Lead and uranium isotopic behavior in diagenetic and epigenetic manganese nodules, Timna Basin, Israel, determined by MC-ICP-MS

Isotope ratios of U and Pb were measured in two types of Mn nodules from the Cambrian Timna Formation, Israel. Type A nodules are mainly composed of pyrolusite and hollandite, with Mn, Ba, Pb and U concentrations of 30–60%, 0.2–2.5%, 0.2–1.0% and 500–3500 ppm, respectively, whereas type B nodules were formed by alteration of the former, and contain mainly coronadite, with Mn, Ba, Pb and U concentrations of 7–48%, 0.2–7%, 0.6–5% and 10–160 ppm, respectively. The isotopic composition of U and Pb was measured by MC-ICP-MS on Mn-rich solutions (up to 100 mg/L) without and with chromatographic separation. The values for the 207/206 and 208/206 ratios have been determined with precisions of up to 50 ppm and those of 206/204, 207/204 and 208/204 – up to 200 ppm. The values for the 234/238 ratios have been determined with precisions of 0.4–1%. The results of the separated and unseparated solutions were shown to be equal within the error. Thus there is no significant matrix effect while measuring U and Pb in Mn rich solution using the MC-ICP-MS.The isotopic composition of Pb and U support the distinction between the two types of Mn nodules. Type A nodules have a wide range of ²⁰⁶Pb/²⁰⁴Pb ratios (18.278–19.776), and an almost constant ratio of ²⁰⁸Pb/²⁰⁴Pb. In contrast, type B nodules have almost constant ²⁰⁶Pb/²⁰⁴Pb ratios and a wide range of ²⁰⁸Pb/²⁰⁴Pb ratios (37.986–38.079). Type A nodules form a linear array on a ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb diagram, while type B nodules form a tight group characterized by lower Pb isotope ratios that slightly deviate from the type A array. The ²³⁴U/²³⁸U ratio differs between the two types of nodules; type A nodules exhibit a uniform and close to equilibrium ²³⁴U/²³⁸U ratio while type B nodules show a wide range of ²³⁴U/²³⁸U ratios above and below the equilibrium value. The isotopic composition of Pb in type A nodules might reflect Pb contributions from plutonic rock weathering, exposed at the time of deposition or later, to the Cambrian sea. These nodules have remained unaffected by processes that occurred since the Cambrian. The higher ²⁰⁸Pb/²⁰⁴Pb values of type B indicate that these nodules were formed from a Th-enriched solution probably during epigenetic processes which occurred also during the last 1 Ma.Thus the two isotopic systems of U and Pb can record formation, leaching and redeposition of Mn ores.     

Predicted equilibrium constants for solid and aqueous selenium species to 300 °C: applications to selenium-rich mineral deposits

In this study, an attempt has been made to assess aqueous speciation of selenium and solubility product constants of common selenides at elevated temperatures (up to 300 °C) by using various extrapolation methods. This study predicts that reduced selenium species are dominant species in many geological processes even under relatively oxidized conditions such as those dictated by the magnetite–hematite buffer. On the basis of extrapolated equilibrium constants and solubility product constants for common Se-bearing mineral phases, critical ∑Se/∑S ratios (molal ratios) in mineralizing fluids are proposed for independent selenium mineralization. The minimum ∑Se/∑S ratios in mineralizing fluids for independent selenium mineralization should be at least 10⁻⁶, 10⁻⁵ and 10⁻⁴ at 100, 200 and 300 °C, respectively. For giant independent selenium deposits such as the La'erma and Qiongmo Au–Se deposits in the western Qingling mountains, and Yutangba Se deposits in Hubei Province, China, the mineralizing fluids have reached much higher ∑Se/∑S ratios ranging from 10⁻¹ to 10⁻³ at 200 °C. This study also suggests that the equilibrium assemblage of pyrite–ferroselite among the common ore minerals requires the highest ∑Se/∑S ratios in mineralizing fluids, followed in decreasing order by the assemblages of stibnite–antimonselite, galena–clausthalite, cinnabar–tiemannite, and acanthite/argentite–naumannite. The assemblage of pyrite–ferroselite can also be formed under relatively oxidizing conditions where [∑H₂Se]/[∑H₂S] ratios can be high enough for the formation of independent ferroselite.     

