperatures (mean=112°C, n=4). To assess these competing hypotheses, we systematically investigated the petrography, carbonate carbon and oxygen isotopes, and carbonaceous material (CM) Raman spectroscopy of the Doushantuo cap carbonate. Raman geothermometry suggests that CM associated with different microfabrics and δ13C signatures—including sedimentary fabrics with moderately negative δ13C values (e.g., peloids, stromatolite laminae, microbial mat fragments), diagenetic fabrics with moderately negative δ13C values (e.g., some void-filling or crack-filling cements), and diagenetic fabrics with extremely negative δ13C values (e.g., void-filling or crack-filling blocky and bladed calcites)—experienced temperatures generally <300°C (largely 200–300°C). Contrary to temperature estimates based on carbonate clumped isotopes, CM Raman geothermometry reveals that different microfabrics show largely overlapping paleotemperatures and that there is no apparent relationship between δ13C signatures and paleotemperatures. Whereas the origins of these extremely negative δ13C values remain unresolved, the Raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500°C that only affected the thermal history of the calcite with extremely negative δ13C values. Instead, the new data suggest that the analyzed microfabrics experienced the same peak temperatures <300°C, probably related to the burial history of the host rocks or pervasive hydrothermal activity that affected all microfabrics in the Doushantuo cap dolostone.

Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany

AbstractThe ~15 Ma, 26 km diameter Ries impact structure in south‐central Germany was one of the first terrestrial impact structures where evidence of impact‐associated hydrothermal alteration was recognized. Previous studies suggested that pervasive, high‐temperature hydrothermal activity was restricted to the area within the “inner ring” (i.e., the crater‐fill impactite units). Here we present mineralogical evidence for localized hydrothermal activity in the ejecta beyond the crater rim in two previously unstudied settings: a pervasively altered lens of suevite ejecta directly overlying the Bunte Breccia at the Aumühle quarry; and suevite ejecta at depth overlain by ~20 m of lacustrine sediments sampled by the Wörnitzostheim 1965 drill core. A comprehensive set of X‐ray diffraction analyses indicates five distinct alteration regimes (1) surficial ambient weathering characterized by smectite and a minor illitic component; (2) locally restricted hydrothermal activity characterized by an illitic component and minor smectite; (3) hydrothermal activity at depth characterized by smectite, a minor illitic component, and calcite; (4) hydrothermal activity at depth characterized by smectite, a minor illitic component, calcite, zeolites, and clinochlore; and (5) pervasive hydrothermal activity at depth characterized by smectite, a minor illitic component, and minor clinochlore. These data spatially extend the Ries postimpact hydrothermal system suggesting a much more extensive, complex, and dynamic system than previously thought. Constraining the mineralogical alteration regimes at the Ries impact structure may also further our understanding of impact‐associated phyllosilicate formation on Mars with implications for climate models and habitability.

Major and trace element composition of surface sediments from the Southwest Indian Ridge: evidence for the incorporation of a hydrothermal component

Hydrothermal materials in deep-sea sediments provide a robust tracer to the localized hydrothermal activity at mid-ocean ridges. Major, trace and rare earth element (REE) data for surface sediments collected from the ultraslow spreading Southwest Indian Ridge are presented to examine the existence of hydrothermal component. Biogenic carbonate oozes dominate all the sediment samples, with CaO content varying from 85.5% to 89.9% on a volatile-free basis. The leaching residue of bulk sediments by ~5% HCl is compositionally comparable to the Upper Continental Crust (UCC) in SiO2, Al2O3, CaO, MgO, alkali elements (Rb, Cs) and high field strength elements (Nb, Ta, Zr, Hf, Ti). These detritus-hosted elements are inferred to be prominently derived from the Australian continent by means of eolian dust, while the contribution of local volcaniclastics is insignificant. In addition, the residual fraction shows a clear enrichment in Fe, Mn, and Ba compared with the UCC. Combining the positive Eu anomaly of residual fraction which is opposed to the UCC but the characteristic of hydrothermal fluids and associated precipitates occurred at mid-ocean ridges, the incorporation of localized hydrothermal component can be constrained. REE mixing calculations indicate that more than half REE within the residual fraction (~55%–60%) are derived from a hydrothermal component, which is inferred to be resulted from a diffuse fluid mineralization. The low-temperature diffuse flow may be widely distributed along the slow-ultraslow spreading ridges where crustal faults and fissures abound, and probably have a great mineralization potential.

