918 resultados para 920 Biography


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The purpose of this study was to investigate a selection of children's historical nonfiction literature for evidence of coherence. Although research has been conducted on coherence of textbook material and its influences on comprehension there has been limited study on coherence in children's nonfiction literature. Generally, textual coherence has been seen as critical in the comprehensibility of content area textbooks because it concerns the unity of connections among ideas and information. Disciplinary coherence concerns the extent to which authors of historical text show readers how historians think and write. Since young readers are apprentices in learning historical content and conventions of historical thinking, evidence of disciplinary coherence is significant in nonfiction literature for young readers. The sample of the study contained 32 books published between 1989 and 2000 ranging in length from less than 90 pages to more than 150 pages. Content analysis was the quantitative research technique used to measure 84 variables of textual and disciplinary coherence in three passages of each book, as proportions of the total number of words for each book. Reliability analyses and an examination of 750 correlations showed the extent to which variables were related in the books. Three important findings emerged from the study that should be considered in the selection and use of children's historical nonfiction literature in classrooms. First, characteristics of coherence are significantly related together in high quality nonfiction literature. Second, shorter books have a higher proportion of textual coherence than longer books as measured in three passages. Third, presence of the author is related to characteristics of coherence throughout the books. The findings show that nonfiction literature offers students content that researchers have found textbooks lack. Both younger and older students have the opportunity to learn the conventions of historical thinking as they learn content through nonfiction literature. Further, the children's literature, represented in the Orbis Pictus list, shows students that authors select, interpret, and question information, and give other interpretations. The implications of the study for teaching history, teacher preparation in content and literacy, school practices, children's librarians, and publishers of children's nonfiction are discussed.

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The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated d34S_sulfide (3.7 to 12.7?). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400°C alone cannot account for both the high sulfur contents and high d34S_sulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (~400°C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ~300°C. Serpentinization produces highly reducing conditions and significant amounts of H2 and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in 13C (up to 4.5?) at temperatures above 250°C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 mln ton seawater S per year. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates.

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Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt-186Os and 187Re-187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353 +/- 7, identical to the mean of 0.1198340 +/-12 for Os-Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about +/- 30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing D(Re), reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128-0.129, consistent with estimates for the primitive upper mantle.

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The compatibility of vanadium (V) during mantle melting is a function of oxygen fugacity (fO2): at high fO2's, V becomes more incompatible. The prospects and limitations of using the V content of peridotites as a proxy for paleo-fO2 at the time of melt extraction were investigated here by assessing the uncertainties in V measurements and the sensitivity of V as a function of degree of melt extracted and fO2. V-MgO and V-Al2O3 systematics were found to be sensitive to fO2 variations, but consideration of the uncertainties in measurements and model parameters indicates that V is sensitive only to relative fO2 differences greater than ~2 log units. Post-Archean oceanic mantle peridotites, as represented by abyssal peridotites and obducted massif peridotites, have V-MgO and -Al2O3 systematics that can be modeled by 1.5 GPa melting between FMQ - 3 and FMQ - 1. This is consistent with fO2's of the mantle source for mid-ocean ridge basalts (MORBs) as determined by the Fe3+ activity of peridotitic minerals and basaltic glasses. Some arc-related peridotites have slightly lower V for a given degree of melting than oceanic mantle peridotites, and can be modeled by 1.5 GPa melting at fO2's as high as FMQ. However, the majority of arc-related peridotites have V-MgO systematics overlapping that of oceanic mantle peridotites, suggesting that although some arc mantle may melt under slightly oxidizing conditions, most arc mantle does not. The fact that thermobarometrically determined fO2's in arc peridotites and lavas can be significantly higher than that inferred from V systematics, suggests that V retains a record of the fO2 during partial melting, whereas the activity of Fe3+ in arc peridotitic minerals and lavas reflect subsequent metasomatic overprints and magmatic differentiation/emplacement processes, respectively. Peridotites associated with middle to late Archean cratonic mantle are characterized by highly variable V-MgO systematics. Tanzanian cratonic peridotites have V systematics indistinguishable from post-Archean oceanic mantle and can be modeled by 3 GPa partial melting at ~FMQ - 3. In contrast, many South African and Siberian cratonic peridotites have much lower V contents for a given degree of melting, suggesting at first glance that partial melting occurred at high fO2's. More likely, however, their unusually low V contents for a given degree of melting may be artifacts of excess orthopyroxene, a feature that pervades many South African and Siberian peridotites but not the Tanzanian peridotites. This is indicated by the fact that the V contents of South African and Siberian peridotites are correlated with increases in SiO2 content, generating data arrays that cannot be modeled by partial melting but can instead be generated by the addition of orthopyroxene through processes unrelated to primary melt depletion. Correction for orthopyroxene addition suggests that the South African and Siberian peridotites have V-MgO systematics similar to those of Tanzanian peridotites. Thus, if the Tanzanian peridotites represent the original partial melting residues, and if the South African and Siberian peridotites have been modified by orthopyroxene addition, then there is no indication that Archean cratonic mantle formed under fO2's significantly greater than that of modern oceanic mantle. Instead, the fO2's inferred from the V systematics in these three cratonic peridotite suites are within range of modern oceanic mantle. This also suggests that the transition from a highly reducing mantle in equilibrium with a metallic core to the present oxidized state must have occurred by late Archean times.