Search

High-temperature annealing experiments on metamorphic kalsilite (KA1SiO(sub 4)) indicate that there is an irreversible phase transition from the P31c to P6(sub 3) structure at approximately 200 degrees C. Powder X-ray investigations indicate that the transformation is incomplete. The proportion of P6(sub 3) phase increases as a function of temperature, attaining 70% for the powdered sample at approximately 500 degrees C. At higher temperatures, the fraction of sample transforming to P6(sub 3) phase seems to be independent of temperature, and probably depends on the structural state of the starting material as influenced by mechanical grinding. Transformation proceeds by tetrahedral rotation such that successive (001) sheets undergo opposite-sense rotations. The transition behavior differs for single crystals, which had not been mechanically ground; at 500 degrees C single crystals transformed completely into the P6(sub 3) phase. Single-crystal structure refinements of the P6(sub 3) phase indicate that the structure is similar to that of volcanic kalsilite. Both apical and basal O atoms show large anisotropic displacement parameters, but the displacement of the apical oxygen does not exhibit the trigonal distribution that has been observed in volcanic kalsilite. The more pronounced positional disorder in volcanic kalsilite is due to the presence of sodium.

The major and trace element compositions of individual fluid inclusions from a range of magmatic-hydrothermal ore deposits were analyzed by laser-ablation inductively coupled plasma-mass spectrometry, to explore the behavior of ore-forming components during fluid phase separation ('boiling') in high-temperature saline fluid systems. Data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior. Na, K, Fe, Mn, Zn, Rb, Cs, Ag, Sn, Pb, and Tl preferentially partition into the brine (probably as Cl complexes), whereas Cu, As, Au (probably as HS complexes), and B selectively partition into the vapor. Fluid phase separation is probably a major, previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic-hydrothermal systems, from deep plutons through porphyry-style and greisen deposits to epithermal mineralization and volcanic fumaroles.

This study focuses on the rare occurrence of high-Fe, low-Si tholeiitic basaltic rocks of a Paleoproterozoic (1.9 Ga) backarc-arc succession (Amisk Lake, Flin Flon Greenstone Belt, Canada) and their implications for subarc mantle processes and the global elemental budget of Ta, Nb, and Ti. Volcanism at Amisk Lake is characterized by interbedded tholeiitic and boninite-like backarc basalts, overlain by intermediate to felsic calc-alkalic arc volcanic rocks. The change from dominantly backarc to arc volcanism is marked by picritic flows overlain by the high-Fe tholeiites. The high-Fe tholeiites are characteristically high in [Mathematical Expression Omitted] (15-21 wt%), low in Si[O.sub.2] (39-45 wt%), and slightly light rare earth element enriched ([[La/Sm].sub.n] = 1.3-1.93). They are unusually high in Ta, Nb, and Ti (normally depleted in arc-related rocks), and low in Zr, Hf, and Y, relative to normal-type mid-ocean ridge basalt. The petrogenesis of these rocks requires high degrees of partial melting of a dehydrated subducting slab, at low pressure. The high-Fe tholeiites represent either direct partial melts of the slab, or partial melts of the overlying mantle wedge, which has been metasomatized by this slab melt. These rocks provide evidence for the retention of Ta, Nb, and Ti in subducting slabs, and the subsequent isolation of these elements in the deep mantle, thus explaining the lack of complementarity of Ta, Nb, and Ti between the continental crust and depleted mantle. Conditions of higher heat flow and rapid subduction of young oceanic crust during the Paleoproterozoic could have resulted in partial melting of the subducted slab at shallow depths.

Andesitic arcs are sited over convergent lithospheric plate junctions. Aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and/or the overlying mantle wedge, and the ascent of calc-alkaline melts. Various hydrous minerals have been proposed to be the source of this [H.sub.2]O. Experimental equilibrium studies show that, under subduction-zone geothermal gradients of 5-7 [degrees] C/km, clinoamphibole constitutes a major phase in deep-seated (>75 km) metabasalts; other hydrous minerals are absent or are of very minor abundance. Clinoamphiboles dehydrate at pressures of [approximately]2.2-2.4 GPa under equilibrium conditions (pressure overstepping is probable), so mafic blueschists and amphibolites expel [H.sub.2]O at magmagenic depths. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures. However, micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4.0 GPa, so at subduction depths >100 km, micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant [H.sub.2]O. Deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and/or above a subduction zone prior to continental collision. As large masses of quartzofeldspathic material enter a suture zone, volatile evolution at deep levels nearly ceases. While small amounts of peraluminous, S-type anatectic melts may be produced, I-type calc-alkaline arcs - and the continents - owe their formation over geologic time to the sustained underflow of oceanic lithosphere.

