A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume.Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.

Chemical proxy models are based upon the assumption that isotopic signatures and concentrations of minor and trace elements reflect equilibrium fractionation processes that occurred during mineralization. This picture is rooted in the fundamental assumptions of BCF crystal growth theory— a thermodynamicbased model that was derived for step growth at very near equilibrium conditions. However, the applicability of these assumptions are being called into question with the realization that many carbonate biominerals form by non-classical processes. Here, mineralization begins with accumulation of amorphous calcium carbonate (ACC) in a localized environment that subsequently transforms to the crystal/organic composites we know as skeletal structures. It is not yet known 1) if the transformation involves classical microscopic dissolution-reprecipitation or an altogether different type of process and 2) the consequences of this process for composition and isotopic signatures. This study investigates the influence of the ACC to calcite pathway on the Mg content and isotopic signature of calcites. For low solution levels of Mg/Ca, Mg content is insufficient to inhibit step growth and ACC transforms into crystallites of Mg calcites that exhibit the expected linear fractionation with Mg/Ca of initial solutions (0-20 mol% MgCO3). In contrast, when initial Mg levels are above the threshold for step growth, ACC transforms to nanoparticle aggregates of very high Mg calcite (30-50 mol% MgCO3). The Mg content of calcites formed by this process is independent of solution chemistry, without evidence of fractionation. The data suggest mineralization is biased to the alternative pathway when the Mg level in the local environment is too high for significant calcite growth beyond nanoparticle sizes. This pathway is allowed because high levels of supersaturation render thermodynamic barriers to nucleation less significant than the larger kinetic barriers. Thus, the alternative pathway is a consequence of interplays between kinetic and thermodynamic factors. Parallel experiments used enriched isotope labels (43Ca and 25Mg) to distinguish dissolution-reprecipitation from direct conversion during the ACC to calcite transformation. When solution Mg/Ca is low calcite assumes the isotopic label of the growth media consistent with a dissolution-repreciptiation pathway. In contrast, very high Mg calcites that grow from high Mg solutions retain a portion of the ACC isotope signature. This suggests the transformation of high Mg ACC occurs by an alternative pathway that involves a substantial fraction of direct transformation. Collectively our data show that mineralization pathway dramatically affects composition. This type of mechanistic understanding of mineralization processes will be necessary to explain proxy behavior and more accurately reconstruct past environental conditions. Constraining dehydration rates during regional metamorphism, Townshend Dam, Vermont, U.S.A. BESIM DRAGOVIC, MATTHEW GATEWOOD, ETHAN F. BAXTER, HAROLD STOWELL, DAVID M. HIRSCH AND ROSE BLOOM Boston University, Deparrtment of Earth Sciences, Boston, MA, U.S.A, dragovic@bu.edu (* presenting author), efb@bu.edu University of Alabama, Department of Geological Sciences, Tuscaloosa, AL, U.S.A, matthewpgatewood@gmail.com, hstowell@geo.ua.edu Western Washington University, Department of Geology, Bellingham, WA, U.S.A., hirschd@geol.wwu.edu, bloomr3@students.wwu.edu

Mode estimation, the visual determination of the proportions of the components of a rock, is a valuable tool in geology, providing one of the most efficient means of describing rock mineralogy and chemistry. However, current methods of teaching mode estimation suffer from the often-mediocre mode-estimation skills of the instructors. Fifty-nine academic and professional geologists was surveyed online to assess their mode estimation skills, and they scored 65.9 ± 7.5 (SD) out of a possible 100. Instructors with poor skills cannot provide the correct answers to mode questions and, thus, cannot create valid assessments of student mode-estimation skills. Two computer programs may help address this problem. ModeMaker is a tool for the instructor to use to create images with up to five phases, each with known modes. Each phase can have a number of additional specified properties, such as shape, size, and orientation. The instructor can, thus, provide valid assessments of student abilities to estimate modes. ModeQuiz is an interactive training application for the student. It creates images of the sort ModeMaker creates but randomizes the properties. The student makes mode estimations on the computer, and the program reveals the correct modes and scores the student's estimates. By repeating this process, the student may improve his or her mode-estimation skill over time. Assessments of ModeQuiz show that as little as 2 h of practice with the program may lead to improvement in student skill at mode estimation.