Pyritic event beds and sulfidized Fe (oxyhydr)oxide aggregates in metalliferous black mudstones of the Paleoproterozoic Talvivaara formation, Finland

The Paleoproterozoic, 2.0–1.9 Ga Talvivaara formation of Finland was deposited during the Shunga Event, a worldwide episode of enhanced accumulation of organic-rich sediments in the aftermath of the Lomagundi–Jatuli carbon isotope excursion. Sulfidic carbonaceous mudstones in the Talvivaara formation contain one of the largest known shale-hosted nickel deposits. In order to gain new insight into this Shungian sedimentary environment, sedimentological, petrographical and in situ S and Fe isotopic microanalyses were carried out on samples representing depositional and early-diagenetic conditions. The event-bedded lithology with tidal signatures in the organic-rich mudstones strongly indicates deposition from predominantly river-delivered mud on a highly-productive coastal area, below storm-wave base. The riverine supply of phosphorus, sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water. The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds. The pyrite beds were deposited by seaward excursions of the concentrated, acidic Fe-rich river plume subsequent to droughts or dry seasons, which led to intense pyrite precipitation upon mixing with euxinic waters. δS34 and δFe56 values of the bedded pyrite (median δS34=−10.3‰ and δFe56=−0.79‰) are consistent with the reaction of dissolved Fe(II) with H2S from bacterial sulfate reduction. Organic-rich clayey Fe-monosulfide-bearing granules were transported from the muddy estuary, and enclosed in Fe (oxyhydr)oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river-delivered iron. The isotopic composition of these presently pyrrhotitic inclusions (median δS34=−3.3‰ and δFe56=−1.6‰) indicates microbial iron reduction. The Fe (oxyhydr)oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor. Their Fe (oxyhydr)oxide matrix was replaced by pyrite (median δS34=+5.8‰ and δFe56=+0.81‰) at shallow sediment depths with 34S and 56Fe-enriched porewater. Wavy-crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation. These results indicate episodic deposition at seasonal to multiannual time scales. δS34 and δFe56 values in the studied Fe-sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early-diagenetic origin, finding no support for the previously proposed local hydrothermal activity in the Talvivaara mudstones.

Lithium and strontium isotopic systematics in playas in Nevada, USA: constraints on the origin of lithium

Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (δ7Li) have been identified as a tool for investigating water–rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial δ7Li values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with δ7Li values of the possible sources. In samples from the playas, δ7Li values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial δ7Li value in water flowing into each playa. However, all δ7Li values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water–rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that δ7Li may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.

Geology and Age of Solander Volcano, Fiordland, New Zealand

AbstractThe Solander Islands are the eroded remnants of a once much larger volcanic complex, the only known subduction-related magmatism on the Pacific-Australia plate boundary south of New Zealand. The largest island, Hautere, preserves an eruptive sequence of subaerial adakitic andesite domes and related block and ash flows, followed by a phase of phreatomagmatic deposits. Ar-Ar plagioclase ages, detrital U-Pb zircon ages, and magnetostratigraphic data from Hautere indicate that magmatism probably occurred in the interval 100–350 ka. In contrast, an andesite dome on nearby Little Solander Island gives an age of 20–50 ka. Local hydrothermal activity is manifested as orange-colored alteration and tuffisite dikes. There is no evidence, at least in the small preserved remnants of the Solander Islands, for tectonic uplift or subsidence; most of Solander Volcano was eroded away during Late Pleistocene marine planation.

Magmatic origin of giant ‘Kiruna-type’ apatite-iron-oxide ores in Central Sweden

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the ‘Kiruna-type’ apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish ‘Kiruna-type’ deposit at Grängesberg have δ18O values between −0.4 and +3.7‰, while the 1.90−1.88 Ga meta-volcanic host rocks have δ18O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ18Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ18Omgt ≤ +0.9‰), in turn, equilibrated with high-δ18O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish ‘Kiruna-type’ ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

Selective leaching studies of sediments from a seamount flank in the Central Indian Basin: Resolving hydrothermal, volcanogenic and terrigenous sources using major, trace and rare-earth elements

We have investigated the sediments from a site where a newly discovered submarine hydrothermal alteration of recent origin is located to identify the source components and estimate their relative contribution. The site is situated on the flank of a seamount in the Central Indian Basin along the 76° 30′ E fracture zone which traces the movement of the Rodriguez triple junction. Detailed analyses of major, trace and rare earth elements (REE) on the bulk, leach, residue and clay fraction, and infra red spectroscopy studies of the clay fraction helped characterize the hydrothermal nature of the sediments. Selective dissolution carried out using HCl has allowed the discrimination between a leach phase (leachable Fe–Mn oxide) and a residue phase (refractory). The sediments are iron-rich, majority of Fe, Mn is in leachable fraction and are compositionally comparable to ridge-crest metalliferous sediments. Elemental ratios and geochemical discrimination plots of major element data reveal that the sediments are a mixture of Fe-rich mineral phases on one hand apparently from hydrothermal sources and aluminosilicates such as illite, zeolites, micaceous clays and smectites contributed from the Bengal fan, weathering of basinal basalts and authigenic processes. Ternary mixing calculations of bulk sediment major element data show that 35–45% metalliferous sediments could reproduce Fe and Mn values of our sediments. The shale normalized REE patterns of the bulk sediments are remarkably similar to those of the HCl soluble leach fraction, implying that the REEs in the HCl soluble leach fraction dominate the bulk fraction. HCl leacheates exhibit characteristics of vent fluids as well as seawater with pronounced Eu anomaly with HREE enrichment and negative Ce anomaly. The residue on the other hand has a high concentration of Fe, V, Zn, and Pb indicating a sulfidic input to the sediments. Ternary mixing calculations of REEs carried out on the residual fraction indicate that the sulfide-like material dominates (64–72%) over terrigenous (10–20%) and MORB derived (16–18%) components. This study helped us identify and quantify two types of hydrothermal material in the sediments, dominated by Fe–Mn phases (ferruginous oxides and silicates; Mn-oxides) in leachable fraction and sulfide-like material in residual phases. The clay fraction is mainly composed of dioctahedral smectites and has a composition similar to Alvin nontronite. The geochemical and mineralogical characteristics collectively suggest that these sediment-hosted precipitates formed at relatively low temperatures ranging between 50 and 100 °C from a mixture of seawater and hydrothermal fluids. The presence of hydrothermal sediments in the vicinity of a fracture zone suggests the occurrence of localized hydrothermal activity associated with intra plate volcanism. This is probably induced by reactivation of tectonic forces along the fracture zones.

Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E

We report the discovery of active and fossil hydrothermal systems during R/V Knorr Cruise 162, Legs VII and IX along a 400 km long segment of the ultra‐slow Southwest Indian Ridge (SWIR) between 10 and 16°E, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr−1 full rate). Eight of forty‐one optical/temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others. Fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias, four deposits ofsepiolite and silica, as well as Mn‐oxide and nontronite cemented breccias. The massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep‐seated normal fault bounding the southern rift valley wall. Silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low‐ to moderate‐temperature ultramafic‐hosted hydrothermal activity. The abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates, and hence the magmatic heat input, along this section of the SWIR are lower than on any other explored segment of the global mid‐ocean ridge system. This observation suggests that high mantle upwelling and magma supply rates are not required to drive mid‐ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra‐slow end of the ridge spreading spectrum. The frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation, with hydrothermal vent sites being preferentially associated with long‐lived faults that provide fluid pathways. Our results suggest the contribution of magma‐starved ridge segments to the global ocean‐lithosphere geochemical budget is potentially much larger than previously thought.