Organic geochemical characteristics and depositional environments of the Jurassic coals in the eastern Taurus of Southern Turkey

In this study, organic matter content, type and maturity as well as some petrographic and physical characteristics of the Jurassic coals exposed in the eastern Taurus were investigated and their depositional environments were interpreted.The total organic carbon (TOC) contents of coals in the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc areas are 24.54, 66.78 and 49.15%, respectively. The Feke–Akkaya and Kozan–Kizilinc coals have low Hydrogen Index (HI) values while the Kozan–Gedikli coals show moderate HI values. All coal samples display very low Oxygen Index (OI) values. The Kozan–Gedikli coals contain Type II organic matter (OM), the Feke–Akkaya coals contain a mixture of type II and type III OM; and the Kozan–Kizilinc coals are composed of Type III OM. Sterane distribution was calculated as C₂₇ > C₂₉ > C₂₈ from the m/z 217 mass chromatogram for all coal samples.T_max values for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 439, 412 and 427 °C. Vitrinite reflectance values (%R_o) for the Feke–Akkaya and Kozan–Kizilinc coal samples were measured as 0.65 and 0.51 and these values reveal that the Feke–Akkaya and Kozan–Kizilinc coals are at subbituminous A or high volatile C bituminous coal stage. On the basis of biomarker maturity parameters, these coals have a low maturity.The pristane/phytane (Pr/Ph) ratios for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 1.53, 1.13 and 1.25, respectively. In addition, all coals show a homohopane distribution which is dominated by low carbon numbers, and C₃₅ homohopane index is very low for all coal samples. All these features may indicate that these coals were deposited in a suboxic environment.The high sterane/hopane ratios with high concentrations of steranes, low Pr/Ph ratios and C₂₅/C₂₆ tricyclic ratios > 1 may indicate that these coals formed in a swamp environment were temporarily influenced by marine conditions.     

Modeling outflow from the Ernest Henry Fe oxide Cu–Au deposit: implications for ore genesis and exploration

The Ernest Henry Fe oxide Cu–Au (IOCG) deposit (>ca. 1.51 Ga) is hosted by breccia produced during the waning stages of an evolving hydrothermal system that formed a number of tens of metres to a kilometre scale, pre- and syn-ore alteration halos, although no demonstrable patterns have been attributed to fluids expelled through the outflow zones. However, the recognition of a population of hypersaline fluid inclusions representing the ‘spent’ fluids after Cu–Au deposition at Ernest Henry provides the basis to model the geochemical characteristics of the deposit's outflow zones. Geochemical modeling at 300 °C was undertaken at both high and low fluid/rock ratios via FLUSH models involving three host rock types: (1) granite, (2) calc–silicate rock, and (3) graphitic schist. In models run at high fluid/rock ratios, all rock types are essentially fluid-buffered, and produce an albite–quartz–hematite–barite-rich assemblage, although in low fluid–rock environments, the pH, redox, and geochemical character of the host rock exerts a greater influence on the mineralogy of the alteration assemblages (e.g., andradite, Fe–chlorite, and magnetite). Significant sulphide mineralization was predicted in graphitic schist where sphalerite occurred in both low- and high-porosity models, which indicates the possibility of an association between high-temperature IOCG mineralization and lower temperature base metal mineralization.Cooling experiments (from 300 to 100 °C) using the ‘spent fluids’ predict early high-T (300–200 °C) Na-, Ca-, Fe-, and Mn-rich, magnetite-bearing hydrothermal associations, whereas with cooling to below 200 °C, and with progressive fluid–rock interaction, the system produces rhodochrosite-bearing, hematite–quartz–muscovite–barite-rich assemblages. These results show that the radical geochemical and mineralogical changes associated with cooling and progressive fluid influx are likely to be accompanied by major transformations in the geophysical expression (e.g., spectral and magnetic character) of the alteration in the outflow zone, and highlight the potential link between magnetite- and hematite-bearing IOCG hydrothermal systems.     

Geochemistry of transition elements in garnet lherzolite nodules in kimberlites

The clinopyroxenes and garnets from garnet lherzolite nodules in kimberlites were analyzed for the major and trace elements (Sc, Ti, V, Cr, Mn, Co, Sr, and Zr) with the secondary ion mass spectrometry (SIMS) techniques using an ion-microprobe. The concentration ranges for clinopyroxenes are: 12–90 ppm Sc, 60–2540 ppm Ti, 110–350 ppm V, 2400 ppm-1.68% Cr, 470–1100 ppm Mn, 18–70 ppm Co, 85–710 ppm Sr, and 6.3–120 ppm Zr. Those for garnets are: 71–180 ppm Sc, 140–6200 ppm Ti, 220–450 ppm V, 1.05–6.24% Cr, 1600–5220 ppm Mn, 33–92 ppm Co, and 66–250 ppm Zr.These data were used to assess a possible use of distribution of transition elements between clinopyroxene and garnet for geothermometry/geobarometry. The distribution coefficients of V and Mn show good correlations with (Ca/Ca+Mg+Fe)_CPX, suggesting their potential usefulness.The abundances of these elements in minerals (and estimated whole-rocks) suggest that garnet lherzolite nodules in kimberlites can be classified into three groups. It is suggested that one of the groups (Group I) could be close to the pristine mantle on the basis of Ti and Zr abundances as well as REE systematics. Arguments on Ti/Zr ratios among different nodule types suggest that abundant granular garnet lherzolites could be cumulates crystallized from a residual liquid after ilmenite was extracted.IPG Contribution No. 281     

The distribution of Hg, Ba, Cu and Zn in the vicinity of cupriferous sulfide deposits, Troodos complex, Cyprus

A geochemical rock- and soil-sampling program was carried out in the vicinity of eight concealed “Cyprus type” deposits, occurring in marginal mafic to intermediate metapillow lavas of the Troodos Ophiolite Complex. The mineralization of massive and stockwork sulfide ore is characterized by the predominance of pyrite, intergrown with less chalcopyrite and minor amounts of sphalerite.Background values of Hg are in the range of 8–12 ppb for soils and 3–6 ppb for surface rocks. Anomaly/background ratios of 10:1 (soils) and 5:1 (rocks) have been found only, where Hg migrated along channels formed by faults cutting shallow-seated mineralization. Here, Hg sometimes shows significant correlations with Cu, Zn, Ba and exceptionally with Co. However in one case an Hg anomaly in soils and surface rocks was detected directly over a deposit. The use of Hg as indicator element for these types of deposits is therefore limited. Buried mineralization may be delineated more distinctly by Cu, Zn and Ba.     

He, Sr and Nd isotopes of mantle-derived xenoliths in volcanic rocks of NE China

Abundances and isotopic ratios of He and isotopic ratios of Sr and Nd have been measured for 18 mantle-derived xenoliths from three Cenozoic volcanic provinces in NE China. The xenoliths are characterized by low He concentrations (2.3×10⁻⁹–2.5×10⁻⁷ cm³ STP g⁻¹) and large variations of ³He/⁴He ratios (1.6×10⁻⁷–15.8×10⁻⁶ or R/Ra=0.1–11). ³He/⁴He, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios in xenoliths from Kuandian Holocene volcanic provinces are consistent with those observed in mid-oceanic ridge basalt (MORB), indicating that the source of He–Sr–Nd in the lithospheric mantle is chemically and isotopically identical to that of MORB reservoir. However, xenoliths in Huinan Pleisteocene–Holocene volcanic province are characterized by slightly low ³He/⁴He ratios (R/Ra=4.3–6.5) and more radiogenic Sr and Nd. The age-corrected ³He/⁴He ratios suggest that their time-integrated ratio of (U+Th)/³He is slightly elevated compared to the depleted upper mantle. The Hannuoba xenoliths in the Miocene volcanic province show much lower and wider variable ³He/⁴He ratios, which is most likely caused by in situ radiogenic ⁴He accumulation since it erupted on to the surface. The He–Sr–Nd results suggest chemical and isotopical heterogeneity in the subcontinental mantle beneath NE China.     