Both the measurement of crystal sizes and locations in three dimensions and the simulation of crystallization produce data sets that require characterization. Reduce3D is a new computer program for measuring spatial statistics of interest to kinetic studies of crystallization. The principal statistical measures of such interest are the L′ function, the pair correlation function, and the mark correlation function. Reduce3D implements the ability to create a large number of pseudorandom interface controlled crystal arrays designed to mimic the rock being studied but with interface control of nucleation and growth, forming a null-hypothesis envelope. Values that exceed this envelope in the direction of ordering of crystal centers and/or growth suppression of nearby crystals may be interpreted as significantly ordered, which for metamorphic crystallization may imply diffusional control of nucleation and growth. Reduce3D, together with utility programs GraphCFs for visualizing relevant statistics and Render3D for visualizing the data set, form a suite that makes such interpretation rapid and straightforward.

The 16 km thick early to middle Eocene Crescent Formation exposed on the Olympic Peninsula represents one of the thickest stacks of basalt on Earth. It is variably metamorphosed at low grade, with evidence of both medium- and high-P/T conditions. Metasomatism is localized within a small number of layers, with variation relative to average values of as much as five standard deviations. The upper section tends to be lower grade but few systematic trends in mineralogy or mineral composition correlate with stratigraphic position. The boundary region between the lower and upper members contains the most dramatic metasomatic alteration as well as the best examples of barroisitic amphibole. Observations are consistent with a model in which extrusion of the basalt stack led to medium-P/T burial recrystallization in the zeolite and prehnite-pumpellyite facies, followed by localized, possibly fluid-facilitated lower-blueschist facies metamorphism; this high-P/T metamorphism may reflect the lowering of isotherms caused by subduction-zone refrigeration to the west of these rocks.

Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens, respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.

Differences in rates of nucleation and diffusion-limited growth for biotite porphyroblasts in adjacent centimetre-scale layers of a garnet-biotite schist from the Picuris Mountains of New Mexico are revealed by variations in crystal size and abundance between two layers with strong compositional similarity. Relationships between fabrics recorded by inclusion patterns in biotite and garnet porphyroblasts are interpreted to reflect garnet growth following biotite growth, without substantial alteration of the biotite sizes. Sizes and locations of biotite crystals, obtained via high-resolution X-ray computed tomography, document that of the two adjacent layers, one has a larger mean crystal volume (9.5 x 10(-4)v. 2.4 x 10(-4) cm(3)), fewer biotite crystals per unit volume (232 v. 576 crystals cm(-3)), and a higher volume fraction of biotite (23%v. 14%). The two layers have similar mineral assemblages and mineral chemistry. Both layers show evidence for diffusional control of nucleation and growth. Pseudosection analysis suggests that the large-biotite layer began to crystallize biotite at a temperature similar to 67 degrees C greater than the small-biotite layer. Diffusion rates differed between layers, because of their different temperature ranges of crystallization, but this effect can be quantified. The bulk compositional difference between the layers, manifested in different modal amounts of biotite, has an effect on the biotite sizes that is also quantifiable and insufficient to account for the difference in biotite size. After these other possible causes of variation in crystal sizes have been eliminated, variability in nucleation and diffusion rates remain as the dominant factors responsible for the difference in porphyroblastic textures. Numerical simulations suggest that relative to the small-biotite layer, the large-biotite layer experienced elevated diffusion rates because of the higher crystallization temperature, as well as increased nucleation rates in order to achieve the observed size and number density of crystals. The simulations can replicate the observed textures only by invoking unreasonably large values for the thermal dependence of nucleation rates (activation energies), strongly suggesting that the observed textural differences arise from variations between layers in the abundance and energetics of potential nucleation sites.

