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The mineralogy of terrestrial planets evolves as a consequence of a range of physical, chemical, and biological processes. In pre-stellar molecular clouds, widely dispersed microscopic dust particles contain approximately a dozen refractory minerals that represent the starting point of planetary mineral evolution. Gravitational clumping into a protoplanetary disk, star formation, and the resultant heating in the stellar nebula produce primary refractory constituents of chondritic meteorites, including chondrules and calcium-aluminum inclusions, with ~60 different mineral phases. Subsequent aqueous and thermal alteration of chondrites, asteroidal accretion and differentiation, and the consequent formation of achondrites results in a mineralogical repertoire limited to ~250 different minerals found in unweathered meteorite samples. Following planetary accretion and differentiation, the initial mineral evolution of Earth's crust depended on a sequence of geochemical and petrologic processes, including volcanism and degassing, fractional crystallization, crystal settling, assimilation reactions, regional and contact metamorphism, plate tectonics, and associated large-scale fluid-rock interactions. These processes produced the first continents with their associated granitoids and pegmatites, hydrothermal ore deposits, metamorphic terrains, evaporites, and zones of surface weathering, and resulted in an estimated 1500 different mineral species. According to some origin-of-life scenarios, a planet must progress through at least some of these stages of chemical processing as a prerequisite for life. Biological processes began to affect Earth's surface mineralogy by the Eoarchean Era (~3.85-3.6 Ga), when large-scale surface mineral deposits, including banded iron formations, were precipitated under the influences of changing atmospheric and ocean chemistry. The Paleoproterozoic 'Great Oxidation Event' (~2.2 to 2.0 Ga), when atmospheric oxygen may have risen to > 1% of modern levels, and the Neoproterozoic increase in atmospheric oxygen, which followed several major glaciation events, ultimately gave rise to multicellular life and skeletal biomineralization and irreversibly transformed Earth's surface mineralogy. Biochemical processes may thus be responsible, directly or indirectly, for most of Earth's 4300 known mineral species. The stages of mineral evolution arise from three primary mechanisms: (1) the progressive separation and concentration of the elements from their original relatively uniform distribution in the pre-solar nebula; (2) an increase in range of intensive variables such as pressure, temperature, and the activities of [H.sub.2]O, C[O.sub.2], and [O.sub.2]; and (3) the generation of far-from-equilibrium conditions by living systems. The sequential evolution of Earth's mineralogy from chondritic simplicity to Phanerozoic complexity introduces the dimension of geologic time to mineralogy and thus provides a dynamic alternate approach to framing, and to teaching, the mineral sciences. Keywords: Pre-solar minerals, meteorite minerals, biominerals, organominerals, teaching mineralogy

Veins and adjacent alteration selvages in the Waits River Formation were investigated to determine whether associated mass transfer was due primarily to large-scale advection or small-scale diffusion. Samples of the vein, selvage, and adjacent wall rock were collected from the earliest and most numerous generation of veins in pelite and carbonate hosts from outcrops in the chlorite and the kyanite zones. Bulk compositions of selvages and unaltered wall rocks were compared using a reference frame defined by a combination of Zr, Ti, REEs, and U. Selvages from both outcrops typically exhibit losses of Si, K, Ba, and Rb relative to unaltered wall rock. Kyanite zone selvages show losses in Mg and Cs in addition. Differences in K, Ba, Rb, Cs, and Mg between the vein-selvage system as a whole and adjacent unaltered wall rock are possibly accounted for by development of micas in veins. The addition of Si and Ca to veins is not balanced by removal of Si and Ca from the selvages. Vein-selvage systems contain more Si than wall rock, with an overall addition of [approximately equal to] 40 mg Si/[cm.sup.3] to the chlorite zone and [approximately equal to] 55 mg Si/[cm.sup.3] to the kyanite zone. Mass balance of Si at the outcrop scale requires that >90% of the Si in quartz veins was derived externally. Quartz veins studied formed primarily by fluid flow and large-scale advective mass transfer with a relatively minor component of local mass transport by diffusion. The estimated time-integrated fluid flux necessary to produce the observed amount of quartz in veins in an entire outcrop is [approximately equal to]2-6*[10.sup.6] [cm.sup.3] fluid/[cm.sup.2] rock. Mineral inclusions in garnet and fracturing of garnets adjacent to veins indicate that formation of selvages and veins initiated prior to formation of garnet and continued after the end of garnet growth. Keywords: Mass transfer, regional metamorphism, veins, fluid flow

