Melt Events in A Nascent Mantle Wedge: Implication from the Luobusa Ophiolite,Tibet

The compositions of minerals and whole rocks of the Luobusa ophiolite in South Tibet, a fragment of Neo‐Tethyan forearc lithosphere, is used to investigate the magmatic evolution of nascent mantle wedges in newly‐initiated subduction zones. Clinopyroxenes in the Luobusa peridotites all have diopsidic compositions, and their Al₂O₃ contents vary from ～ 2% in the dunites and refractory harzburgites to 2‐4% in the cpx‐bearing harzburgites. The REE of clinopyroxenes in the harzburgites have left‐sloping patterns with contents comparable to those in abyssal peridotites that have experienced 5‐15% partial melting. Chromites in the Luobusa chromitites have the highest Cr#s (～ 80) and TiO2 contents (0.1‐0.2%), and those in the cpx‐bearing harzburgites have the lowest Cr#s (20‐60) and TiO2 contents (0‐0.1%), whereas those in refractory harzburgites and dunites have intermediate compositions. Cpx‐bearing and refractory harzburgites show spoon‐and U‐shaped REE patterns, respectively, and their HREE distribution patterns suggest at least 15%‐ 20% partial melting. The REE patterns of dunites and high‐Cr chromitites vary from spoon‐ to U‐shaped and require 15‐30% partial melting in their mantle sources to produce their parental melts. Our dataset reveals that the nascent Luobusa mantle wedge was first infiltrated by slab‐derived fluids and later refertilized by transitional lava‐like melts, resulting in cpx‐bearing harzburgites. Partial melting in the deeper cpx‐bearing mantle generated high‐Ca boninitic to arc picritic melts, which interacted with the peridotites in the uppermost mantle to generate high‐Cr chromitites, dunites and some refractory harzburgites. Lithological variation from cpx‐bearing to refractory harzburgites in forearc ophiolites is the result of multi‐stage melt events rather than increasing degrees of partial melting. Intermittent slab rollback during subduction initiation induces asthenospheric upwelling and high heat flux in nascent mantle wedges. Elevated geothermal gradients play a more important role than slab dehydration in triggering Mg‐rich magmatism in newly‐initiated subduction zones.     

Factors Controlling Sulfide Geochemistry in Sub-tropical Estuarine and Bay Sediments

The primary factors that control the concentration of total reduced (inorganic) sulfide in coastal sediments are believed to be the availability of reactive iron, dissolved sulfate and metabolizable organic carbon. We selected nine sites in shallow (<3 m), close to sub-tropical, estuaries and bays along the central Texas coast that represented a range in sediment grain size (a proxy for reactive iron), salinity (a proxy for dissolved sulfate), and total organic carbon (a proxy for metabolizable organic carbon). Based on these parameters a prediction was made of which factor was likely to control total reduced sulfide at each site and what the relative total reduced sulfide concentration was likely to be. To test the prediction, the sediments were analyzed for total reduced sulfide, acid volatile sulfide, and citrate dithionate-extractable, HCl-extractable and total Fe in the solid phase. Using solid-state gold–mercury amalgam microelectrodes and voltammetry, we determined pore water depth profiles of Fe(II) and ΣH₂S and presence or absence of FeS_(aq). At five of the nine sites the calculated degree of sufildization of citrate dithionite-reactive-iron was close to or greater than 1 indicating that rapidly reactive iron was probably the limiting factor for iron sulfide mineral formation. At one site (salinity = 0.9) dissolved Fe(II) was high, ΣH₂S was undetectable and the total reduced sulfide concentration was low indicating sulfate limitation. At the last three sites a low degree of sulfidization and modest total reduced (inorganic) sulfide concentrations appeared to be the result of a limited supply of metabolizable organic carbon. Fe(II)–S(-II) clusters (FeS_(aq)) were undetectable in 10 out of 12 bay sediment profiles where ΣH₂S was close to or below detection limits, but was observed in all other porewater profiles. Acid volatile sulfide, but not total reduced sulfide, was well correlated with total organic carbon and ranged from being undetectable in some cores to representing a major portion of total reduced sulfide in other cores. Although predicted controls on total reduced sulfide were good for very low salinity water or sandy sediments, they were only right about half the time for the other sediments. The likely reasons for the wrong predictions are the poor correlation of total organic carbon with grain size and differing fractions of metabolizable organic carbon in different sedimentary environments. Differences in sediment accumulation rates may also play a role, but these are difficult to determine in this region where hurricanes often resuspend and move sediments. This study demonstrates the need to examine more complex and often difficult to determine parameters in anoxic “normal marine” sediments if we are to understand what controls the concentration and distribution of sulfides.     

