Ap Jones

In-situ measurement of viscosity and density of carbonate melts at high pressure

We present the first measurements of carbonate melt viscosity and density at mantle pressures and temperatures and provide important data for modelling carbonatite behaviour within the mantle. Synchrotron radiation was used to observe falling spheres with high atomic number in situ, allowing precise determination of high terminal velocities over short fall distances. The measured viscosities of 1.5 (5) X 10m2 to 5 (2.5) X 10e3 Pas are the lowest of any known terrestrial magma types and these measurements extend the region of measurable viscosity at high pressure by at least 2 orders of magnitude. Accurate measurements of K,Ca(CO,), melt density were performed at atmospheric pressure:

Impact induced melting and the development of large igneous provinces

Abstract We use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater. This mechanism may generate sufficient quantity of melt to auto-obliterate the crater. Melting would initiate almost instantaneously, but the effects of such massive mantle melting may trigger long-lived mantle up-welling that could potentially resemble a mantle hotspot. Decompression melting is well understood; it is the main method advocated by geophysicists for melting on Earth, whether caused by thinned lithosphere or hot rising mantle plumes. The energy released is largely derived from gravitational energy and is outside (but additive to) the conventional calculations of impact modelling, where energy is derived solely from the kinetic energy of the impacting projectile, be it comet or asteroid. The empirical correlation between total melt volume and crater size will no longer apply, but instead there will be a discontinuity above some threshold size, depending primarily on the thermal structure of the lithosphere. We estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km/s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces (∼106 km3); similar melting of the mantle beneath an oceanic impact was also modelled by Roddy et al. [Int. J. Impact Eng. 5 (1987) 525]. The mantle melts will have plume-like geochemical signatures, and rapid mixing of melts from sub-horizontal sub-crater reservoirs is likely. Direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record. We suggest that the end-Permian Siberian Traps should be reconsidered as the result of a major impact at ∼250 Ma. Auto-obliteration by volcanism of all craters larger than ∼200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system.

In situ measurement of viscosity of liquids in the Fe-FeS system at high pressures and temperatures

Abstract The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 × 10-2 to 4 × 10-3 Pa-s in FeS at 1450 to 1700 °C and 2 × 10-2 to 8 × 10-3 Pa-s in Fe-Seut at 1150 to 1380 °C. These results are consistent with recent viscosity measurements in Fe-Seut at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vočadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 × 10-2 Pa-s for an outer core composition with ~10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity

Physical basis of colors seen in Congo red-stained amyloid in polarized light

Amyloid stained by Congo red is traditionally said to show apple-green birefringence in polarized light, although in practice various colors may be seen between accurately crossed polarizing filters, called polarizer and analyzer. Other colors are seen as the polarizer and analyzer are uncrossed and sometimes when the slide is rotated. Previously, there has been no satisfactory explanation of these properties. Birefringence means that a material has two refractive indices, depending on its orientation in polarized light. Birefringence can change linearly polarized light to elliptically polarized, which allows light to pass a crossed analyzer. The birefringence of orientated Congo red varied with wavelength and was maximal near its absorption peak, changing from negative (slow axis of transmission perpendicular to smears or amyloid fibrils) on the shortwave side of the peak to positive (slow axis parallel) on the longwave side. This was explained by a property of any light-absorbing substance called anomalous dispersion of the refractive index around an absorption peak. Negative birefringence gave transmission of blue, positive gave yellow, and the mixture was perceived as green. This explains how green occurs in ideal conditions. Additional or strain birefringence in the optical system, such as in glass slides, partly or completely eliminated blue or yellow, giving yellow/green or yellow, and blue/green or blue, which are commonly seen in practice and in illustrations. With uncrossing of polarizer or analyzer, birefringent effects declined and dichroic effects appeared, giving progressive changes from green to red as the plane of polarization approached the absorbing axis and from green to colorless in the opposite way. This asymmetry of effects is useful to pathologists as a confirmation of amyloid. Rather than showing ‘apple-green birefringence in polarized light’ as often reported, Congo red-stained amyloid, when examined between crossed polarizer and analyzer, should more accurately be said to show anomalous colors.

Metamorphism, Partial Melting, and K-Metasomatism of Garnet-Scapolite-Kyanite Granulite Xenoliths from Lashaine, Tanzania

Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase (An28–41, Or1–2) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and CO2. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses (SiO2 38–56 wt %, Al2O3 15–23, K2O 0.5–8) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to melting of garnet, clinopyroxene, and plagioclase, followed by quench crystallization, and finally by K replacement of Na. One clinopyroxenite with 15% glass and quench spinel lacks garnet and scapolite. All properties are consistent with rapid decompression, quenching and K metasomatism. The estimated pre-emption temperature (1200 K) for the Lashaine granulites lies well above the temperatures at ∼14 kb for a standard shield geotherm (850 K) and even an oceanic geotherm (1060 K). Pressure-temperature estimates for granulite xenoliths from three sites (Delegate, Engeln, Lashaine) fall on a single trend tentatively called an alkaline-province geotherm.

Molecular dynamics simulations of CaCO3 melts to mantle pressures and temperatures: implications for carbonatite magmas

Carbonatite magmas have been suggested to be important agents of metasomatism of the lithospheric mantle. However, the structures and properties of this important class of melt have been only poorly constrained at mantle pressures and temperatures. In the present study a molecular dynamics approach is adopted to constrain carbonatite magma properties and structure, since experimental difficulties preclude the direct study of alkaline carbonate melts at pressure. Simulation results suggests that CaCO3 melt densities increase from 2000 kg m−3 at P ≈ 0.1 GPa to 2900 kg m−3 at P ≈ 10.0 GPa. Estimates of the constant pressure heat capacity of 1.65–1.90 J g−1 K−1, isothermal compressibilities of 0.0120-0.002 kbar−1 and thermal expansivities of 1.886-0.589 × 10−4 K−1 for CaCO3 melts to mantle pressures and temperatures are also provided from simulation results. Self-diffusion coefficients, calculated from simulation results, qualitatively suggest that CaCO3 melts have very low viscosities even at high pressures. The molecular dynamics simulations suggest octahedral Ca2+ coordination in carbonatite melts up to at least 11.5 GPa and the presence in the melt phase of associative metal-carbonate clusters. Fluid-flow calculations, based on the derived density data, suggest carbonatite ascent rates of 20–65 m s−1, which imply that mantle derived carbonatites should be capable of transporting mantle xenoliths of dimensions up to 0.25 that of their conduits.

An infrared and Raman study of carbonate glasses: implications for the structure of carbonatite magmas

place constraints on the structures of these glasses and natural carbonatite magmas. The activity of the fundamental modes of the carbonate ion indicate that at least two structural populations of CO:- exist in carbonate glass structure, one of which, by virtue of the large vibrational splitting of its v) mode, is suggested to occupy a highly asymmetric site. The spectral activity of the O-H stretching region suggests that water exists both as molecular Hz0 and OH, interacting variably with carbonate ions and as metal complexes occupying relatively high symmetry sites in these glasses. The presence of bicarbonate groups, however, is prohibited by the absence of characteristic O-H stretching frequencies. It is suggested on the basis of the vibrational spectra that carbonate glass structures represent “flexible” frameworks constructed by the “bridging” of carbonate ions by strongly interacting metal cations and that the flexible framework is supported by framework modifying cations and molecular groups. The presence of at least two structural populations of CO:- in carbonate glasses implies a level of medium range order and the existence of extended structural units in carbonate melts and it is suggested that such groups represent metal-carbonate complexes. The possible effects of such complexes on the geochemical behaviour of elements in carbonatite melts is discussed and a general model by which variations in elemental solubility could be understood is proposed.

Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

Abstract We present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90−92), orthopyroxene (~En89, Wo2.0), clinopyroxene (Mg90−92, 3−6% Al2O3, 1−1.5% Cr2O3), and chrome-spinel (14−20% MgO, ~30−40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with ~4% FeO, 1.8% Cr2O3, 1.4−2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (~0.5−1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14−22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm-2) but it compares well with the high-pressure end of a typical alkaline continental rift.

Glasses in Mantle Xenoliths from Olmani, Tanzania

Nineteen mantle xenoliths from Olmani, Tanzania, have deformation textures and fluid inclusions like those in olivine-rich peridotites from Hawaii. Group A comprises 7 barren harzburgites (spinel, TiO2 <. 1 wt %) and group B (spinel, TiO2.8–2.6 wt %) contains 6 clinopyroxene-dunites, 2 lherzolites, 1 wehrlite, 1 two-pyroxene-dunite, and 1 olivine-orthopyroxenite. Both groups lack garnet and porphyroclastic textures and display fingerprint intergrowths of spinel with ortho-or clinopyroxene. Many secondary inclusions of several generations within olivine are rich in Co2. Alkali-rich glasses (Na2O.4–7.3 wt %, K2O 3.3–5.1, P2O5 ∼1) in B xenoliths are unrelated chemically to the olivine-augite-phyric ankaramite host. Aphyric inter-granular glass occurs along with larger patches of residual glasses associated with euhedral olivine, clinopyroxene and spinel. Clinopyroxenes of B xenoliths show decreasing Na2O (2.1–.3 wt %) and Cr2O3 from olivine-brown cores to clear margins of larger grains, and to finer grains associated with residual glass. These properties of the xenoliths are consistent with establishment of a coarse texture during prolonged metamorphism in the mantle (∼28 kb, 1100 K?), partial melting in the presence of a CO2-rich fluid (∼26 kb, 1400 K?), transport by ankaramite magma to the Earths surface, and rapid crystallization of some of the melts at 1300–1200 K.

SYNTHESIS OF CUBIC DIAMOND IN THE GRAPHITE-MAGNESIUM CARBONATE AND GRAPHITE-K2MG(CO3)2 SYSTEMS AT HIGH PRESSURE OF 9-10 GPA REGION

Cubic diamond was synthesized with two systems, (1) graphite with pure magnesium carbonate (magnesite) and (2) graphite with mixed potassium and magnesium carbonate at pressures and temperatures above 9.5 GPa, 1600 degrees C and 9 GPa, 1650 degrees C, respectively. At these conditions (1) the pure magnesite is solid, whereas (2) the mixed carbonate exists as a melt. In this pressure range, graphite seems to be partially transformed into hexagonal diamond. Measured carbon isotope delta(13)C values for all the materials suggest that the origin of the carbon source to form cubic diamond was the initial graphite powder, and not the carbonates.