Fluid Inclusion research, volume 20 : edited by Edwin Roedder, Andrzej Kozlowski and Harvey E. Belkin. University of Michigan Press, 1987, 559p. (ISBN 0-472-02020-X)

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Introduction

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Dynamic Modeling of In-Use Cement Stocks in the United States

A dynamic substance-flow model is developed to characterize the stocks and flows of cement utilized during the 20th century in the United States, using the generic cement life cycle as a systems boundary. The motivation for estimating historical inventories of cement stocks and flows is to provide accurate estimates of contemporary cement in-use stocks in U.S. infrastructure and future discards to relevant stakeholders in U.S. infrastructure, such as the federal and state highway administrators, departments of transportation, public and private utilities, and the construction and cement industries. Such information will assist in planning future rehabilitation projects and better life cycle management of infrastructure systems. In the present policy environment of climate negotiations, estimates of in-use cement infrastructure can provide insights about to what extent built environment can act as a carbon sink over its lifetime. The rate of addition of new stock, its composition, and the repair of existing stock are key determinants of infrastructure sustainability. Based upon a probability of failure approach, a dynamic stock and flow model was developed utilizing three statistical lifetime distributions—Weibull, gamma, and lognormal—for each cement end-use. The model-derived estimate of the “in-use” cement stocks in the United States is in the range of 4.2 to 4.4 billion metric tons (gigatonnes, Gt). This indicates that 82% to 87% of cement utilized during the last century is still in use. On a per capita basis, this is equivalent to 14.3 to 15.0 tonnes of in-use cement stock per person. The in-use cement stock per capita has doubled over the last 50 years, although the rate of growth has slowed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72233/1/JIEC_055_sm_SuppMat.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/72233/2/j.1530-9290.2008.00055.x.pd

Fluid inclusion chemistry in the exploration for Mississippi Valley-type deposits: an example from East Tennessee, U.S.A.

Scanning electron microscopy and energy dispersive analysis of fluid inclusion decrepitates in sparry dolomite cements associated with sphalerite mineralization in the Mascot-Jefferson City Mississippi Valley-type district of East Tennessee show a markedly lower Na/Ca ratio than those in dolomites unassociated with ore. The exploration potential of this variation in fluid inclusion chemistry has been evaluated by analyzing inclusion decrepitates from white sparry dolomite along a 150 m traverse in the New Market West mine area. The traverse extends from the central portion of a large domal collapsebreccia orebody with greater than 3% Zn, into the dolomite-veined, but unmineralized Knox Group limestones. Ratios of Na/Ca as low as 0.8-1.2 were common in dolomite-hosted decrepitates from the high-grade collapse ores. This anomalous decrepitate chemistry was not observed in sparry dolomites less than 12 m from the margins of the breccia bodies where Na/Ca ratios ranged from 1.8 to 3.6. Dolomite cements in low grade (<1.0% Zn) parts of the collapse bodies showed the greatest variation in Na/Ca ratio, with values ranging from 0.9 to 3.5. No systematic trends were observed in K/Na and S/Cl ratios from the same samples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26717/1/0000267.pd

Lead isotopic composition of the oldest volcanic rocks of the eastern greater antilles island ARC

Pb isotopic analyses for rocks from the early Cretaceous Los Ranchos Formation, which is the basal volcanic assemblage in the Dominican Republic, fall on a short linear array of shallow slope in a 207Pb/204Pb vs. 206Pb/204Pb plot. The array is interpreted to have formed by decay of local U since the time of ore and rock formation ~ 110 Ma ago. Previously reported analyses of sulfide samples from the Pueblo Viejo Au---Ag deposit in the upper part of the Los Ranchos Formation fall on the lower half of the same linear array. In a 208Pb/204Pb vs. 206Pb/204Pb plot the Los Ranchos Formation can be divided into the lower member with an apparent Th/U ratio of 1.2 and the upper three members with an apparent Th/U ratio of 2.6. A cluster of analyses falls at the least radiogenic end of these arrays and is interpreted to represent the initial Pb isotopic composition of the Los Ranchos magmas. This cluster falls close to the Stacey-Kramers growth curve. For the Los Ranchos Pb, a homogeneous source with [mu]=9.6 and Th/U near 3.8, since 3.7 Ga ago, is implied.Previously reported analyses for correlative rocks of the Water Island Formation, the basal volcanic complex in the Virgin Islands at the east end of the Greater Antilles, suggest a steep linear array extending downwards from the Los Ranchos cluster, after correction of the ratios for U decay since formation of the rocks. This array has a slope of 0.250 +/- 0.023, which could have resulted from magma derivation from sources that had been isolated for ~ 3.0 Ga. Younger rock Pb, from the island of Martinique, has a similar slope, although with larger 206Pb/204Pb and 207Pb/204Pb ratios. The data points extend upward from the radiogenic end of the Los Ranchos array.These relationships indicate a complex derivation of magmas in the area. In the Greater Antilles, the proportion of continental material decreases eastward, while even farther east in the Lesser Antilles at Martinique, the proportion of crustal material, as indicated by Pb isotopic composition, is again very high.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26769/1/0000321.pd

Fluid inclusion gas chemistry in east Tennessee Mississippi Valley-type districts: Evidence for immiscibility and implications for depositional mechanisms

Analyses of fluid inclusion gases from Mississippi Valley-type districts in east Tennessee reveal the presence of several distinct aqueous solutions and vapors that were part of the mineralizing process. Inclusion contents were released by crushing 5 to 25 mg mineral samples and by decrepitating individual inclusions; all analyses were obtained by quadrupole mass spectrometry. Most analyzed inclusion fluids consist of H2O with significant amounts of CH4 (0.3 to 2.9 mol%), CO2 (0.1 to 4.7 mol%), and smaller amounts of C2H6, C3H8, H2S, SO2, N2, and Ar. In general, inclusion gas abundances are greatest for sphalerite from the Mascot-Jefferson City district, lower for the Sweetwater district, and lowest for the Copper Ridge district. Compositional similarities in the inclusion fluids from the three districts imply that mineralization probably formed from fluids that permeated the entire region, rather than from completely separate fluids at each site.Saturation pressures calculated for these fluid compositions range from 300 to 2200 bars. Burial depths for the host unit have been estimated to be about 2 to 3 km in the east Tennessee area during Devonian time, the age of mineralization indicated by recent isotopic ages. Pressures at these depths, whether hydrostatic or lithostatic, would not have been adequate to prevent phase separation. Thus, our gas analyses represent either a mixture of vapor-rich and liquid-rich inclusions, or liquid-rich inclusions that trapped excess vapor. A lack of visible vapor-rich inclusions, high gas contents in individual fluid inclusion gas analyses obtained by decrepitation, and a positive correlation between decrepitation temperature and gas content for individual inclusions strongly suggest that the samples contain liquid-rich inclusions that trapped varying amounts of excess vapor. This excess gas probably accounts for the anomalously high homogenization temperatures and the wide range of homogenization temperatures observed in fluid inclusions in these ores. The vapor phase could have formed either by phase separation resulting from over-pressured aqueous fluids migrating into a region of hydrostatic pressure, or by incorporation of a pre-existing gas cap at the sites of deposition into the invading aqueous fluid.Exsolution of a vapor phase from the mineralizing brines should cause precipitation of carbonate and sulfide minerals, but reaction path modelling indicates that the resulting sparry dolomite:sphalerite ratios would be too high to form an ore-grade deposit. On the other hand, if the vapor phase was from a pre-existing sour gas cap that was intercepted by a Zn-rich brine, large amounts of sphalerite would precipitate in a fairly small region. Preliminary mass balance calculations suggest that a gas cap of dimensions similar to the individual districts in east Tennessee could have contained enough H2S to account for the total amount of sphalerite precipitated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30308/1/0000710.pd

Magmatic contributions to hydrothermal systems

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Sulfur- and strontium-isotopic geochemistry of celestite, barite and gypsum from the Mesozoic basins of northeastern Mexico

The Mesozoic sedimentary basin of northern Mexico contains large limestone-hosted stratiform "mantos" of celestite and barite that appear to have formed by carbonate replacement. We have measured the isotopic compositions of sulfur and strontium in these mantos in order to determine whether they formed by diagenetic processes and, if so, what fraction of the diagenetically released Sr is fixed in celestite deposits. S- and Sr-isotopic analyses of gypsum from layers in the limestone confirm its Cretaceous sedimentary origin. Isotopic compositions of most celestite and barite differ from values observed for the gypsum. Division of the deposits into two geologically and geochemically distinct groups shows that there is a general positive correlation between [delta] 34S and 87Sr/86Sr compositions of celestite and barite in the two systems, with celestite containing lighter S and less radiogenic Sr in both groups of deposits. Less radiogenic Sr in the celestite probably came from nearby Cretaceous limestones. More radiogenic Sr, which is found largely in the barite, probably came from basement-derived arkoses in the Cretaceous basin. Sulfur was derived from evaporites in one deposit group and from some other lighter source, perhaps coal or petroleum, in the other. These isotopic constraints, considered in the light of available solubility data, require that at least two distinct solutions existed and mixed locally during formation of the celestite and barite mantos. No more than 10% of the diagenetically-released Sr is concentrated in the celestite deposits.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23386/1/0000331.pd

Source of lead in Central American and Caribbean mineralization, II. Lead isotope provinces

In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher 206Pb/204Pb and 207Pb/204Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing 207Pb/204Pb and 208Pb/204Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in 207Pb/204Pb and 208Pb/204Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280+/-310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24169/1/0000428.pd

Relationship of fluid inclusion geochemistry to wall-rock alteration and lithogeochemical zonation at the Hollinger-McIntyre gold deposit, Porcupine District, Canada

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