Synthetic epidotes were produced using a Tuttle type hydrothermal vessel in the temperature range from 500 to 700 [degrees] C and at 4.5 to 5.0 kbars pressure. Single-crystal X-ray diffraction structure refinements yielded intracrystalline cation distributions for two crystals grown at 600 [degrees] C (0.68 and 0.73 Fe apfu) and five crystals grown at 700 [degrees] C (0.88 to 1.08 Fe apfu). The resulting Fe occupancies were compared with those calculated according to a thermodynamic model: The samples formed at 700 [degrees] C display a Fe distribution between M3 and M1 sites that match the calculation; nevertheless, we found the presence of a small but appreciable amount of Fe in the M2 sites that increases with the total Fe content (up to 0.08 Fe apfu). The crystals formed at 600 [degrees] C show a much higher disorder than expected, but have no Fe in the M2 site. Crystal-chemical features of the samples studied were compared with those of a previously published set of natural epidotes displaying a much lower Fe disorder: In particular, the unit-cell volume vs. Fe content relationship is not affected by the intracrystalline Fe distribution. This relation suggests that the Al-Fe intracrystalline distribution is independent of pressure.

Catalysis at organophilic silica-rich surfaces of zeolites and feldspars might generate replicating biopolymers from simple chemicals supplied by meteorites, volcanic gases, and other geological sources. Crystal-chemical modeling yielded packings for amino acids neatly encapsulated in 10-ring channels of the molecular sieve silicalite-ZSM-5-(mutinaite). Calculation of binding and activation energies for catalytic assembly into polymers is progressing for a chemical composition with one catalytic Al-OH site per 25 neutral Si tetrahedral sites. Internal channel intersections and external terminations provide special stereochemical features suitable for complex organic species. Polymer migration along nano/micrometer channels of ancient weathered feldspars, plus exploitation of phosphorus and various transition metals in entrapped apatite and other microminerals, might have generated complexes of replicating catalytic biomolecules, leading to primitive cellular organisms. The first cell wall might have been an internal mineral surface, from which the cell developed a protective biological cap emerging into a nutrient-rich 'soup.' Ultimately, the biological cap might have expanded into a complete cell wall, allowing mobility and colonization of energy-rich challenging environments. Electron microscopy of honeycomb channels inside weathered feldspars of the Shap granite (northwest England) has revealed modern bacteria, perhaps indicative of Archean ones. All known early rocks were metamorphosed too highly during geologic time to permit simple survival of large-pore zeolites, honeycombed feldspar, and encapsulated species. Possible microscopic clues to the proposed mineral adsorbents/catalysts are discussed for planning of systematic study of black cherts from weakly metamorphosed Archaean sediments.

Amphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 [degrees] C and 1 kbar P([H.sub.2]O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local [Mathematical Expression Omitted] ([Mathematical Expression Omitted] = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) 03 anions either through the O3-O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH-[.sup.A]Na-OH and MgMgMg-OH-[.sup.A]Na-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite-potassic-fluororichterite series.

Dated sediment cores from Cayuga Lake, New York State, document that biologically mediated precipitation of calcite has been controlled by environmental change, both natural and anthropogenic, over the past 10 000 yr. During the Holocene Hypsithermal ([approximately]9-4 ka [14C]), Milankovitch forcing of summer insolation in the Northern Hemisphere resulted in a broad increase (to 55%), then decrease (to

A study of the phase relations of the phosphate end-member in the MgO-P2O5-H2O system was conducted to examine the stability of pure phosphoellenbergerite. It was found that the system included the phases of Mg2PO4OH polymorphs, holtedahlite, althausiste, Mg3(PO4)2 polymorphs, farringtonite and phosphoellenbergerite. The latter, although a high-pressure phase, is stable to much lower pressures and higher temperatures.

A set of 34 laser total-fusion 40Ar/39Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 [+ or -] 0.1 Ma. This age is about 9.0 m.y. older than 40Ar/39Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40Ar/39Ar ages of the sanidine show that it is essentially free of excess 40Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40Ar/39Ar age of 74.1 [+ or -] 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40Ar/39Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 [+ or -] 0.1 Ma age for the Manson impact structure and limit its age to the interval [+ or -] 74.5 0.1 to 73.8 [+ or -] 0.1 Ma. Recently, two origins for the Crow Creek have been proposed - eastward transgression of the Late Cretaceous sea and a Manson impact-triggered tsunami. We conclude that most data are in accord with an impact origin for the Crow Creek Member and are at odds with the marine transgression hypothesis.