Infiltration-driven decarbonation reactions produced biotite in pelites and psammites but not in micaceous carbonate rocks in the biotite zone during metamorphism. In one large exposure, the amount of biotite varies greatly in rocks that retain reactants of biotite-forming reactions (0.01-1.10 mol/L in pelites; 0.15-0.53 mol/L in psammites; 0 in carbonates). Conventionally, the variations are interpreted as reactive fluid flow in chemically isolated channels that correspond to individual metasedimentary layers. Measured values of In[Q.sub.s] for the equilibria: K[Al.sub.3][Si.sub.3][O.sub.10][(OH).sub.2] + 3 CaMg[(C[O.sub.3]).sub.2] + 2 Si[O.sub.2] = K[Mg.sub.3]Al[Si.sub.3][O.sub.10][(OH).sub.2] + Ca[Al.sub.2][Si.sub.2][O.sub.8] + 2 CaC[O.sub.3] + 4 C[O.sub.2] and 3 K[Al.sub.3][Si.sub.3][O.sub.10[(OH).sub.2] + [Mg.sub.5][Al.sub.2][Si.sub.3][O.sub.10][(OH).sub.8] + 4 CaMg[(C[O.sub.3]).sub.2] + 5 Si[O.sub.2] = 3 K[Mg.sub.3]Al[Si.sub.3][O.sub.10][(OH).sub.2] + 4 Ca[Al.sub.2][Si.sub.2][O.sub.8] + 8 C[O.sub.2] + 4 [H.sub.2]O are uniform within error of measurement in all analyzed samples (-1.67 [+ or -] 0.08, MSWD = 0.89, and -4.17 [+ or -] 0.26, MSWD = 0.52, respectively). The uniformity in [Q.sub.s] values is explained by homogenization of the activities of C[O.sub.2] and [H.sub.2]O by diffusion and dispersion over a distance >70 m across layering during metamorphism. The uniformity in [Q.sub.s] is inconsistent with the conventional interpretation of differences in the abundance of biotite. The differences are better explained in terms of layer-by-layer variations in the amounts and compositions of minerals prior to reaction. Measured progress of the biotite-forming reactions is reproduced by mass-balance calculations that consider different initial amounts and compositions of reactant minerals in different samples but the same or nearly the same value of [Q.sub.s] at the end of reaction. Mass transport of volatiles both by advection and by diffusion and dispersion are essential controls on the progress of infiltration-driven reactions. Keywords: Fluid flow, diffusion, dispersion, biotite, regional metamorphism, Gile Mountain Formation, Vermont, reaction progress

The [X.sub.C[O.sub.2]] recorded by mineral-fluid equilibria in contact metamorphosed siliceous carbonates commonly defines two groups of rocks in the same aureole. One group records relatively low [X.sub.C[O.sub.2]], that results from infiltration of chemically reactive [H.sub.2]O-rich fluid. The other records relatively high [X.sub.C[O.sub.2]], up to 0.99, that results from decarbonation reactions with little or no infiltration. A complementary dichotomy in apatite compositions exists in five contact aureoles in Italy, Scotland, and the U.S.A. Apatite in the low-[X.sub.C[O.sub.2]] group is close to an F-OH solid solution. Apatite in the high-[X.sub.C[O.sub.2]] group is a relatively Cl-rich Cl-F--OH solution. The halogen content of fluid coexisting with analyzed apatite was characterized in two aureoles to determine the origin and significance of the dichotomy in apatite composition. Calculated [a.sub.HF]/[[a.sub.[H.sub.2].sub.O], [a.sub.HF]/[a.sub.HCl], [a.sub.HF], and [m.sub.[F.sub.T]] (the total F molality of fluid) are systematically higher in fluid coexisting with the low-[X.sub.[co.sub.2]] group. In contrast, [a.sub.HCl]/[[a.sub.[H.sub.2].sub.O] in the high-[X.sub.C[O.sub.2]] group may be higher than or overlap with [[a.sub.HCl]/[a.sub.H].sub.2].sub.O] in the low-[X.sub.C[O.sub.2]] group. Calculated [a.sub.HCl] and [m.sub.Cl].sub.T] in the high-[X.sub.[co.sub.2]] group are lower than or overlap with [a.sub.HCl] and [m.sub.Cl].sub.T] in the low-[X.sub.[co.sub.2]] group. The Cl-rich apatites in the high-[X.sub.C[O.sub.2]] group are explained by crystallization at relatively low [[a.sub.[H.sub.2].sub.O], any, and [m.sub.F].sub.T]] rather than at high [a.sub.HCl] or [m.sub.Cl].sub.T]]. In comparison, the F-OH apatites in the low-[X.sub.[co.sub.2]] group formed by infiltration of rock by and equilibration with relatively [H.sub.2]O-rich, high [m.sub.F].sub.T]]/[m.sub.Cl].sub.T]] fluid, reflecting the same metasomatic process responsible for F-rich humite-group minerals and skarns in many contact aureoles. Calculated halogen contents indicate that the non-C[O.sub.2] fraction of fluid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive [H.sub.2]O- and F-rich fluid that infiltrated the low-[X.sub.C[O.sub.2]] group had a plutonic source.

The P-T conditions of both equilibria were determined precisely by reversal experiments in a piston-cylinder apparatus. On the basis of 8 experiments, brackets for the rutile-magnesite-geikielite equilibrium are 7.0-7.1 kbar at 800 [degrees]C, 8.6-8.7 kbar at 850[degrees]C, and 10.5-10.7 kbar at 900[degrees]C. On the basis of 9 experiments, brackets for the zircon-magnesite-baddeleyite-forsterite equilibrium are 7.1-7.7 kbar at 800[degrees]C, 9.2-9.4 kbar at 850[degrees]C, and 10.7-10.9 kbar at 900[degrees]C. Considering experimental uncertainties in P ([+ or -]300 bars) and T ([+ or -]3[degrees]C), equilibrium curves calculated from both the Berman and the Holland and Powell databases pass through all brackets. Molar Gibbs free energy of formation from the elements at 1 bar and 298 K for geikielite and zircon, derived from the experiments and consistent with the Berman database, are -1481.94 [+ or -] 0.67 kJ and -1917.54 [+ or -] 1.25 kJ, respectively. Corresponding values consistent with the Holland and Powell database are -1479.30 [+ or -] 0.74 kJ and -1918.47 [+ or -] 1.49 kJ. Application of the two equilibria indicate that: (1) the mole fraction of C[O.sub.2] in fluid was 0.54-1.00 when geikielite and baddeleyite formed during contact metamorphism of siliceous dolomites in the Ballachulish aureole, Scotland; (2) the activity of C[O.sub.2] could have been as low as 2 [10.sup.-5] during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of C[O.sub.2] was

Inverse calculations reveal the three-dimensional geometry of time-integrated fluid flux over a 120 [km.sup.2] area during peak Barrovian regional metamorphism in southeastern Vermont. Prograde changes in whole-rock C[O.sub.2], [sup.18]O, and [sup.13]C and calculated fluid compositions at the peak of metamorphism were inverted assuming tracer mass balance to obtain the time-integrated fluid flux in three dimensions. Peak metamorphic fluid flow was spatially nonuniform with flux magnitudes ranging from ~0 to 3*[10.sup.5] mol fluid/[cm.sup.2] rock and flux directions ranging from vertical (upward and downward) to horizontal. Averaged over the entire study area, the magnitude of the time-integrated metamorphic fluid flux vector is ~3.4*[10.sup.4] mol fluid/[cm.sup.2] rock. The average flux vector trends 45[degrees] to the southwest and points upward at 36[degrees] from the present horizontal, parallel to formation boundaries on a regional scale. Fluids in the terrain carried ~3*[10.sup.3] mol C[O.sup.2]/[cm.sup.2] rock toward Earth's surface during the peak of metamorphism. Results suggest that local cross-layer transport processes are secondary to terrain-scale metamorphic fluid flow in driving prograde decarbonation reactions. Regional structure exerts a first-order control on the gross geometry of peak metamorphic fluid flow. Keywords: metamorphism, fluid flow, inverse problem, C[O.sup.2], [[delta].sup.18]O, [[delta].sup.13]C.

Isograds were mapped in siliceous dolomites from the Ballachulish aureole on the basis of the formation of geikielite (MgTi[O.sub.3]), baddeleyite (Zr[O.sub.2]), and qandilite ([Mg.sub.2]Ti[O.sub.4]) by the following model reactions: rutile + dolomite = geikielite + calcite + C[O.sub.2], zircon + 2 dolomite = baddeleyite + forsterite + 2 calcite + 2 C[O.sub.2], and geikielite + periclase = qandilite. The (T, [X.sub.C[O.sub.2]]) conditions of reaction inferred from (1) mineral equilibria in pelitic rocks, (2) calcite + dolomite thermometry, and (3) the diopside + dolomite + forsterite + calcite + tremolite and dolomite + periclase + calcite equilibria are: geikielite isograd (640-655 [degrees] C, 0.76-0.80); baddeleyite isograd (660-710 [degrees] C, 0.76-0.95); qandilite isograd (725-755 [degrees] C, < 0.08). These T-[.sub.C[O.sub.2]] conditions for the geikielite and baddeleyite isograds are the same within error of those independently estimated from rutile + dolomite + geikielite + calcite and zircon + dolomite + baddeleyite + forsterite + calcite equilibria using Berman's thermodynamic data base. Rutile, zircon, geikielite, and baddeleyite are common in dolomites from at least two other contact aureoles in Scotland and Montana. Although the minerals occur in concentrations < 0.01%, they appear to have been in local equilibrium during metamorphism both with each other and with coexisting carbonates and silicates and are potentially useful but currently unexploited records of the physical conditions of metamorphism.

The extent and direction of fluid flow during Barrovian-type regional metamorphism in east-central Vermont and during Buchan-type regional metamorphism in south-central Maine are determined from the modal, whole-rock and mineral chemical data of rocks collected from these two regions. The rocks consist of 266 samples of micaceous sandstone, micaceous limestone and pelitic schist. The data indicates that metamorphosis occurred in Vermont at a pressure of 7 to 8 kilobars and at 3.5 kilobars in Maine.

Examination of seven case studies of contact metamorphism of siliceous dolomitic limestones shows that less than 14% porosity leads to fluid flow in all cases. The geometry of the flow is different for different cases. Three types of contact metamorphisms and fluid-rock interactions are explained. The temperature, composition of rock and fluid, pressure and amount and the direction of fluid flow regulate the mineral assemblages in metacarbonate rocks.

Models were developed to simulate contact metamorphism and fluid-rock interaction effects on calcareous argillite mineralogical and O isotopic evolution. A model employing metamorphism with flow of specified time-integrated flux provided a successful simulation of the main petrologic and isotopic features of the aureole. The results show how isotopic and petrological data for contact aureoles can be utilized in providing quantitative constraints on the geometry and magnitude of metamorphic flow.

A general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time-integrated fluid flux. The model is applied to regional metamorphism, quartz veins, and a metasomatized ductile fault zone.

Ferry's model for studying fluid flow during contact metamorphism of siliceous dolomitic limestones differs from R. Brooks Hanson's suggestion regarding the importance of heat transfer, and energy balance over mass balance. Ferry's model considers mineral assemblages on an equal temperature basis. Energy conservation is irrelevant since its value is consistent with the observed assemblages, inferred temperature, and time-integrated flux. Petrologic and chemical information on fluid-rock interaction are essential for evaluating Ferry's model.

Technique based on inverse methods for directly inverting mapped patterns of whole-rock chemical alteration finding magnitude and three-dimensional (3D) geometry of the associated reactive fluid flow is discussed.

The geologic carbon cycle plays a fundamental role in controlling Earth's climate and habitability. For billions of years, stabilizing feedbacks inherent in the cycle have maintained a surface environment that could sustain life. Carbonation/decarbonation reactions are the primary mechanisms for transferring carbon between the solid Earth and the ocean–atmosphere system. These processes can be broadly represented by the reaction: CaSiO 3 (wollastonite) + CO 2 (gas) ↔ CaCO 3 (calcite) + SiO 2 (quartz) . $\text{CaSi}{{\text{O}}_{\text{3}\,\,\left(\text{wollastonite} \right)}}+\text{C}{{\text{O}}_{\text{2}\,\,\left(\text{gas} \right)}}\leftrightarrow \text{CaC}{{\text{O}}_{\text{3}\,\,\left(\text{calcite} \right)}}+\text{Si}{{\text{O}}_{\text{2}\,\,\left(\text{quartz} \right)}}.$ This class of reactions is therefore critical to Earth's past and future habitability. Here, we summarize their significance as part of the Deep Carbon Obsevatory's "Earth in Five Reactions" project. In the forward direction, carbonation reactions like the one above describe silicate weathering and carbonate formation on Earth's surface. Recent work aims to resolve the balance between silicate weathering in terrestrial and marine settings both in the modern Earth system and through Earth's history. Rocks may also undergo carbonation reactions at high temperatures in the ultramafic mantle wedge of a subduction zone or during retrograde regional metamorphism. In the reverse direction, the reaction above represents various prograde metamorphic decarbonation processes that can occur in continental collisions, rift zones, subduction zones, and in aureoles around magmatic systems. We summarize the fluxes and uncertainties of major carbonation/decarbonation reactions and review the key feedback mechanisms that are likely to have stabilized atmospheric CO2 levels. Future work on planetary habitability and Earth's past and future climate will rely on an enhanced understanding of the long-term carbon cycle. [ABSTRACT FROM AUTHOR]

Forsterite in metamorphosed siliceous dolomites commonly contains calcite inclusions that are significantly more magnesian than even the most-magnesian calcite grains in the matrix of the same rock. Calcite inclusions in forsterite from dolomite-bearing rocks therefore have an unexploited potential for providing more-accurate estimates of the peak T of metamorphism than do matrix calcite grains. Some consequences for estimating fluid pressure and for the development of models of heat and fluid transport during contact metamorphism are reviewed. Measured compositions of calcite inclusions in forsterite from five contact aureoles indicate that significant growth of forsterite grains continues over a 50-200 [degrees] C range in T after the initial nucleation and growth of the mineral during heterogeneous mineral-fluid reaction. The continued growth may occur by an annealing mechanism driven by reduction in interfacial free energy.

Patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism. Three such patterns are reviewed. First, trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds. Examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites. Recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the P-T conditions of those reactions. Second, the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral-fluid equilibrium. Examples include the distribution of retrograde calcite, quartz, and tremolite in siliceous limestones and of retrograde tremolite, dolomite, brucite, and serpentine in siliceous dolomites from contact aureoles. Among other things, application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool. Third, pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks. The distribution may be explained by the effect of 'force of crystallization.' The pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes.

Research over the last four decades on carbonate rocks of the Waterville limestone, Maine, U.S.A., has contributed to the development of both concepts and methodologies for understanding fluid-rock interaction during regional metamorphism, including: (1) buffering of fluid composition by mineral reactions, (2) infiltration of carbonate rocks by aqueous fluids, (3) petrologic fluid-rock ratios, (4) infiltration-driven metamorphism, (5) one- and two-dimensional continuum models for coupled fluid flow and mineral reaction, (6) time-integrated fluid fluxes, and (7) channelized, horizontal fluid flow in the direction of increasing temperature within chemically isolated layers. Disagreement between the last concept and both hydrodynamic models for metamorphic fluid flow and empirical evidence for homogenization of fluid composition at a scale much larger than layer thickness motivated development of the latest models for coupled fluid flow and mineral reaction in the Waterville limestone. The new models consider a flow medium composed of layers that differ in the initial amounts and compositions of minerals, both horizontal flow in the direction of increasing temperature and vertical flow in the direction of decreasing pressure and temperature, significant but imperfect homogenization of fluid composition across layering by CO2-H2O interdiffusion, and infiltration by fluids that are spatially variable in composition on the kilometer scale with CO2 content increasing with increasing grade of metamorphism. The new models reproduce measured progress of the biotite-forming reaction in the Waterville limestone over a range of spatial scales spanning six orders of magnitude, from differences in reaction progress of up to a factor of ~100 between adjacent centimeter-thick layers to the coexistence of mineral reactants and products over a distance of ~13 km parallel to the metamorphic field gradient. Results imply that the present spatial distributions of reaction progress represent a steady state achieved when rocks closely approached equilibrium with the infiltrating fluid during metamorphism. The new models resolve what for many years appeared to be fundamental discrepancies among petrologic data for regionally metamorphosed carbonate rocks, hydrodynamic models of regional metamorphism, and the length scales of mass transport of volatiles across layering by diffusion. [ABSTRACT FROM AUTHOR]