Strontium isotopic studies of alkalic rocks: Localities from Australia,Spain, and the Western United States

Initial Sr⁸⁷/Sr⁸⁶ ratios and rubidium and strontium contents have been measured in 83 specimens from 8 suites of alkalic and ultrabasic rocks. The range of initial Sr⁸⁷/Sr⁸⁶ ratio observed and the number of specimens (in parentheses) analysed for each suite are: West Kimberley, Australia (8), 0.7125–0.7215; Jumilla, Spain (6), 0.7136–0.7158; Bearpaw Mountains, Montana (9), 0.7062–0.7086; Highwood Mountains, Montana (10), 0.7072–0.7087; Hopi Buttes, Arizona (8), 0.7038–0.7094; Leucite Hills, Wyoming (17), 0.7055–0.7070; Montana diatremes (13), 0.7034–0.7073; Navajo Province, Arizona and New Mexico (12), 0.7052–0.7099.The initial Sr⁸⁷/Sr⁸⁶ ratios of the West Kimberley and Jumilla rocks are the highest yet found in strontium-rich basic rocks. Some of the individual specimens from West Kimberley have initial ratios as high as some estimates of the present Sr⁸⁷/Sr⁸⁶ ratio of average crustal material. This is interpreted to mean that the West Kimberley and Jumilla rocks contain substantial amounts of radiogenic strontium, and possibly other elements, from a crustal source.     

In-situ U–Pb geochronology and Hf isotope analyses of the Rayner Complex,east Antarctica

In-situ zircon U–Pb and Hf isotopic analysis via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of samples from Kemp and MacRobertson Lands, east Antarctica suggests that the Kemp Land terrane evolved separately from the rest of the Rayner Complex prior to the ca. 940 Ma Rayner Structural Episode. Several Archaean metamorphic events in rocks from western Kemp Land can be correlated with events previously reported for the adjacent Napier Complex. Recently reported ca. 1,600 Ma isotopic disturbance in rocks from the Oygarden Group may be correlated with a charnockitic intrusion in the Stillwell Hills before ca. 1,550 Ma. Despite being separated by some 200 km, T^Hf_DM ages indicate felsic orthogneiss from Rippon Point, the Oygarden Group, Havstein Island and the Stillwell Hills share a ca. 3,660–3,560 Ma source that is indistinguishable from that previously reported for parts of the Napier Complex. More recent additions to this crust include Proterozoic charnockite in the Stillwell Hills and the vicinity of Mawson Station. These plutons have distinct ¹⁷⁶Hf/¹⁷⁷Hf ratios and formed via the melting of crust generated at ca. 2,150–2,550 Ma and ca. 1,790–1,870 Ma respectively.     

Dependence of earthquake recurrence times and independence of magnitudes on seismicity history

The fulfillment of a scaling law for earthquake recurrence–time distributions is a clear indication of the importance of correlations in the structure of seismicity. In order to characterize these correlations we measure conditional recurrence–time and magnitude distributions for worldwide seismicity as well as for Southern California during stationary periods. Disregarding the spatial structure, we conclude that the relevant correlations in seismicity are those of the recurrence time with previous recurrence times and magnitudes; in the latter case, the conditional distribution verifies a scaling relation depending on the difference between the magnitudes of the two events defining the recurrence time. In contrast, with our present resolution, magnitude seems to be independent on the history contained in the seismic catalogs (except perhaps for Southern California for very short time scales, less than about 30 min for the magnitude ranges analyzed).     

Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia)

Geochronological data, major and trace element abundances, Nd and Sr isotope ratios, δ¹⁸O whole rock values and Pb isotope ratios from leached feldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the granites from outcrop 10 km N′ Omaruru, reversely discordant U–Pb monazite data give ²⁰⁷Pb/²³⁵U ages of 511±2 Ma and 517±2 Ma, similar to previously published estimates for the time of regional high grade metamorphism in the Central Zone. Based on textural and compositional variations, garnets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P–T estimates from these xenocrystic garnets are significantly higher (800°C/9–10 kbar) than regional estimates (700°C/5 kbar), the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites. Sm–Nd garnet–whole rock ages are between 500 and 490 Ma indicating the age of extraction of the granites from their deep crustal sources. For the granites from Oetmoed, both Sm–Nd and Pb–Pb ages obtained on igneous garnets range from 500 to 490 Ma. These ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 Ma for these granites based on Rb/Sr whole rock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial ⁸⁷Sr/⁸⁶Sr ratios (>0.716), high δ¹⁸O values of >13.8‰, negative initial _Nd values between −4 and −7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intruded. The large range of Pb isotope ratios and the lack of correlation between Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat-producing elements. Very limited fluxing of volatiles from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that have been derived at least in part from a mantle source supports this model.     

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda,Africa

Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.     

Lead isotope constraints on the genesis of Pb–Zn deposits in the Neoproterozoic Vazante Group, Minas Gerais, Brazil

The Neoproterozoic Vazante Group at the western border of the São Francisco Craton, Brazil, hosts the largest Zn–Pb district in South America. Several authors have classified this mineral district as Mississippi Valley-type (MVT), based on the intimate association with carbonates and the epigenetic character of most ore bodies. In this paper, we present 47 new lead isotope data from four deposits located along the 300 km N–S Vazante–Paracatu–Unai linear trend. Pb isotope ratios indicate sources with relatively high U/Pb and Th/Pb ratios. Considering the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios as indicative parameters for the source, we suggest an upper crustal source for the metals. The small variation on the Pb isotope ratios compared to those observed in the classical MVT deposits, and other geological, fluid inclusion and sulphur isotopic data indicates a metallogenic event of long duration. It was characterized by focused circulation of hydrothermal fluids carrying metals from the basement rocks and from the sedimentary pile. The data obtained are more compatible with an evolution model similar to that of IRISH-type deposits. The existence of three Pb isotopic populations could be the result of regional differences in composition of the source rocks and in the fluid–rock interaction since the mineralization is a long-term process.     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Recognition of non-Milankovitch sea-level highstands at 185 and 343 thousand years ago from U–Th dating of Bahamas sediment

Thirty-one new bulk-sediment U–Th dates are presented, together with an improved δ¹⁸O stratigraphy, for ODP Site 1008A on the slopes of the Bahamas Banks. These ages supplement and extend those from previous studies and provide constraints on the timing of sea-level highstands associated with marine isotope stages (MIS) 7 and 9. Ages are screened for reliability based on their initial U and Th isotope ratios, and on the aragonite fraction of the sediment. Twelve ‘reliable’ dates for MIS 7 suggest that its start is concordant with that predicted if climate is forced by northern-hemisphere summer insolation following the theory of Milankovitch. But U–Th and δ¹⁸O data indicate the presence of an additional highstand which post-dates the expected end of MIS 7 by up to 10 ka. This event is also seen in coral reconstructions of sea-level. It suggests that sea-level is not responding in any simple way to northern-hemisphere summer insolation, and that tuned chronologies which make such an assumption are in error by ≈10 ka at this time. U–Th dates for MIS 9 also suggest a potential mismatch between the actual timing of sea-level and that predicted by simple mid-latitude northern-hemisphere forcing. Four dates are earlier than that predicted for the start of MIS 9. Although the most extreme of these dates may not be reliable (based on the low-aragonite content of the sediment) the other three appear robust and suggest that full MIS 9 interglacial conditions were established at 343 ka. This is ≈8 ka prior to the date expected if this warm period were driven by northern-hemisphere summer insolation.     

