Xenolith-derived mantle geotherms: whither the inflection?

Three possibilities exist for the geometry of the upper mantle geotherm determined from study of garnet lherzolite xenoliths from Cretaceous kimberlites of northern Lesotho, southern Africa: (1) the geotherm is inflected to a lower thermal gradient at greater depth (low-T inflection); (2) the geotherm is uninflected; and (3) the geotherm is inflected to a higher thermal gradient at greater depth (high-T inflection). In the past two years all three possibilities have been advocated. Finnerty and Boyd (1984, 1987) found that many independent thermometers yield similar P-T arrays, so that features of xenolith geotherms cannot be artifacts of the method of temperature estimation. Hence the current controversy centers on the barometers used for pressure estimation. Bertrand et al. (1986) calibrated a new aluminous-enstatite barometer using 50–100 kbar data of Yamada and Takahashi (1983), and presented a southern Africa geotherm displaying a low-T inflection. The high-P alumina solubility data are incompatible with lower-P data, however, with the result that the new barometer underestimates pressure: a diamond-bearing xenolith falls at least 5.7 kbar into the stability field of graphite. Thus, the Bertrand et al. (1986) barometer does not adequately test the reality of inflected geotherms. Carswell and Gibb (1987 a, b) modified the aluminous enstatite barometer of Nickel and Green (1985) to account for Jadeite molecule in orthopyroxenes containing relatively high concentrations of Na. When applied to xenoliths of northern Lesotho the apparent inflection is minimized but still evident. In this suite Na content of orthopyroxenes increases systematically with greater T or greater depth. Sodium correlates poorly with T (and depth) in a suite of xenoliths from Farm Louwrencia, Namibia, and application of the Nickel and Green (1985) barometer (with or without modification) destroys the correlation of T with P expected for a geotherm. The decorrelation of P from T in the Louwrencia suite is caused by errors in the Na correction. The minimization of the inflection in the northern Lesotho suite is caused by the correlation of Na with T (and depth) in that suite and does not result from an improved correction scheme for the aluminous enstatite barometer. Hence, the Carswell and Gibb (1987a, b) formulation of the barometer does not support the absence of an inflection in the northern Lesotho geotherm. Adams and Bishop (1986) recalibrated the olivine barometer (Finnerty and Boyd 1978) and presented a southern African P-T array that appears uninflected. Their barometer, however, underestimates pressure by 10–20 kbar: all xenoliths from the southern African diamond-bearing kimberlites plot well within the graphite stability field. This barometer is also too imprecise to judge whether an inflection is present or absent. Finnerty (1989), employing an empirical fit to data for Ca solubility in olivine and several different independent thermometers, presented northern Lesotho P-T arrays that satisfy the diamond-graphite constraint with minimal scatter and display high-T inflections. Because the inflection is evident with independent thermometers and independent barometers, it cannot be an artifact of the method of P-T estimation. The arguments contesting such an interpretation are flawed and so it is concluded that a high-T inflection is a fundamental property of the Cretaceous upper mantle geotherm beneath southern Africa.     

Effects of low dissolved oxygen events on the macrobenthos of the lower Chesapeake Bay

The effects of low dissolved oxygen or hypoxia (<2 mg l^?1) on macrobenthic infaunal community structure and composition in the lower Chesapeake Bay and its major tributaries, the Rappahannock, York, and James rivers are reported. Macrobenthic communities at hypoxia-affected stations were characterized by lower species diversity, lower biomass, a lower proportion of deep-dwelling biomass (deeper than 5 cm in the sediment), and changes in community composition. Higher dominance in density and biomass of opportunistic species (e.g., euryhaline annelids) and lower dominance of equilibrium species (e.g., long-lived bivalves and maldanid polychaetes) were observed at hypoxia-affected stations. Hypoxia-affected macrobenthic communities were found in the polyhaline deep western channel of the bay mainstem north of the Rappahannock River and in the mesohaline region of the lower Rappahannock River. No hypoxic effects on the infaunal macrobenthos were found in the York River, James River, or other deep-water channels of the lower Chesapeake Bay.     

Second-order elasticity theory: Explanation for the high poisson's ratio of the inner core

Terms in conventional elasticity theory that are normally neglected become important when the ambient pressure is not negligible in comparison with the elastic moduli. These terms give a pressure dependence to the elastic moduli in addition to that resulting from the pressure dependence of atomic forces. The net effect is to reduce the rigidity moduli and to increase incompressibility, resulting in an increase in Poisson's ratio. It is shown that the value of Poisson's ratio for the inner core (～0.445) is compatible with a rigid metal, such as iron, which would yield the observed high value when subjected to core pressures.     

Modelling of igneous fractionation and other processes using Pearce diagrams

Geochemical data can be quantitatively modelled by means of Pearce diagrams. These are graphs of A/Z vs B/Z where A, B and Z are compositional abundances (e.g. wt.% SiO₂, wt.% MgO, and ppm La) and Z has the additional property of having constant absolute abundance. In the terminology of igneous petrology, Z (the common denominator variable) could be an incompatible element. The numerators (A and B) may be complex algebraic combinations of elements, or even CIPW normative abundances. The utility of Pearce diagrams lies in the fact that slopes of data distributions equal the bulk AB ratio of minerals lost or gained from a suite of cogenetic rocks. There is no distortion because these plots correct for data closure. Terms of the form A_i·Z₀/Z_i (where Z₀ is the abundance in a reference sample) remove the scaling to A_i caused by the abundance of a particular choice of Z. Subtraction of these terms for different samples (e.g. A_i·(Z₀/Z_i)-A_j· (Z₀/Z_j)) quantifies mineral losses and gains. Mathematical analysis shows that limited compatibility of the denominator variable is permitted. A bulk partition value (D) of 0.1 introduces an error of only 10% in values of A_i***-Z₀/Z_i, and 10° in slope-angle on Pearce diagrams over a crystallization interval of 50%. For D0.01 the error is minimal for a crystallization interval over 90%.     

Where are the rings of Neptune?

Stable rings can exist at inclinations of 0–15°, 165°–180°, or ～90° to Neptune's equator, but perturbations due to the massive satellite Triton would produce a severe “warping” of the ring plane. If Neptune possesses rings, they may not lie in the plane of its equator.     

A review of experimental studies of crystal/liquid trace element partitioning

Available data on silicate/liquid, phosphate/liquid and oxide/liquid trace element partition coefficients from experimental studies show an encouraging degree of consistency, although much more work is required before a set of coefficients of wide-ranging application to planetary problems can be compiled. The complex dependences of most coefficients on bulk chemical composition (and liquid structure) are important aspects which remain to be fully resolved. Further determinations of coefficients for a number of elements for phosphates, zircon, spinels, amphiboles and garnets are especially desirable and investigations into the effects of pressure and volatiles on the magnitudes of partition coefficients are also needed. There is a major discrepancy among existing data regarding the upper concentration limits of Henry's Law dilute solution behavior under different experimental conditions. Data obtained at 1 atmosphere under dry conditions can apparently be reconciled with data from highpressure, H₂O-saturated experiments only if Henry's Law limits are themselves functions of variables such as pressure, temperature, H₂O activity and chemical composition. Further experiments (including studies bearing on the role of defect substitution at very low trace element concentrations) are required to resolve this question.     

Late Quaternary palaeohydrology of Lake Pergusa (Sicily,southern Italy) as inferred by stable isotopes of lacustrine carbonates

Ten meters of lacustrine deposits retrieved from Lake Pergusa (Sicily, southern Italy) were investigated through stable isotope composition (carbon and oxygen) of authigenic carbonate (calcareous muds) and freshwater shells. The core chronology was established through three AMS dates, and by correlation with a previously dated nearby core. Stable isotope data show that the lake water evolution was mainly dominated by evaporation. Between ca. 20 and 28 ka the recovered sediments have very high δ¹⁸O values, likely corresponding to very dry climatic conditions. The observed rapid oscillations in the δ¹⁸O of the recovered sediments during this period also suggest important climatic fluctuations. More humid conditions dominated during the Holocene period, with the wettest interval occurring between ca. 9000 and 3000 years BP. Late Holocene sediments represent a substantial return to drier conditions. The available pollen data from a nearby core substantially confirm this general climatic trend during the Holocene. The positive correlation between δ¹³C of the calcareous muds and carbonate content suggests that biological activity played a key role in the carbon isotope evolution of dissolved inorganic carbon. However, a clear climatic signal is not evident from the δ¹³C record.     

A new chronology for the age of Appalachian erosional surfaces determined by cosmogenic nuclides in cave sediments

The relative chronology of landscape evolution across the unglaciated Appalachian plateaus of Kentucky and Tennessee is well documented. For more than a century, geomorphologists have carefully mapped and correlated upland erosional surfaces inset by wide‐valley straths and smaller terraces. Constraining the timing of river incision into the Appalachian uplands was difficult in the past due to unsuitable dating methods and poorly preserved surface materials. Today, burial dating using the differential decay of cosmogenic ²⁶Al and ¹⁰Be in clastic cave sediments reveals more than five million years of landscape evolution preserved underground. Multilevel caves linked hydrologically to the incision history of the Cumberland River contain in situ sediments equivalent to fluvial deposits found scattered across the Eastern Highland Rim erosional surface. Cave sediments correlate with: (1) thick Lafayette‐type gravels on the Eastern Highland Rim deposited between c. 5·7 and c. 3·5 Ma; (2) initial incision of the Cumberland River into the Eastern Highland Rim after c. 3·5 Ma; (3) formation of the Parker strath between c. 3·5 Ma and c. 2·0 Ma; (4) incision into the Parker strath at c. 2 Ma; (5) formation of a major terrace between c. 2·0 Ma and c. 1·5 Ma; (6) shorter cycles of accelerated incision and base level stability beginning at c. 1·5 Ma; and (7) regional aggradation at c. 0·85 Ma. Initial incision into the Appalachian uplands is interpreted as a response to eustasy at 3·2–3·1 Ma. Incision of the Parker strath is interpreted as a response to eustasy at 2·5–2·4 Ma. A third incision event at c. 1·5 Ma corresponds with glacial reorganization of the Ohio River basin. Widespread aggradation of cave passages at c. 0·85 Ma is interpreted as the beginning of intense glacial–interglacial cycling associated with global climate change. Copyright © 2006 John Wiley & Sons, Ltd.     

Alteration, HFSE mineralisation and hydrocarbon formation in peralkaline igneous systems: Insights from the Strange Lake Pluton, Canada

Two characteristics of peralkaline igneous rocks that are poorly understood are the extreme enrichment in HFSE, notably Zr, Nb, Y and REE, and the occurrence of fluid inclusions dominated by methane and higher hydrocarbons. Although much of the HFSE enrichment can be explained by magmatic processes, the common intense alteration of the parts of the peralkaline intrusions most enriched in these elements suggests that hydrothermal processes also play an important role in HFSE enrichment. Likewise, although the origin of the higher order hydrocarbons that occur as inclusions in these rocks is still debated, there is strong evidence that at least in some cases their formation involved hydrothermal processes. The issues of HFSE enrichment and hydrocarbon formation in peralkaline intrusions are examined using data from the Strange Lake pluton, a small, middle-Proterozoic intrusion of peralkaline granite in northeast Canada. This pluton contains some of the highest concentrations of Zr, REE and Y ever reported in an igneous body, and is characterised by abundant hydrocarbon-dominated fluid inclusions in rocks that have been hydrothermally altered, including those that form a potential HFSE ore zone. We show that HFSE at Strange Lake were partly concentrated to near exploitable levels as a result of their transport in a high salinity magmatic aqueous liquid, and that this fluid coexisted immiscibly with a carbonic phase which reacted with hydrogen and iron oxides generated during the associated hydrothermal alteration to produce hydrocarbons via a Fischer–Tropsch synthesis. As a result, hydrocarbons and HFSE mineralization are intimately associated. We then go on to show that hydrothermal alteration, HFSE mineralisation and hydrocarbons are also spatially associated in other peralkaline complexes, and present a model to explain this association, which we believe may be applicable to any peralkaline intrusion where HFSE enrichment was accompanied by calcium metasomatism, hematisation and hydrothermal fluorite. We also suggest that, even where these criteria are not satisfied, hydrothermally enriched HFSE and hydrocarbons will be intimately associated simply because they are products of the same initial magmatic fluid. Finally, we speculate that the association of HFSE and hydrocarbons may in some cases actually be genetic, if, as seems possible, unmixing or effervescence of a reduced carbonic fluid from the original magmatic fluid caused changes in temperature, pH, fO₂ or the activity of volatile ligands sufficient to induce the deposition of HFSE minerals.