Cummingtonite from central Massachusetts, which has experienced pervasive exsolution of hornblende, contains numerous hornblende precipitates (lamellae and discs) ranging in thickness from 2.5 [[micro]meter] to 4 nm that represent multiple generations of exsolution. The smallest precipitates were investigated using transmission electron microscopy and analytical electron microscopy to characterize the coherent cummingtonite-hornblende solvus at low temperature. In contrast to older (coarser) generations of exsolution lamellae, which are 0.5-2.5 [[micro]meter] in width, planar, and extend across entire grains, the youngest generation consists of nanometer-scale discs (4-80 nm thick and 10-1000 nm long) that represent exsolution at approximately 300 [degrees] C (Klein et al. 1996). Quantitative energy-dispersive X-ray analyses were obtained from discs as small as 20 nm in thickness, providing compositional data for coherent exsolution of hornblende at [approximately]300 [degrees] C. Surprisingly, these tiny discs have a lower Ca content and a higher Al content than the coarser, higher temperature lamellae. These compositions appear to lie well within miscibility gaps of the equilibrium and coherent solvi for cummingtonite-hornblende (in terms of Ca [equivalence] [Fe.sup.2+] exchange) at low temperatures, and may represent a more tschermakitic hornblende coexisting with cummingtonite with excess Al relative to Ca. The composition of the cummingtonite host between nanometer-scale discs is very low in both Ca and Al relative to bulk cummingtonite analyses that were obtained from electron microprobe analyses of 1-2 [[micro]meter] areas. Compositional profiles across host areas were measured to help interpret the unexpected chemistries of the nanometer-scale discs. A profile across the precipitate-free zone between a coarse hornblende lamella and a group of nanometer-scale hornblende discs shows downward concavity for Ca and Al, as expected for incomplete diffusion between the precipitation zone and the pre-existing hornblende lamellae. A profile measured between two nanometer-scale discs shows downward concavity for Al and nearly constant to slight upward concavity for the Ca content, suggesting incomplete Al diffusion before completion of the exsolution process, but a near-equilibrium state for Ca. However, the Ca profile could also represent incomplete diffusion, if the hornblende discs had started coarsening at the expense of other (shrinking) discs by means of the Gibbs-Thomson (capillary) effect. The observed profiles suggest that, relative to Al, Ca was the faster diffusing element when the exsolution and coarsening processes stopped.

Normal grain growth at elevated temperatures under isostatic stress conditions is an important process in the control of grain size of rock-forming minerals. The normal grain growth kinetics of pure calcite powders from different sources was investigated, to explore the possible poor state of existing calcite grain growth experimental data. Rates of pore drag exhibited differences for the different materials, indicating that the process is very sensitive to trace solute impurity concentrations.

Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD, SAED). SAED patterns were also obtained for synthetic Sr-exchanged birnessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic structure of NaBi and the one-layer hoexagonal structure of HBi with subcell parameters a = 5.172 [Angstrom], b = 2.849 [Angstrom], c = 7.34 [Angstrom], [Beta] = 103.3[degrees] and a = 2.848 [Angstrom], c = 7.19 [Angstrom], [Gamma] = 120 [degrees], respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental observations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of [Mn.sup.3+] for [Mn.sup.4+]. The supercell A = 3a parameter arises from the ordered distribution of [Mn.sup.3+]-rich rows parallel to [010] and separated from each other along [100] by two [Mn.sup.4+] rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when [Mn.sup.3+]-rich rows are free of [Mn.sup.4+]. The idealized structural formula proposed for NaBi type II is [Na.sub.0.333]([Mathematical Expression Omitted])[O.sub.2]. NaBi type I has a lower amount of [Mn.sup.3+] and its ideal composition would vary from [Na.sub.0.167]([Mathematical Expression Omitted])[O.sub.2] to [Na.sub.0..25]([Mathematical Expression Omitted])[O.sub.2]. Satellites in SAED patterns of NaBi crystals result from the ordered distribution of [Mn.sup.4+] and [Mn.sup.2+] pairs in [Mn.sup.3+]-rich rows with a periodicity of 6b. The structure of HBi consists of hexagonal octahedral layers containing predominantly [Mn.sup.4+] with variable amounts of [Mn.sup.3+] and layer vacancies. The distribution of layer vacancies is inherited from the former [Mn.sup.3+] distribution in NaBi. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of [Mn.sup.3+]-rich rows at low pH.

Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal ([H.sup.+]-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural [Na.sup.+] with solution [H.sup.+] and the partial loss of [Mn.sup.2+] to the aqueous phase. The desorption of [Na.sup.+] in two temporally distinct steps indicates the existence of two types of binding sites for this cation. [Mn.sup.2+] appears to originate from a partial disproportionation of [Mn.sup.3+] in the NaBu layers, according to the sequence [Mathematical Expression Omitted] EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing Mn[O.sub.6] octahedra, with no evidence for triple-corner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly [Mn.sup.3+]. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer [Mn.sup.3+] to interlayer TC-sharing positions and re-adsorption of [Mn.sup.2+] from solution onto layer vacancy sites. The migration of [Mn.sup.3+] cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of [Zn.sup.2+]-exchanged birnessite shows that [Zn.sup.2+] also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

We studied structural and dynamical properties of [H.sup.+] absorbed in Mg2Si[O.sub.4] by ab-initio molecular dynamics. We first calculated the T = 0 equation of state of pure forsterite as a function of pressure, and we determined the relative stabilities of the olivine, [Beta]-spinel, and spinel polymorphs. The results show that the ab-initio model successfully reproduces the known structural properties of the system. In the protonated phases, in agreement with experimental evidence, our computations show that [H.sup.+] is absorbed preferentially in the [Beta]-spinel phase. The most stable absorption site is located close to the O1 atom, which is coordinated by five [Mg.sup.2+] cations and not directly bound to Si. In addition to this stable absorption site, the computation reveals other low-energy positions, forming an extended network of hydrogen bonds, that could play an important role in the diffusion of [H.sup.+] in [Beta]-spinel. We analyze the dependence of structure and dynamics of the pure and protonated phases as a function of temperature and pressure.

Crystal-structure refinements were performed on clintonite-1M crystals [ideal composition [6]Ca[6][([Mg.sub.2]Al).sub.[4](Si[Al.sub.3])[O.sub.10][(OH).sub.2]] from skarns of the Predazzo-Monzoni area and Adamello Massif (northern Italy) with the aim of characterizing some aspects of their crystal chemistry and their relationships with closely associated phlogopite-1M. In the clintonite samples examined, the tetrahedral composition ranges from [Si.sub.1.19][Al.sub.2.78][Fe.sub.0.03] to [Si.sub.1.28][Al.sub.2.70][Fe.sub.0.02] indicating that the extent of the exchange vector [4][Al.sub.-1][6][Mg.sub.-2][4]Si[6](Al, ??), which links trioctahedral with dioctahedral Ca-bearing brittle micas, was very limited. Single-crystal X-ray diffraction data were collected and structure refinements completed in space group C2/m converging to [R.sub.obs] from 0.027 to 0.037 for six samples. The [4][Al.sup.3+] for [4][Si.sup.4+] substitution, which is close to 70%, produces more regular and flatter tetrahedra than in the case of phlogopite, together with an increase in the thickness and in the lateral dimensions of the sheet; the presence of [Al.sup.3+] in octahedral coordination, on the other hand, reduces the dimensions of both M1 and M2 sites with a consequent decrease in the thickness of the sheet. The volume, the flattening angle [Psi], and the central cation off-center shift (BLD) of the trans M1 octahedral site are greater than those of the cis M2 site, thus indicating a normal octahedral ordering. The high misfit value (from 1.187 to 1.326 [Angstrom]) between tetrahedral and octahedral sheets is mostly compensated by the distortion of the tetrahedral ring (tetrahedral rotation angle [Alpha]: 23.1 [less than or equal to] [Alpha] [less than or equal to] 24.9[degrees]). Relative to phlogopite, the interlayer separation in clintonite is reduced by about 0.6 [Angstrom].

The intracrystalline distribution of Mg and [Fe.sup.2+] on the M1 and M2 octahedral sites in high-P C2/c (Mg,Fe)Si[O.sub.3] clinopyroxene has been determined on quenched samples using 57Fe Mossbauer spectroscopy. Although the recovered samples that were measured had the low-P ([P2.sub.1]/c) structure, the ordering state of the high-P polymorph is believed to have been preserved because the high-P C2/c [right arrow] low-P [P2.sub.1]/c transformation occurred at room temperature during the final stages of decompression of the experiments. Under such conditions significant cation diffusion is unlikely. Low temperature (81 K) Mossbauer spectra indicate that [Fe.sup.2+] is strongly ordered onto the M2 sites, with an average [K.sub.D] [Angstrom] 4.0(8) for the intersite exchange reaction: [Fe.sup.2+](M1) + [Mg.sup.2+](M2) = [Fe.sup.2+](M2) + [Mgs.up.2+](M1) (1) Because the samples were quenched from high temperature (1200-1275 [degrees]C) at 9.5 GPa, the temperature to which the measured ordering state corresponds remains problematic. However, a sample re-annealed at 1000 [degrees]C is more ordered than the original sample synthesized at 1275 [degrees]C, suggesting that high-temperature cation distributions in high-P clinopyroxene can be quenched in the multi-anvil experiments. Therefore, we conclude that the measured cation distributions correspond approximately to the conditions of the experiments. A small compositional dependence of cation ordering also exists, indicating subregular behavior across the binary join. A fit to the data yields [Mathematical Expression Omitted] = -11.9 (2.0) kJ/mol at 1275 [degrees]C for the exchange Reaction 1, which describes the ordering behavior on the octahedral sites in the clinopyroxenes. Comparison with data for orthopyroxene, which were obtained mostly at 1 atm, indicates that the state of ordering in orthopyroxene and high-P clinopyroxene is similar, especially if cation ordering in the high-P clinopyroxene has a small pressure dependence. The similarities are consistent with the observed structural similarities between the two polymorphs.

The activity-composition relation indicates a negative deviation from ideality in case of Tschermak's substituted Fe biotites on the join annite-siderophyllite. The experiments are conducted at 700 degree centigrade and 2 kbar to measure the Ann component and show that the fine-grained assemblage of Fe-Al biotites acts as a sliding scale buffer. Mg-Al and Fe-Al mixing in biotite shows positive deviations and Fe-Mg shows least deviation from ideality. However, this simplistic treatment in the experiments is insufficient to explain the natural biotite data.

Calcium is the slowest volume diffusing species in natural in hot-pressed calcite aggregates. The calcium grain boundary diffusion rates in natural and hot-pressed calcite aggregates are similar. Activation energy for calcium grain boundary diffusion is nearly 2/3 of the activation energy for volume diffusion. Calcium grain boundary diffusion rates are six times faster than the volume diffusion rates over the temperature range of the experiments.

The quenched cation ordering states of five orthopyroxene crystals have been determined from the marginal border group and the lower zone a and b of the Skaergaard intrusion. These data help in retrieving the closure temperatures of Fe-Mg ordering and cooling rates. Cooling rates strictly apply to temperatures around 340 to 400 degree celsius of the ordering states. The results of cooling rates agree with a thermal model computed using an average permeability of 10 to the -12th power square centimeters.

Equations of state and equations for activity coefficients were used to analyze the thermodynamic properties of aqueous species of the Al2O3-H2O-NaCl system at high temperatures and pressures. The solubility data between 40 and 350 degrees celsius produces the dissociation constant of NaAl(OH)(super 0)(sub 4). Further solubility measurements will better constrain the relative stabilities to reduce the discrepancies in the observations at temperatures above 200 degree celsius.

Two types of chromite deposits occur in the Hongguleleng ophiolite in Xinjiang, northwest China. One is located in the mantle sequence, the other occurs in the transition zone between the mantle sequence and layered cumulates. Abundant primary silicate inclusions such as phlogopite, pargasite, clinopyroxene, orthopyroxene, and olivine are found in the segregated chromite, but silicate inclusions occur only rarely in accessory chromite of the ulttamafic rocks from the transition zone and cumulates. These silicate phases are considered to have been entrapped as discrete and rare composite inclusions during magmatic precipitation of chromite rather than formed by postmagmatic entrapment. Phlogopite is the most abundant mineral found as inclusions in chromite in the Hongguleleng ophiolite. There are two types of substitutions for K in the phlogopite inclusions: (1) Na substitutes for K, and phlogophe shows a continuous range from almost pure sodium phlogopite to phlogopite; (2) Ca partially substitutes for K, resulting in the formation of Ca-bearing phlogopite. It is proposed mat the alkalic aqueous liquid (melt) responsible for the formation of the phlogopite inclusions was derived from the mixture of the K-rich aqueous liquid related to the subduction of the oceanic slab and the Na-rich aqueous liquid from the primary magma of the ophiolite. Two types of phlogopite hydrates, hydrate I and possibly a new hydrate (hydrate E[), occur as inclusions in the chromite and result from later hydrothermal processes. The fractures from brittle deformation provided passage for meteoric water to enter and react with the phlogopite inclusions. Compared with those in the transition zone, the inclusions of phlogopite and phlogopite hydrates in the mantle sequence are characterized by (1) smaller grain size and greater abundance, (2) undulatory extinction, (3) higher Si, Cr, Ni, and Ca, and (4) lower Ti and Al. These differences are possibly due to (1) P, T, and composition of chromite-precipitating magma, (2) subsolidus reequilibration with the host chromite, and (3) postmagmatic deformation and hydrothermal processes.

The Zr-bearing minerals baddeleyite (Zr[O.sub.2]) and zircon (ZrSi[O.sub.4]) occur within plagioclase-rich (61-95% plagioclase) cumulates of the Laramie anorthosite complex (LAC), southeastern Wyoming. In each of the examined samples, zircon is present as relatively coarse (1-2 mm) interstitial grains, and baddeleyite occurs as small (0.05 mm) inclusions within cumulus plagioclase. Zircon crystallized between cumulus plagioclase crystals near solidus temperatures from highly fractionated, Zr-saturated liquids. The resultant shape of zircon was controlled by the form of the remaining pore space. The origin of baddeleyite in the anorthositic rocks of the LAC is less well constrained. It may have crystallized early from the anorthositic parental magmas at relatively low silica activities; however, this would require baddeleyite saturation at extremely low Zr concentrations in the parental magmas (<< 100 ppm). Baddeleyite and zircon U-Pb ages reveal that several petrologically distinct intrusions were emplaced and crystallized in the LAC over a relatively restricted 1-3 m.y. interval at ca. 1434 Ma. The [sup.207]Pb/[sup.206]Pb ages obtained for the baddeleyite and zircon in each sample are identical within error ([+ or -] 1-3 m.y.), and U concentrations are uniformly low (

The occurrence of sector zoning in minerals of regional metamorphic or low-temperature intrusive origin implies that rapid growth is not required for the development of this particular form of homogeneous disequilibrium. It is shown here that sector zoning can be a natural consequence simply of slow lattice diffusion. Given anisotropic surface enrichrnent coupled with the low diffusivities typical of highly charged elements in refractory accessory minerals such as zircon and titanite, sector zoning can arise even in cases of growth rates as low as a few micrometers per million years. According to the proposed model, the development of sector zoning depends upon the competition between growth rate (V) and lattice diffusion ([D.sub.i]) within the near-surface layer (1), such that above a critical value of Vl/[D.sub.i] (~0.5-3), sector zoning is unavoidable in crystals that exhibit selective enrichment on some growth surfaces. Known diffusivities of rare earth elements in Zircon and titanite lead to the expectation of sector zoning in these minerals with respect to REEs for reasonable geologic growth rates. Even in the case of clinopyroxene, diffusion of REEs and high field-strength elements may be slow enough to contribute to the development of sector zoning in laboratory-grown crystals.

A study of the progress of the metamorphic reaction calcite + quartz resulting in wollastonite + CO2 in natural wollastonite-hosted fluid inclusions involves the knowledge of bulk fluid compositions and densities. The calculations use unaltered inclusions in quartz. The calcite + quartz formation occurs during retrogression, inside the wollastonite-hosted fluid inclusions. The reaction reversal is possible in the microstage. The study provides important petrogenetic information.

A thermodynamic dataset is obtained for the phases nepheline, jadeite, alpha-quartz, albite and monalbite by considering two reversed heterogeneous reactions of the rock-forming minerals in the system NaAlSiO4-SiO2. The refined dataset along with their uncertainties and correlation are used to form the NaAlSiO4-SiO2 phase diagram involving 2-sigma confidence interval for each P-T curve.

The hornblende barometer, in its current form, is not as sophisticated as other barometers calibrated for metamorphic rocks thus restricting its applications, since the temperature changes in the barometer lead to pressure reading variations. Temperature and fO2 parameters should be evaluated before application of the barometer. Lack of Tschermark-buffering assembly in the barometer is critical and chemical analysis or calculations should be done for the determination of the Fe3+ and Fe2+ of hornblende.

The kinds of mineral interfaces preserved in rock fragments of modern plutoniclastic sand are closely correlated with climate and topography in the source area. Stepwise linear-regression analysis of the relationship among environmental and petrographic parameters for sand collected from 16 drainages of varying morphology developed on five granodiorite plutons demonstrates that sand composition varies systematically with temperature and precipitation, and drainage steepness and area, for small drainages in temperate climates. The durability of each kind of mineral interface in sand-sized rock fragments derived from the plutons depends on mechanical, chemical, and textural factors. Interfaces are mechanically stressed by differential expansion of minerals during unroofing; interfaces between minerals of similar expansibility/compressibility are more likely to remain uncracked and thus survive weathering. The rate and extent of chemical dissolution of interfaces depends on the crystallography and chemistry of adjoining minerals; interfaces between minerals of similar structure that have short atomic-bond lengths are more likely to survive. Mineral inclusions are texturally isolated from rock-disaggregating processes and thus more likely to survive. Interfaces of opaque with tectosilicate minerals are enriched relative to other interfaces by progressive disaggregation, the rate and extent of which increase with increasing temperature. Interfaces of biotite with tectosilicates are depleted relative to other interfaces with increasing precipitation. Regression analysis yields statistically plausible, testable models of the dependence of modern sand composition on modern climate. Such models can be used to quantitatively estimate ancient mean annual air temperature and precipitation based on ancient sand composition.

Modern tropical peats require continuity of precipitation for accumulation, whereas calcretes and calcic vertisols require strongly seasonal conditions. The 20-30-m-thick cyclothems of the Sydney basin, Nova Scotia, Canada, show a systematic alternation of coals and other hydromorphic paleosols with calcretes and calcic vertisols. This implies strong variation in seasonality during the duration of a cyclothem, estimated at 200 ka. In at least one cyclothem, calcic paleosols formed on an interfluve adjacent to a paleovalley cut through marine strata, suggesting that a more seasonal and probably drier climate prevailed during sea-level lowstand. The calcic paleosols are estimated to have formed during periods of [10.sup.4] yr, possibly indicative of partial control by obliquity and precessional cycles.

The thermal effect in the southern portion of the Alta aureole is influenced by advective heat transfer and the fluid flow involved in this advection is lateral to the intrusive contact during prograde metamorphism. The silicon dolomites in the contact aureole of the Alta stock undergo progressive metamorphic sequence, which can be observed by the application of calcite + dolomite geothermometry.

Single-crystal X-ray diffraction and microprobe examination provide insights into the crystal chemistry of 1M phlogopite. Selective partitioning of high-change cations in the cis M2 regions is evident from the geometric and chemical characteristics of the octahedral designs. Elevation in high-change cation content is accompanied by distinct responses of the pair of octahedral regions.

Pedogenesis can be studied by analyzing the sequence of loess and interbedded paleosols of north-central China. The paleosols, which have accumulated over many years, can be differentiated from unaltered parent material zones by their higher low-field mass magnetic susceptibility.

The isothermal doubly ionized charged ferrous ion-magnesium disordering in the two orthopyrorene samples of Opx 4 and Opx 5 is investigated by single-crystal X-ray methods. The heating is implemented till the cation distribution attained an equilibrium state. The data from other researches on orthopyrorene crystals and synthetic end members agrees with the data obtained from the present investigations. An internal retardation on the disordering functions is provided by the different crystallographic variations.

A study on the viscosity and heat capacities of rhyolite and andesite glass and melt was conducted through relative enthalpy measurements between 400 and 800 Kelvin. The melts were used due to the relative simplicity of their chemical compositions. The results confirmed that the heat capacity and enthalpy of silicate glasses can be predicted using the oxide empirical coefficients. Heat capacities of the silicate melts were dependent on the melt composition and exhibited complicatedtemperatures due to the reaction between the aluminum and alkali elements at different temperatures.

The effect of increasing the concentration of Boron, Fluorine and Phophorus elements which usually occur as small amounts in granitic melts on the solubility of water wasdetermined by introducing F, B2O and P2O3 in an initially haplogranitic liquid 0.5,1.2 and 3 kb and 800, 850 and 900 degrees C. It was shown that increasing Band F concentrations increased water solubility in the granitic melt and was more remarkable at high pressure conditions. Addition had negligible effects inthe system. The results of this study were compared and found to be very different from the findings of another study investigation the Macusani melt.

Darken's quadratic formalism (DQF) expresses the thermodynamic behavior of amphiboles by mixing real and fictive end-members. DQF is used in modeling the activity-composition relationships of calcic amphibole, sodic amphibole, cummingtonite and orthoamphibole solid solutions. Comparison of a-x relationships with P-T-x conditions indicates the applicability of DQF in determining a-x relationships in multicomponnent amphiboles.

A study on the P1-I1 transition of anorthite having the same composition but with different states of A1,Si disorder is discussed. Transition behavior changes of anorthites subjected to high pressure are thermodynamically analyzed. Results show changes in both the transition character and the P1-I1 transition pressure accompanying A1,Si disorder. These changes are induced by a decrease in the length of the correlation of the state of A1,Si order.

Predominantly sand-sized spherules of K-feldspar with spherulitic, vesicular, and other crystalline textures help to define a unique marker bed in the 2.6 Ga Wittenoom Formation which persists for more than 300 km across the Hamersley Basin of Western Australia. These spherules usually occur as a minor constituent of a single turbidite, but locally they are restricted to centimeter-thick lenses of pure spherules along a single horizon in the argillite close beneath it. In the shallower northeastern part of the Basin, similar spherules (as well as larger, more heterogeneous particles) likewise occur at only one horizon, but here they are a minor constituent of a thick dolomitic debris-flow deposit. as the Wittenoom Formation and the Carawine Dolomite are thought to be stratigraphically equivalent, this debris-flow deposit is believed to be a proximal equivalent of the spherule-bearing turbidite. On the basis of their similarity to microtektites and microkrystites in shape, size, and mode of occurrence, the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the Hamersley Basin by a major bolide impact. Moreover, the mass of the spherules preserved in the Hamersley Basin is of the same order of magnitude as the estimated masses of microtektite glass in major Cenozoic strewn fields, despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller. The layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial. The flow, however, is not believed to have been a direct result of the proposed impact. If the spherule-bearing layers constitute a chronostratigraphic marker, as suggested here, there are important implications for the deposition of carbonates and banded iron formations (BIF) within the Hamersley Basin. Similar spherules also occur at a single horizon [is congruent to] 250 m higher stratigraphically in the Brockman Iron Formation, suggesting that two impacts occurred near the Hamersley Basin in relatively rapid succession. Correlative strata may be present in the Transvaal Basin of South Africa.

Phengitic muscovites undergoing thermal decomposition due to contact with a metamorphic aureole at low-pressure were examined using transmission electron microscopy to study the various aspects of the breakdown process. Partial breakdown of muscovites yields a combination of cordierite, spinel, sillimanite, mullite, potassium-feldspar, biotite and phengite. Various regions within individual phengite grains undergoing partial decomposition show gradation in reaction and different end-products.

Petrographic analysis of the mass transfer of major elements in the evolution of monzogranite to basic feldspar granite and stable isotope examination of fluids altering rock composition and depositing ores were undertaken to evaluate the course of interaction between fluids and rocks in the Las Chacras batholith. Results indicate that the addition of potassium and sodium to and the loss of calcium and strontium from biotite-containing monzogranite produced fenite. Further, perthite had replaced plagioclase and microcline. Biotite was also converted into aggregates of various minerals.

The evolution of basalts at atmospheric pressure was investigated in experiments performed on the low-temperature fields of liquid + anorthite + augite + olivine (liq + an + aug + ol) assemblages. Together with a previous work on high-temperature fields, the divariant field of liq + an + aug + ol can be contoured and some of its boundaries demarcated. The boundary of the liq + an + aug + ol assemblage is composed of six segments with one additional phase. The stability of the assemblage and the composition of phases are dependent on temperature and the silica content of the melt.

A residual-source model for the origin of A-type granites is assessed by examining the likely mineral content and geochemistry of the residue remaining from generation of an I-type granite. Although this model may explain some characteristics of A-type granite, available data suggest that a residual source is unlikely to generate a partial melt with the appropriate major element characteristics. An alternative model for the origin of some A-type granites involves partial melting of crustal igneous rocks of tonalitic to granodioritic composition. Modeling the partial melting of these source rocks suggests that partial melts with water contents appropriate for A-type granites may be generated by 15% to 40% melting. This model can predict many other characteristics of A-type granites.

Dolomite, CaMg[([CO.sub.3]).sub.2], is found in much greater abundance in ancient rocks than in modern ones, but why this is true has been one of geology's enduring mysteries. Part of this [...]

Next to volcanoes or earthquakes, mudstones are hardly a glamorous subject for geologists. But these widespread strata are an important source of hydrocarbons that migrate into petroleum deposits, and they [...]