Late Quaternary climatic control on erosion and weathering in the eastern Tibetan Plateau and the Mekong Basin

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: ⁸⁷Sr/⁸⁶Sr = 0.7206–0.7240 and ε_Nd(0) = −11.1 to −12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite + chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5–6.5 μm/15–55 μm siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial–interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.     

Syn-metamorphic gold mineralisation,Invincible Vein,NW Otago Schist,New Zealand

The Invincible Vein fills a fault zone which strikes northeast and dips steeply southeast in the lower Rees Valley, NW Otago. The vein cuts north striking foliation in lower greenschist facies Otago Schist. Structures associated with the fault zone are both brittle and ductile, and the fault zone has had a complex history of post-mineralisation reactivation. Mineralised vein material filling parts of the fault zone consist of quartz, albite, muscovite, chlorite, calcite, pyrite, arsenopyrite and minor gold. These minerals have been strained and locally recrystallised during ductile deformation. Fluid inclusion homogenisation temperatures (140–175°C) and ice melting temperatures (0 to –1°C) indicate that the mineralising fluid was low salinity, low CO₂ water with a density between 0.88 and 0.93 g/cm₃. Arsenopyrite geothermometry implies a temperature of mineralisation of 370 ± 70°C. Mineralisation pressure lay between 2 and 5 kbar. Mineralisation pressure-temperature conditions and mineralogy are essentially the same as for metamorphism of the host schist. Vein calcite oxygen isotope ratios (+12 to +15 per mil) are similar to host schist values. Carbon isotope ratios of vein calcite (– 3 to –5 per mil) are distinctly different from ratios in host schist (–7 to –10 per mil). Elevated vein Cr contents, and isotopically depleted carbon data, are consistent with some degree of equilibration with metavolcanic rocks. It is inferred that metavolcanic rocks of the underlying Aspiring Terrane were a significant source for mineralising fluid and metals. Invincible mineralisation occurred in the latter stages of metamorphism, and is the earliest recognised gold-bearing vein system in the Otago Schist.     

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K₂O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)_n ratios are particularly high (3.4–7.8). ⁸⁷Sr/⁸⁶Sr (0.70290–0.70368), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51284) and ²⁰⁶Pb/²⁰⁴Pb (18.708–19.564) isotopic ratios further indicate the geochemically enriched nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.     

Adsorption of platinum, gold and silver by filter paper and borosilicate glass and its relevance to biogeochemical studies

Platinum, gold and silver are lost from solution by the filtration procedure usually followed in the pre-analytical treatment of resuspended ashed or acid-digested plant tissues. Sorption losses however, may be reduced by the presence of other cations in the filtrate. Low concentration gold solutions (1–10 ng ml⁻¹) cannot be stored in borosilicate glass containers for more than 48 hours, even with acidification. Synthetic solution data from this study suggest that some published values for these precious metals may have underestimated plant tissue concentrations.     

Post-collisional Plio-Pleistocene shoshonitic volcanism in the western Kunlun Mountains, NW China: Geochemical constraints on mantle source characteristics and petrogenesis

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO₂, Al₂O₃ and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low ε_Nd (−1.8 to −8.7) at more restricted ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of ²⁰⁷Pb/²⁰⁴Pb (15.48–15.74) and ²⁰⁸Pb/²⁰⁴Pb (38.30–39.12) ratios at relatively invariant ²⁰⁶Pb/²⁰⁴Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high ⁸⁷Sr/⁸⁶Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet.