Oded Navon

Oxygen isotope variations in phosphate of biogenic apatites, I. Fish bone apatite—rechecking the rules of the game

Abstract The major advantage of the oxygen in phosphate isotope paleothermometry is that it is a system which records temperatures with great sensitivity while bone (and teeth) building organisms are alive, and the record is nearly perfectly preserved after death. Fish from seven water bodies of different temperatures (3–23°C) and different δ 18 O (values −16 to +3) of the water were analysed. The δ 18 O values of the analysed PO 4 vary from 6 to 25. The system passed the following tests: (a) the temperatures deduced from isotopic analyses of the sequence of fish from Lake Baikal are in good agreement with the temperatures measured in the thermally stratified lake; (b) the isotopic composition of fish bone phosphate is not influenced by the isotopic composition of the phosphate which is fed to the fish, but only by temperature and water composition. Isotopic analysis of fossil fish in combination with analysis of mammal bones should be a useful tool in deciphering continental paleoclimates.

Hydrous and carbonatitic mantle fluids in fibrous diamonds from Jwaneng, Botswana

Abstract Fluid-inclusions in fibrous diamonds from Jwaneng (Botswana) contain water, carbonates, silicates, apatite, and CO 2 . Average compositions of fluids trapped in individual diamonds span a wide range, and vary linearly and continuously between two endmember compositions, a carbonatitic fluid rich in carbonate, CaO, FeO, MgO, and P 2 O 2 , and a hydrous fluid rich in water, SiO 2 , and Al 2 O 2 . K 2 O contents are high in both endmembers. The mg numbers (Mg/(Mg + Fe)) of the trapped fluids are low (0.55-0.44) and decrease towards the hydrous endmember. Fluid compositions are broadly similar to those reported for Zairean diamonds, but cover a wider range. Intra-diamond compositional variation is limited. We examine three simple models for the formation and evolution of the fluid in the earths mantle: 1. (1) Mixing of hydrous and carbonatitic fluids, 2. (2) partial melting of a carbonate-bearing source rock, and 3. (3) fractional crystallization of a carbonatitic melt at depth. The low mg numbers of both endmembers suggest that the source rocks for the melting scenario must be more Fe-rich than common mantle peridotites. Fractional crystallization of ferroan dolomite and magnesite with small amounts of rutile and apatite can account for the observed variation of most elements. Crystallization of an additional K-rich phase is needed to explain the potassium trend. Recent experimental studies have demonstrated that carbonatitic melts and hydrous fluids may exist in equilibrium with metasomatized peridotite. The data presented here provide the first direct evidence for the existence of both fluids in the diamond stability field, deep in the upper mantle.

Bubble nucleation in rhyolitic melts: experiments at high pressure, temperature, and water content

Abstract We report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release. Two rhyolitic obsidians, one containing less than 1% of microlites of Fe—Ti oxides and the other about 20% of various crystals were hydrated at 150 MPa and 780–850°C. After saturation was reached (5.3–5.5 wt% water), pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time, before the final, rapid quenching of the experiments. The results demonstrate the importance of heterogeneous nucleation. Microlites of Fe—Ti oxides are very efficient as sites for bubble nucleation. In their presence, modest nucleation was observed even after decompression by 6 –10 8 bubbles cm −3 ). In the absence of microlites, no nucleation occurred at ΔP MPa. At ΔP > 10 MPa, bubbles also nucleated on crystals of biotite, zircon and apatite. Modest nucleation (10 3 –10 5 cm −3 ) took place even in crystal-free samples, but it was still heterogeneous. When ΔP exceeded 80 MPa, nucleation in crystal-free samples became extensive (10 5 –10 7 cm −3 ). The lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous. Nucleation rates were controlled mainly by the availability of sites. Rates were faster than 10 6 cm −3 s −1 when microlites were present, and faster than 10 5 cm −3 s −1 in the absence of microlites at ΔP > 70 MPa. Narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop. The experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation. When a large number of efficient sites (e.g., Fe—Ti oxide) are present, bubble nucleation requires very little supersaturation, and to a good approximation, gas and magma are in equilibrium. In magmas that are crystal-free or contain crystals that are inefficient at nucleating bubbles, very high degrees of supersaturation are required in order to initiate nucleation. These two modes of exsolution may lead to contrasting styles of convection, pressure build up and eruption.

Brine inclusions in diamonds: a new upper mantle fluid

Micro-inclusions in cloudy diamonds from Koffiefontein consist of three main types: silicates, carbonates and brine inclusions. The silicates belong to either the eclogitic or the peridotitic paragenesis and both are associated with carbonates and brine. The brine composition is roughly (K,Na)8(Ca,Fe,Mg)4SiO(CO3)4Cl10(H2O)28–44. Average mass proportions are about 30–42% water, 19–22% chlorine, 14–17% sodium and potassium, 22–25% Fe–Ca–Mg–carbonates, and 3–4% silica. The brine composition is distinct from that of fluids trapped in fibrous diamonds mainly by its high chlorine and low silica content. The close association of carbon-bearing brine, silicate minerals and diamonds suggests that such brines are important for diamond growth in both eclogitic and peridotitic environments. The similarity of brine composition in both environments may indicate that diamonds of both suites grew in a single event.

Bubble growth in highly viscous melts: theory, experiments, and autoexplosivity of dome lavas

We examine the physics of growth of water bubbles in highly viscous melts. During the initial stages, diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation. Growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential. At later stages, internal pressure falls, radial growth decelerates and follows the square-root of time. At this stage it is controlled by diffusion. The time of transition between the two stages is controlled by the decompression, melt viscosity and the Peclet number of the system. The model closely fit experimental data of bubble growth in viscous melts with low water content. Close fit is also obtained for new experiments at high supersaturation, high Peclet numbers, and high, variable viscosity. Near surface, degassed, silicic melts are viscous enough, so that viscosity-controlled growth may last for very long times. Using the model, we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation. We suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation. The presence of such bubbles is an indicator of the conditions at fragmentation. The water content of lavas extruded at lava domes is a key factor in their evolution. Melts of low water content (<0.2 wt%) are too viscid and bubbles nucleated in them will not grow to an appreciable size. Bubbles may grow in melts with ∼0.4 wt% water. The internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows.

Vesiculation processes in silicic magmas

Abstract The physics of vesiculation, i.e. the process of bubble formation and evolution, controls the manner of volcanic eruptions. Vesiculation may lead to extreme rates of magma expansion and to explosive eruptions or, at the other extreme, to low rates and calm effusion of lava domes and flows. In this paper, we discuss the theory of the different stages of vesiculation and examine the results of relevant experimental studies. The following stages are discussed: (1) The development of supersaturation of volatiles in melts. Supersaturation may develop due to a decrease in equilibrium solubility following changes in ambient pressure or temperature or due to an increase in the magma volatile content (e.g. in response to crystallization of water-free or waterpoor mineral assemblages). (2) Bubble nucleation. The classical theory of homogeneous nucleation and some modern modifications, heterogeneous nucleation with emphasis on the nucleation of water bubbles in rhyolitic melts and the role of specific crystals as heterogeneous sites. (3) Bubble growth. The effect of diffusion, viscosity, surface tension, ambient pressure and inter-bubble separation on the dynamics of growth. (4) Bubble coalescence. The theory of coalescence of static foams. factors that may effect coalescence in expanding foams and shape relaxation following bubble coalescence.

Raman barometry of diamond formation

Abstract Pressures and temperatures of the diamond source region are commonly estimated using chemical equilibria between coexisting mineral inclusions. Here we present another type of geobarometer, based on determination of the internal pressure in olivine inclusions and the stresses in the surrounding diamond. Using Raman spectroscopy, pressures of 0.13 to 0.65 GPa were measured inside olivine inclusions in three diamonds from the Udachnaya mine in Siberia. Stresses in the diamond surrounding the inclusions indicated similar pressures (0.11–0.41 GPa). Nitrogen concentration and aggregation state in two of the diamonds yielded mantle residence temperatures of ∼1200°C. Using this temperature and the bulk moduli and thermal expansion of olivine and diamond, we calculated source pressures of 4.4–5.2 GPa. We also derived a linear approximation for the general dependence of the source pressure ( P 0 , GPa) on source temperature ( T 0 , °C) and the measured internal pressure in the inclusion ( P i ): P 0 =(3.259×10 −4 P i +3.285×10 −3 ) T 0 +0.9246 P i +0.319. Raman barometry may be applied to other inclusions in diamonds or other inclusion–host systems. If combined with IR determination of the mantle residence temperature of the diamond, it allows estimation of the pressure at the source based on a non-destructive examination of a single diamond containing a single inclusion.

Bubble growth during decompression of magma: experimental and theoretical investigation

A model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt. The model accounts for the interplay of three dynamic processes: decompression rate of the magma, deformation of the viscous melt around the growing bubble, and diffusion of volatiles into the bubble. Generally, these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically. For a better understanding of the physics of the processes, we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance, diffusion or linear decompression rate. We show that the solutions are a function of time and two dimensionless numbers, which are the ratios of either the diffusive or viscous time scales over the decompression time scale. The conditions for each growth regime are provided as a function of the two governing dimensionless parameters. Analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts. The model solutions, including the division to the growth regimes as function of the two parameters, provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure, supersaturation and gas volume fraction. The model results are in agreement with the conditions of Plinian explosive eruption (e.g. Mount St. Helens, 18 May 1980), where high gas overpressure is expected. The conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions, mostly in equilibrium degassing and partly in overpressure conditions. We show that in a situation of quasi-static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble, insensitive to the diffusivity profile.

TEM imaging and analysis of microinclusions in diamonds: A close look at diamond-growing fluids

Abstract Fluid-bearing microinclusions in diamonds (<1 μm) provide a unique source of information on the diamond-forming medium. Transmission electron microscopy (TEM) investigation of such microinclusions enables the detailed study of their size, external habit, internal morphology, and mineralogy, and yields information on the chemical composition and crystallography of the included phases. Here we present a detailed TEM examination of microinclusions in four fibrous diamonds from Canada and Siberia, each with a distinctive inferred original fluid composition. Most microinclusions contain multi-phase assemblages that include carbonate, halide, apatite, possible pyroxene, and high-silica mica (6.8-7.7 Si atoms per formula unit) whose composition lies along the phlogopite-Al-celadonite join. The TEM results, together with the tight range of composition detected by electron probe microanalysis (EPMA) and the volatiles detected by infrared (IR) spectroscopy, suggest that the microinclusions trapped a uniform, dense, supercritical fluid and that the crystallized minerals grew as secondary phases during cooling. Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely.

Trace element analyses of fluid-bearing diamonds from Jwaneng, Botswana

Abstract Fibrous diamonds from Botswana contain abundant micro-inclusions, which represent syngenetic mantle fluids under high pressure. The major element composition of the fluids within individual diamonds was found to be uniform, but a significant compositional variation exists between different diamond specimens. The composition of the fluids varies between a carbonatitic and a hydrous endmember. To constrain the composition of fluids in the mantle, the trace element contents of thirteen micro-inclusion-bearing fibrous diamonds from Botswana was studied using neutron activation analysis. The concentrations of incompatible elements (including K, Na, Br, Rb, Sr, Zr, Cs, Ba, Hf, Ta, Th, U, and the LREEs) in the fluids are higher than those of mantle-derived rocks and melt inclusions. The compatible elements (e.g., Cr, Co, Ni) have abundances that are similar to those of the primitive mantle. The concentrations of most trace elements decrease by a factor of two from the carbonate-rich fluids to the hydrous fluids. Several models may explain the observed elemental variations. Minerals in equilibrium with the fluid were most likely enriched in incompatible elements, which does not agree with derivation of the fluids by partial melting of common peridotites or eclogites. Fractional crystallization of a kimberlite-like magma at depth may yield carbonatitic fluids with low mg numbers (atomic ratio [Mg/(Mg+Fe)]) and high trace element contents. Fractionation of carbonates and additional phases (e.g., rutile, apatite, zircon) may, in general, explain the concentrations of incompatible elements in the fluids, which preferably partition into these phases. Alternatively, mixing of fluids with compositions similar to those of the two endmembers may explain the observed variation of the elemental contents. The fluids in fibrous diamonds might have equilibrated with mineral inclusions in eclogitic diamonds, while peridotitic diamonds do not show evidence of interaction with these fluids. The chemical composition of the fluids in fibrous diamonds indicates that, at p , T conditions that are characteristic for diamond formation, carbonatitic and hydrous fluids are efficient carriers of incompatible elements.