Ankerite grains with dolomite cores occur in marls, pelites, and psammites from a Buchan terrain in Maine and a Barrovian terrain in Vermont (U.S.A.). Dolomite cores are typically ≤20 µm in diameter, have sharp but irregular contacts with ankerite, and have the same crystallographic orientation as ankerite rims. Ankerite grains with dolomite cores are common in the chlorite zone, less abundant in the biotite and garnet zones, and rare (Vermont) or absent (Maine) at higher grades. The texture and crystallographic orientation of dolomite and ankerite and the sharpness of the dolomite-ankerite contact are consistent with partial replacement of detrital dolomite by ankerite by solution-reprecipitation. Metamorphic biotite is in Fe-Mg exchange equilibrium with ankerite rims but not with dolomite cores, implying that ankerite did not form long after biotite (biotite has no phlogopite cores). Possible sources of iron for the formation of ankerite are reduction of ferric iron hydroxide or the smectite-to-illite reaction during diagenesis. The sharpness of the dolomite-ankerite contact is a diffusion chronometer that constrains timescales of metamorphic process. Relatively low spatial resolution analyses of Fe/Mg across the contact with a NanoSIMS instrument and a FEG TEM give upper bounds on the thickness of the transition from ankerite to dolomite of ~2 and ~0.5 µm, respectively. Higher resolution analysis of BSE grayscale contrast with a FEG SEM gives a thickness ~100 nm. Fit of the grayscale profile to a model of one-dimensional diffusion across an infinite plane gives Dt = 10-15 m² (± a factor of 5), where D is the effective Fe-Mg interdiffusion coefficient and t is the duration of diffusion. Using the published experimental determination of D, upper bounds on the residence time of ankerite grains with dolomite cores at peak T = 400-500 °C, on the duration of linear cooling from peak T to 100 °C, and on the duration of linear heating from 100 °C to peak T followed by linear cooling to 100 °C are all <1 yr. For linear heating and cooling lasting 106 years, peak T could not have been >100 °C. The question is what explains the occurrence of ultrasteep composition gradients between dolomite and ankerite. Regional metamorphism on a timescale of a year or less is unrealistic. No barrier to diffusion at the dolomite-ankerite contact was observed in TEM images. Post-metamorphic formation of ankerite at very low temperature is ruled out by Fe-Mg exchange equilibrium between biotite and ankerite but not dolomite. It is unlikely that the steep composition gradients were preserved by intracrystalline pressure gradients. Alternatively, the steep composition gradients would be consistent with timescales of metamorphic process ~106 years or longer if D values during metamorphism were approximately six orders of magnitude or more smaller than those measured in the laboratory. The error of measurement is much less, approximately ± a factor of 2. A correction to D for the difference in P between measurements (0.1 MPa) and metamorphism (350-800 MPa) is likely an order of magnitude or less. Oxygen activity (...), however, was 17-20 orders of magnitude larger during the laboratory measurements than during metamorphism. A correction to measured D for the difference in ... between experiment and metamorphism appears to be the likeliest way to reconcile the steep composition gradients with realistic timescales of metamorphism. Before ankerite grains with dolomite cores are fully realized as a useful diffusion chronometer for low- and medium-grade metamorphic rocks, the rates of Fe-Mg interdiffusion in ankerite and dolomite need to be calibrated as a function of .... [ABSTRACT FROM AUTHOR]

The oxygen isotope compositions of calcite, diopside, dolomite, forsterite, garnet, K-feldspar, kyanite, plagioclase, quartz, and wollastonite were analyzed in suites of contact and regional metamorphic rocks using an ion microprobe. Spatial resolution was ∼10 μm. Precision, measured as the standard deviation of working standards averaged over the entire project, was 0.13–0.18‰ for three carbonate standards and 0.11–0.12‰ for two silicate standards. A total of 1176 analyses (excluding standards) were made of 73 minerals in 23 samples. Both intercrystalline and intracrystalline variability in δ 18 O is greater in contact than in regional metamorphic rocks. Of 27 minerals analyzed in contact metamorphosed rocks, 70% exhibit statistically significant grain-to-grain variability in δ 18 O over areas ⩽1.41 cm 2 with the largest range in silicates and carbonates in a single sample of 7.4‰ (forsterite) and 10.6‰ (dolomite). Of 88 grains analyzed in two or more places in contact metamorphosed rocks, 32% exhibit statistically significant intracrystalline variability in δ 18 O with the largest range in a single silicate and carbonate grain of 3.1‰ (forsterite) and 10.1‰ (dolomite). In contrast, 44% of 45 minerals in regional metamorphic rocks exhibit significant grain-to-grain variability in δ 18 O over areas ⩽1.17 cm 2 with the largest range in silicates and carbonates in a single sample of only 1.1‰ (plagioclase) and 0.9‰ (calcite). Only 6% of 144 grains analyzed in two or more places in regional metamorphic rocks exhibit significant intracrystalline variability in δ 18 O with the largest range in a single silicate and carbonate grain of only 1.5‰ (diopside) and 0.7‰ (calcite). The difference in intercrystalline and intracrystalline variability in δ 18 O between contact and region metamorphic rocks is explained by the longer duration and slower reaction rates of regional metamorphism rather than to differences in temperature. There is no significant difference in intercrystalline and intracrystalline variability in δ 18 O in regional metamorphic rocks among samples from the biotite, garnet, and kyanite zones. Calcite inclusions in forsterite, and calcite and quartz inclusions in garnet either have δ 18 O that is statistically indistinguishable from δ 18 O of the same mineral occurring as nearby matrix grains or have statistically significant lower δ 18 O. No reversed isotope fractionations were measured between coexisting mineral pairs. Minerals in individual samples, however, exhibit a wide range in the degree to which they attained and preserve oxygen isotope fractionations consistent with metamorphic temperatures recorded by mineral equilibria. Processes that account for grain-scale departures from isotope exchange equilibrium include: (a) overstepping of prograde mineral reactions, (b) growth zoning in low-diffusivity minerals, (c) interaction of rocks with fluids at the peak of metamorphism and/or during cooling, (d) retrograde mineral reactions, and (e) closed-system isotope exchange between coexisting minerals during cooling. This study provides new information about (1) the degree to which a variety of textural changes experienced by rocks during metamorphism are associated with changes in δ 18 O, (2) oxygen isotope homogenization at the outcrop scale among contrasting lithologies, (3) changes in δ 18 O with increasing grade of regional metamorphism, and (4) time scales of metamorphic process. [ABSTRACT FROM AUTHOR]

Progress in understanding mineral evolution, Earth's changing near-surface mineralogy through time, depends on the availability of detailed information on mineral localities of known ages and geologic settings. A comprehensive database including this information, employing the mindat.org web site as a platform, is now being implemented. This resource will incorporate software to correlate a range of mineral occurrences and properties vs. time, and it will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical records of important geophysical, geochemical, and biological events in Earth history. [ABSTRACT FROM AUTHOR]

Replacement dolomite in the Latemar carbonate buildup developed when limestone was infiltrated by reactive fluid. Minor-element, trace-element, and oxygen and carbon isotope compositions of dolomite and precursor limestone constrain the origin of the fluid and fundamental aspects of the flow. Inferred salinity (similar to seawater); temperature (45°- 85°C); 87Sr/86Sr (0.7076 - 0.7079); Ca/Mg (<1.4); and Fe, Mn, and Zn content of the dolomitizing fluid are consistent with a fluid similar to modem diffuse effluent. Modem diffuse effluent itself is approximately a mixture of seawater with up to ̃25 percent high-temperature mid-ocean ridge hydrothennal vent fluid. Time-integrated fluid flux was in the range (2-4) · 106 mol fluid/cm2 rock or (4-7) · 107 cm3 fluid/cm2rock. Estimation of time-integrated flux leads to an internally consistent framework for the appropriate interpretation of the oxygen, strontium, and carbon isotope compositions of replacement dolomite. The oxygen and strontium isotope compositions reflect equilibration with dolomitizing fluid and provide a chemical fingerprint of the fluid. The carbon isotope composition of dolomite, however, was simply inherited directly from the precursor limestone in nearly all cases. A quantitative evaluation of the minor- and trace-element budget of dolomitization verifies that a fluid like modern diffuse effluent, but not unmodified seawater, could supply sufficient Fe, Mn, and Zn to enrich dolomite in these elements compared to limestone. If the flux of dolomitizing fluid was similar to that of modern diffuse effluent, ̃0.02 cm3/cm2 · s, the duration of fluid flow and mineral-fluid reaction was short, ̃100 years. The total duration of dolomitization, however, could have been much longer if fluid flow was episodic, as in modern seafloor hydrothermal systems, depending on the time elapsed between episodes of flow. Conversion of limestone to dolomite likely occurred by a mechanism intermediate between the end-member cases of replacement at constant oxygen and carbon and replacement at constant volume. [ABSTRACT FROM AUTHOR]