An experimental and theoretical determination of oxygen isotope fractionation in the system magnetite-H2O from 300 to 800°C

Oxygen isotope fractionations have been determined between magnetite and water from 300 to 800°C and pressures between 10 and 215MPa. We selected three reaction pathways to investigate fractionation: (a) reaction of fine-grained magnetite with dilute aqueous NaCl solutions; (b) reduction of fine-grained hematite through reaction with dilute acetic acid; and (c) oxidation of fine iron power in either pure water or dilute NaCl solutions. Effective use of acetic acid was limited to temperatures up to about 400°C, whereas oxide-solution isotope exchange experiments were conducted at all temperatures. Equilibrium ¹⁸O/¹⁶O fractionation factors were calculated from the oxide-water experiments by means of the partial isotope exchange method, where generally four isotopically different waters were used at any given temperature. Each run product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and on a limited basis, high-resolution transmission electron microscopy (HRTEM) and Mössbauer spectroscopy. Results from the microscopic examinations indicate the formation of well-crystallized octahedra and dodecahedra of magnetite where the extent of crystallization, grain size, and grain habit depend on the initial starting material, P, T, solution composition, and duration of the run.The greatest amount of oxygen isotope exchange (∼90% or greater) was observed in experiments where magnetite either recrystallized in the presence of 0.5 m NaCl from 500 to 800°C or formed from hematite reacted with 0.5 m acetic acid at 300, 350 and 400°C. Fractionation factors (10³ ln α_mt-H2O) determined from these partial exchange experiments exhibit a steep decrease (to more negative values) with decreasing temperature down to about 500°C, followed by shallower slope. A least-squares regression model of these partial exchange data, which accounts for analytical errors and errors generated by mass balance calculations, gives the following expression for fractionation that exhibits no minimum: 1000lnα_lmt-lw=−8.984(±0.3803)x+3.302(±0.377)x²—0.426(±0.092)x³ with an R² = 0.99 for 300 ≤ T≤ 800°C (x = 10⁶/T²). The Fe oxidation results also exhibit this type of temperature dependence but shifted to slightly more negative 10³ ln α values; there is the suggestion that a kinetic isotope effect may contribute to these fractionations. A theoretical assessment of oxygen isotope fractionation using β-factors derived from heat capacity and Mössbauer temperature (second-order Doppler) shift measurements combined with known β-factors for pure water yield fractionations that are somewhat more negative compared to those determined experimentally. This deviation may be due to the combined solute effects of dissolved magnetite plus NaCl (aq), as well as an underestimation of β_mt at low temperatures. The new magnetite-water experimental fractionations agree reasonably well with results reported from other experimental studies for temperatures ≥ 500°C, but differ significantly with estimates based on quasi-theoretical and empirical approaches. Calcite-magnetite and quartz-magnetite fractionation factors estimated from the combination of magnetite β’s calculated in this study with those for calcite and quartz reported by Clayton and Kieffer (1991) agree very closely with experimentally determined mineral-pair fractionations.     

A re-examination of petrogenesis and 40Ar/39Ar systematics in the Chain of Ponds K-feldspar: “diffusion domain” archetype versus polyphase hygrochronology

K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, upon which thermochronological modeling of Ar diffusion in discrete “domains” was founded. We re-examine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. ⁴⁰Ar/³⁹Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that post-date magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest “diffusion domains.” Heterochemical phases also play an important role in producing nonlinear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick’s Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the “diffusion domain” model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution among different coexisting phases that gives rise to the staircase-shaped age spectrum. Microbeam imaging of textures is as essential for ⁴⁰Ar/³⁹Ar hygrochronology as it is for U–Pb geochronology.     