Bill Carlson’s work has spanned a wide range of metamorphic and mineralogic topics. One overarching theme has been the extension of metamorphic petrology to occurrences dominated by disequilibrium, rather than equilibrium textures. Beginning with his graduate work on calcite-aragonite transition kinetics, continuing with coronal reaction textures, and, for the past 15 years, focusing on the quantitative analysis of porphyroblast textures, Bill has driven the field forward. His advances have been both in the theoretical realm, building on the work of folks like Ralph Kretz, and in the technical realm, in which he pioneered the use of high-resolution computed tomography for the analysis of porphyroblast textures in three-dimensions. While he has not been alone in this effort, his work, along with that of his students and colleagues, has been instrumental in advancing our science from the clean ideality of equilibrium towards the messy but more accurate world of disequilibrium. Oxygen isotope speedometry in the Biwabik iron-formation

New data-processing methods for making three-dimensional measurements in volumetric data sets, described in a companion paper(1), are applied here for quantitative textural analysis of porphyroblastic rocks. In a reexamination of a suite of garnetiferous rocks from the Picuris Mountains, New Mexico, Whitt Ranch, Texas, and Mica Dam, British Columbia, signals that in earlier studies indicated ordering of porphyroblast nucleation sites and competition for nutrients are significantly altered but generally confirmed in the improved data. Better-resolution tomographic imagery greatly enhances the observation and measurement of small crystals, in some cases substantially modifying the shape of the crystal size distributions. The enhanced analysis also enables detection of instances of strong impingement among neighboring porphyroblasts that were probably previously interpreted as single crystals, which may have spuriously enhanced ordering signals. Overall the results of this study corroborate earlier findings of diffusion-controlled nucleation and growth of garnet in the specimens examined. They also document, however, the critical importance of high-quality data, which are required to ensure that subtle mechanistic signals can emerge from statistical noise and to ensure that failure of crystal impingement to be detected or preserved does not generate a bias toward ordering.

The central portions of garnet porphyroblasts from Harpswell Neck, Maine, exhibit small discrete regions of high Mn content, dispersed in three dimensions. Orientation contrast imaging (OCI) and quantitative crystallographic orientation measurements from electron backscatter diffraction (EBSD) patterns show that most garnet porphyroblasts contain no variations in crystallographic orientation. It is difficult to reconcile these data with a model in which each high-Mn region represents a discrete garnet nucleus.An alternative hypothesis invokes a model of garnet growth in which precursor phases rich in Mn are overgrown, and their Mn is incorporated locally into the garnet structure. Although this model requires length scales for equilibration of Mn during the early stages of garnet growth that are shorter than any previously documented, that inference is consistent with the very low temperatures of garnet nucleation in these rocks, and with compositional evidence for progressively larger scales of equilibrium as reaction progressed.Very short length scales for equilibration of Mn, identified in this study for the lowest-grade portion of the history of the rocks, call into question the common assumption of rock-wide equilibrium for Mn during garnet growth at low temperature. The absence of rock-wide equilibrium for Mn at low grade has potential negative implications for all common interpretive methods based in phase equilibria, including thermobarometric studies and thermodynamic analysis of the origin and meaning of zoning in low-grade garnet.

Spatial correlation functions, which quantify spatial relationships among porphyroblasts over a range of length scales, can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks. The utility, reliability, and robustness of these functions, however, depend critically upon correct methods of calculation and application to geological samples. Application of the L ′-function, Pair Correlation Function, and Mark Correlation Function (Stoyan and Stoyan, 1994) to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities. These results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays. Analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways (displacing crystals in an ordered array by increasing amounts, reducing the number of crystals, and increasing the sample’s aspect ratio) demonstrates that these scale-dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms, even in complex circumstances. The effects of clustering of nucleation sites, however, can strongly obscure any underlying signal that might reveal crystallization mechanisms. The L ′-function and the Pair Correlation Function are sensitive to short-range ordering of crystals, which may reflect suppression of nucleation in the vicinity of growing porphyroblasts. The Mark Correlation Function is sensitive to size-isolation correlations, which may reflect retardation of growth among crystals competing for nutrients. Interpretation of these functions, however, requires careful attention to proper calculation of Monte Carlo simulations, which are used to identify values of the functions that constitute a null-hypothesis region for comparison to samples with unknown ordering and clustering characteristics. To yield functional values commensurate with those calculated for a particular natural rock specimen, each simulation must match as closely as possible several critical features of the natural rock, including the set of crystal radii, limitations on the observability of crystals, and the shape and size of the bounding surface of the sample. Crystallization mechanisms in seven previously studied garnetiferous rocks from three localities (Carlson et al. 1995; Denison and Carlson 1997) have been reassessed using both scale-dependent correlation functions and single-valued spatial statistics, both evaluated by comparison to rigorously computed null-hypothesis regions. The results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium.

Intergroup variation in chondrule sizes is commonly attributed to mass or aerodynamic sorting in the solar nebula, a concept that has recently been extended to other chondrite constituents (metal chondrules, metal–troilite grains). Both sorting mechanisms are dependent on grain density and size. Because metal chondrules and metal–troilite grains have smaller average sizes than their coexisting chondrule populations, the assumption of mass equivalence has been made and invoked in support of nebular sorting. We present the results of a quantitative comparison of the sizes, masses, and aerodynamic stopping times of chondrules and metal–troilite grains from three ordinary chondrites: Kelly (LL4), Bjurböle (L/LL4), and Hammond Downs (H4). Chondrule volumes were determined from corrected thin-section measurements, and metal–troilite grain volumes were estimated from X-ray tomographic images. Chondrule and metal–troilite grain populations have similar masses in Hammond Downs but are dissimilar in Bjurböle and Kelly. The difference in the average particle aerodynamic stopping times of the chondrules and metal–troilite grains (15% for Hammond Downs, 16% for Bjurböle, 32% for Kelly) are much smaller than differences in their average masses (28% for Hammond Downs, 73% for Bjurböle, 82% for Kelly). The observed ranges in mass of the two populations are relatively narrow in Hammond Downs and are wider in Bjurböle and Kelly. Furthermore, in all three meteorites the observed range in mass of the chondrules is narrower than that of the corresponding metal–troilite grain populations. It appears that the chondrules were sorted more efficiently than the metal–troilite grains. Our results agree with the idea that aerodynamic stopping times vary with particle size and density and disagree with sorting only by mass. While the average stopping times (expressed as rp ρs) of the two populations correlate better than their average masses, the percent difference in (rp ρs) between the two populations (Hammond Downs, 15%; Bjurböle, 16%; and Kelly, 32%) is greater than that between the chondrules and metal chondrules previously reported for the Acfer 059 carbonaceous chondrite (W. R. Skinner and J. M. Leenhouts 1993, Proc. Lunar Planet. Sci. Conf. 24th, 1315–1316). We attribute this to the small and irregular shapes of the metal–troilite grains, although thermal metamorphism may have also affected the metal–troilite grain data. For these reasons, our results are at variance with the concept of nebular mass sorting but may be in agreement with aerodynamic sorting models. Our results are consistent with the hypothesis that chondrule sorting is related to the phenomena of metal–silicate fractionation. However, these data are only preliminary. Final interpretation should be reserved until more meteorites can be analyzed, the effects of thermal metamorphism on metal grain sizes quantified, and software capable of true three-dimensional analysis developed.

This is a case report concerning a spontaneous 5-year remission of malignant pericardial effusion secondary to metastatic breast carcinoma. The patient received no specific therapy for the pericardial effusion other than one pericardiocentesis. The authors are unable to explain the failure of recurrence of the pericardial effusion especially in the presence of progressive metastatic breast carcinoma in other areas. A review of the literature has not revealed any case of malignant pericardial effusion secondary to metastatic breast carcinoma surviving 5 years.