Volcanogenic–hydrothermal iron-rich materials from the southern part of the Central Indian Ocean Basin

We report the occurrences of magnetite-rich spherules, Ti-rich particles and possibly nontronite recovered at 19°S and 76°E from the pelagic clay in the Central Indian Ocean Basin (CIOB). It is proposed that the magnetite spherules were formed as a result of volcanic activity in this part of the CIOB, while the presence of nontronite suggests localized hydrothermal activity. The magnetic spherules and the associated materials are compared with similar occurrences 555 km to the north in the vicinity of the 76°30′E fracture zone. Considering radiolarian age dating of the sediment, the age of the volcano-hydrothermal episode responsible for the formation of these materials seems to be between 425 and 650 ka. It is inferred that there could have been widespread local volcanism in the CIOB.

The concentration and isotopic composition of osmium in the oceans

Osmium is one of the rarer elements in seawater. Analytical difficulties have previously prevented the direct measurement of the osmium concentration and isotopic composition in seawater. We report a chemical separation procedure that yields quantitative extraction of osmium standard and of osmium tracer by iron hydroxide precipitation from seawater doped with osmium standard, osmium tracer, and FeCl 3. The iron hydroxide precipitate is processed to extract osmium, using techniques developed for iron meteorites. Utilizing this procedure, water samples from the Pacific and Atlantic oceans were analyzed for osmium concentration and isotopic composition. Direct determination of the osmium concentration of seawater gives between 15 and 19 fM kg −1. Detailed experiments on different aliquots of one seawater sample from the North Atlantic Ocean, keeping the amounts of reagents constant, yield concentrations from 16 to 19 fM kg −1. The variability in concentration is outside the uncertainty introduced because of blanks and indicates a lack of full equilibration between the osmium tracer and seawater osmium. The most reliable osmium concentration of the North Atlantic deep ocean water is 19 fM kg −1 with the 187Os 186Os ratio being 8.7 ± 0.2 (2σ). Detailed experiments on one seawater sample from the Central Pacific Ocean indicate that the most reliable osmium concentration of the deep ocean water from the Central Pacific is 19 fM kg −1 with the 187Os 186Os ratio being 8.7 ± 0.3 (2σ). The directly measured osmium isotopic composition of the oceans is in good agreement with that obtained from the analysis of some rapidly accumulating organic rich sediments ( Ravizza and Turekian, 1992). A sample of ambient seawater around the Juan de Fuca Ridge gave 187Os 186Os= 6.9 ± 0.4. This is distinctly lower than the deep-sea water value and may reflect local hydrothermal activity or some analytical difficulty with this sample. The osmium isotopic composition of the deep oceans indicates that ∼80% of the osmium is derived from the continents and the rest from extraterrestrial and hydrothermal sources. Using the iridium data of Anbar et al. (1996, 1997) we find that the Os/Ir ratio is ∼22 for seawater. The mean residence time of osmium (τ Os) cannot be derived directly from these data. However, using the Os/Ir ratio of seawater and the Os/Ir ratio of continental rocks, a lower limit can be established of τ Os ≥ 4.4 × 10 4 years. The Os/Ir in modern deep sea sediments should be relatively constant and is distinctly less radiogenic than seawater osmium due to limited evaporation (26%) of osmium from infalling cosmic dust. Relative to the cosmic dust infall the hydrothermal sources may, however, play a smaller but significant role in the bulk economy of dissolved osmium and other Platinum Group Elements in the oceans.

Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks

Many of the minor elements in seawater today have a concentration-depth profile similar to that of the biologically essential nutrients, NO − 3 and PO 3− 4. They show a relative depletion in the photic zone and enrichment in the deep ocean. The difference between their surface- and deep-ocean values, normalized to the change in PO 3− 4, approaches the average of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plankton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies, particularly of sediments which accumulate in shelf-slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated. The interelement relations observed for some of these sediments approach the average values of plankton. These same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites, rocks which have a high content of marine fractions (e.g., organic matter, apatite, biogenic silica and carbonates). Many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity, and/or seawater with an elemental content different from that of the modern ocean, was required to account for their minor-element contents. However, the similarity in several of the minor-element ratios in many of these formations to minor-element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal seas, of the ocean today. The accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater. The suite of elements in this fraction reflects redox conditions in the bottom water, as determined by bacterial respiration. For example, high Mn, high Cr+V and high Mo concentrations, above those which can be attributed to the accumulation of planktonic matter, characterize accumulation under bottom-water oxidizing, denitrifying and sulfate-reducing conditions, respectively.

Metallogeny of the midcontinent rift system of North America

Abstract The 1.1 Ga Midcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ∼ 1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by rift-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30–40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift.

The distribution of uranium and thorium in the cape columbine granite from the southwestern cape province, South Africa

The coastal outcrops of the Cape Columbine granite were mapped radiometrically on a 10 m grid by gamma-spectrometry. Numerous radioelement-enriched anomalies were found, each a few square metres in outcrop. Some of the anomalies, but by no means all, are associated with small bodies of dequartzified granite (episyenite), chloritized or fluoritized granite, evidently testifying to localized hydrothermal activity of considerable diversity. The uranium and thorium in the relatively unaltered main granite are present in a variety of host minerals: (a) rock-forming minerals; (b) accessory minerals, including U- and Th-minerals, (c) secondary minerals associated with hydrothermal activity; and (d) secondary minerals,associated with weathering. Thin coatings of cryptocrystalline material in micro-fractures within rock-forming minerals may belong to either of the last-mentioned two groups. Variations in the relatively unaltered granite are described to both primary differences in the amount of accessory minerals and the percentage of uranium hosted by secondary minerals. The uranium is present, in order of decreasing abundance, in xenotime, thorite, along grain boundaries and within biotite and zircon, while the thorium occurs in monazite, thorite, xenotime, apatite and epidote, along grain boundaries and within zircon and biotite. The different types of alteration bodies are associated with wide variations in whole-rock radio-element distributions. Quantitative mineralogical study of one occurrence of fluoritized granite has made it possible to calculate the relative importance of local uranium host minerals: zircon (30% of the uranium), xenotime (13%), thorite (13%), along grain boundaries (9%) and in biotite (9%) and other rock-forming minerals (4%). The thorium is present in zircon (28% of the thorium), monazite (20%), thorite (14%), biotite (9%), xenotime (2%) and along grain boundaries (2%). The uranium was apparently liberated from primary minerals during alteration and trapped in secondary minerals, causing a major radio-element redistribution. The redistributed uranium and thorium are more readily leachable during weathering, resulting in some variation in surface samples and outcrops. The demonstrated dekametre-scale anomalies are nevertheless considered to be a pristine feature of the rocks.

Heat flow in the Balearic and Tyrrhenian Basins, western Mediterranean

We present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the Balearic and Tyrrhenian basins in the western Mediterranean. Analysis of 12 measurements within a 10‐km radius from 40°01′, 4°55′E in the Balearic abyssal plain shows a mean heat flow of 92±10 mW m−2. After correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late Oligocene age proposed for the basin. A similar survey with 18 measurements around 40°16′N, 11°19′E in the Western Tyrrhenian gives a flux of 134±8 mW m−2, while the third survey of 26 measurements in the southern Tyrrhenian abyssal plain at 37°17′N, 12°58′E yields a slightly higher heat flow of 151±10 mW m−2. These values are within the range predicted by simple plate cooling models for the late Miocene ages of the deep Tyrrhenian basin. Thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins, the thermal signature is similar. Also, the trend of increasing heat flow from west to east through the Balearic and Tyrrhenian basins is in agreement with models of the formation of the western Mediterranean behind an eastwardly migrating trench system. In all three areas the measured flux shows significant local variability. In the two westernmost surveys this can be attributed to the presence of buried, high‐conductivity salt structures, but in the third area (SE Tyrrhenian) the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone. Instead, evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area.