Fluid evolution in base‐metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

The Dalradian and Ordovician–Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base‐metal sulphide‐bearing vein deposits associated with kilometre‐scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H₂O–CO₂–salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base‐metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician–Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base‐metal sulphide deposition and involved a low to moderate temperature (T_h 70 to 240°C), low to moderate salinity (0 to 20 wt% NaCl eq.), H₂O–salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long‐term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley & Sons, Ltd.     

A Vegetation History of the Far North of New Zealand during the Late Otira (Last) Glaciation

During the latter part of the last (Otira) glaciation the forest cover of New Zealand was much reduced. It has frequently been postulated, however, that diverse mixed forest communities survived in the far north of North Island. Pollen diagrams and radiocarbon dates from two last glacial and postglacial (Aranuian) sits on the Aupouri Peninsula in the far north of New Zealand are compared with other published palynological and plant macrofossil evidence from the region. Mixed kauri/podocarp/angiosperm forest was present at times during the late Otiran (and Aranuian) and no evidence was found for substantial loss of forest. However, radiocarbon samples from one site, at least, seem to have been contaminated with young carbon; this introduces uncertainty into the chronology established at that site. Possibly nondeposition or erosion has obscured part or all of the late Otiran record at all the sites studied so that very much reduced forest cover at that time cannot be ruled out.     

Rapid magmatic processes accompany arc–continent collision: the Western Bismarck arc, Papua New Guinea

New U–Th–Ra, major and trace element, and Sr–Nd–Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc–continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc–continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated ²⁰⁸Pb/²⁰⁴Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. ²³⁸U and ²²⁶Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.     

Intrusion–Related Vein Gold Deposits: Types, Tectono-Magmatic Settings and Difficulties of Distinction from Orogenic Gold Deposits

Abstract: A spectrum of intrusion-related vein gold deposits is recognized. Representative examples are described of the following geochemical associations: Au-Fe oxide–Cu, Au–Cu–Mo–Zn, Au–As–Pb–Zn–Cu, Au–Te–Pb–Zn–Cu and Au–As–Bi–Sb. The associated intrusions range from small outcropping stocks to complex batholiths. The different vein associations are believed to reflect the compositions of related intrusions, which themselves characterize distinct tectonic settings. The Au-Fe oxide–Cu and Au–Cu–Mo–Zn associations belong to two broad groups of deposits, Fe oxide–Cu–Au and porphyry Cu–Au, both of which are related to highly oxidized calc-alkaline intrusions emplaced in sub–duction–related arcs. The Au–As–Pb–Zn–Cu association seems to be linked to somewhat less oxidized intrusions emplaced in a similar setting. The Au–Te–Pb–Zn–Cu association, which possesses well-known epithermal counterparts, is also found with highly oxidized intrusions, but of alkaline composition and back-arc location. In contrast, the Au–As–Bi–Sb association, part of a newly recognized class of intrusion-hosted Au–Bi–W–As deposits, is related to relatively reduced intrusions, spanning the boundary between the magnetite– and ilmenite–series. Such intrusions, which may host major bulk-mineable gold deposits, were emplaced along the landward sides of arcs, possibly during lulls in subduction, as well as in continental collision settings. Therefore, a variety of geological environments is prospective for vein and, by extrapolation, other styles of gold mineralization, not all of them fully appreciated in the past. Several features of vein gold deposits, including imprecise relationships to individual intrusive phases, poorly developed mineral and metal zoning, apparent time gaps between intrusion and mineralization and presence of low–salinity, CO₂–rich fluid inclusions, are commonly taken to indicate a non-igneous origin and to be more typical of orogenic (mesothermal) gold deposits generated during accretionary tectonic events. However, several or all of these features apply equally to some intrusion– related vein gold deposits and, therefore, do not constitute distinguishing criteria. The currently popular assignment of most gold-rich veins to the orogenic category requires caution, because of the geological convergence that they show with some intrusion-related deposits. A proper distinction between intrusion-related and orogenic gold deposits is crucial for exploration planning.     

Diagenetic evolution of Aptian evaporites in the Namibe Basin (south‐west Angola)

The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre‐scale and patchy, dome‐like gypsum accumulations are well‐exposed within strongly incised present‐day river valleys along the eastern margin of the Namibe and Benguela basins (south‐west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution‐related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin‐spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the current at‐surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross‐cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics.