ABSTRACT Dissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries. Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins. Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation. Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large‐scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one‐way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure. Deformation partitioning‐controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well preserved.
ABSTRACT Holocene reef development was investigated by coring on Britomart Reef, a mid‐shelf reef, 23 km long and 8 km wide situated 120 km north of Townsville in the central Great Barrier Reef (GBR). Two holes were drilled, Britomart 1 on a lagoon patch reef, and Britomart 2 on the windward reef crest. The Holocene reef (25·5 m) is the thickest yet recorded in the GBR and overlies an uneven substrate of weathered Pleistocene limestone. Mineralogical and geochemical analyses show that magnesian calcite and aragonite were converted to low Mg‐calcite below the Holocene‐Pleistocene disconformity. Corals above the interface have 7500–8500 ppm Sr, but 1650–1500 ppm just below it, decreasing to 400–800 ppm downwards. The intermediate Sr values could be due to partial replacement of aragonite by calcite or higher original Sr content in the corals. Three units are recognized in the Holocene: (1) coral boundstone unit, (2) coral framestone unit, and (3) coral rudstone unit. The coral boundstone unit forms the top 5 m of both cores and is algal‐bound coral rubble similar to the present reef top. The coral framestone unit is composed of massive head corals Diploastrea heliopora and Porites sp., and is currently forming in patch reefs situated in the lagoon and along the reef front. The coral rudstone unit comprises coral rudstone and floatstone with unabraded, and unbound, coral clasts in muddy matrix. This matrix may be up to 30% sponge chips. Radiocarbon dating indicates the reef grew more rapidly under the lagoon than under the reef front from 7000 to 5000 yr BP. The rate of reef growth matched existing estimates of sea‐level rise, but lagged approximately 1000 years (5–10 m) behind it. Most of the reef mass accumulated between 8500 and 5000 yr BP as a mound of debris, perhaps stabilized by seagrasses or algae. Accretion of the reef top in a windward direction between 5000 and 3000 yr BP created the present, steep reef‐front profile.
Abstract High-resolution transmission electron microscopy of biogenic carbonate minerals is hampered by a lack of stability during exposure to the electron beam. However, aragonite in bivalve shells may be successfully imaged using a modification of the ‘minimal-exposure technique’ of Williams and Fisher (1970). Diffraction patterns taken before and after beam exposure indicate that the aragonite remained stable during imaging. The procedure described here should prove useful for further studies of the ultrastructure and/or the diagenesis of biogenic carbonate minerals.
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Abstract Lower than anticipated gas production rates in coal bed methane (CBM) operations are sometimes related to formation damage in which reduced permeability results from the interaction of bore and fracturing fluids with coal. This study assesses the potential for mineral precipitation to cause formation damage from a Permian coal seam in the Bowen Basin, eastern Australia, using geochemical modelling of coal mineralogy, formation fluid and bore fluid composition. The mineralogical composition of coal was assessed using petrography, X-ray diffraction, X-ray fluoresence and electron beam microanalysis. Geochemical modelling of ambient groundwater and drilling fluid interactions with coal samples was undertaken using Geochemist's WorkBench (GWB). This modelling indicates that likely mineral precipitates/re-precipitates to adversely impact porosity and permeability include a range of clay, carbonate and sulphate minerals. Additionally, these interactions may induce alteration of precursor smectites to new species that reduce permeability. The resultant smectites also have a high propensity for expansion and dispersion in the presence of inappropriate drilling fluids. Precipitation, expansion and dispersion of these fine-grained minerals may potentially lead to large reductions in permeability, with profound impacts upon gas flow. Indications are that reduced permeability can be mitigated by suitable chemical matching of groundwaters with drilling fluids.
Abstract Endoskarns formed where a swarm of diorite dykes have intruded calcite marble at Redcap Creek include an inner melilite-dominated, a wollastonite-dominated, and an outer massive tilleyite zone in contact with marble. The massive tilleyite is unusual in that it is coarse-grained (prisms 2–15 cm in length) and its deep grey colour contrasts with the lighter coloured varieties described elsewhere. A chemical analysis gives a formula close to ideal, with only minor substitution of Al, Ti, and Mg. Refined unit cell parameters are in close agreement with those quoted in the literature. The skarns have clearly formed by transport of Si, Mg, Fe, Al, and Ti from the igneous rocks, and Ca in the reverse direction from the marble. Activity diagrams derived from experimental data are most useful in interpreting the zonal sequence of endoskarns, and preliminary results suggest mass transfer at low X co 2 and temperature of the order of 800°C or higher for the formation of the massive tilleyite.
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