Hora, John Milan

[["dc.bibliographiccitation.firstpage","1681"],["dc.bibliographiccitation.issue","11-12"],["dc.bibliographiccitation.journal","Geological Society of America Bulletin"],["dc.bibliographiccitation.lastpage","1688"],["dc.bibliographiccitation.volume","127"],["dc.contributor.author","Jicha, Brian R."],["dc.contributor.author","Laabs, Benjamin J. C."],["dc.contributor.author","Hora, John Milan"],["dc.contributor.author","Singer, Bradley S."],["dc.contributor.author","Caffee, Marc W."],["dc.date.accessioned","2018-11-07T09:49:37Z"],["dc.date.available","2018-11-07T09:49:37Z"],["dc.date.issued","2015"],["dc.description.abstract","The catastrophic gravitational collapse of the Old Cone of Volcan Parinacota produced a 6 km(3) debris avalanche that traveled similar to 22 km and covered more than 150 km(2). The Upper Lauca drainage, a broad high-altitude basin in the Chilean Altiplano, was permanently altered by the collapse. Although the eruptive history of Parinacota before and after-the debris avalanche is well known from petrologic and geochronologic studies, previous age limits on the debris avalanche (based on multiple chronometers) span ca. 8-20 ka. New cosmogenic surface-exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ Be-10 and indicate that the avalanche occurred at 8.8 +/- 0.5 ka. These data demonstrate that cosmogenic 10Be surface-exposure dating can be successfully applied to quartz-bearing, volcanic debris avalanche deposits, and that this method offers a distinct advantage over C-14 chronologies that provide only minimum or maximum age limits. The 8.8 ka exposure age for the debris avalanche (1) agrees with 14C age limits of paleosol material incorporated in the debris avalanche, (2) requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone-building episodes at most continental arc volcanoes, and (3) suggests that volcano collapse did not result in the formation of Lago Chungara, but instead led to a major expansion of a preexisting closed basin."],["dc.identifier.doi","10.1130/B31247.1"],["dc.identifier.isi","000364200700014"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/35545"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Geological Soc Amer, Inc"],["dc.relation.issn","1943-2674"],["dc.relation.issn","0016-7606"],["dc.title","Early Holocene collapse of Volcan Parinacota, central Andes, Chile: Volcanological and paleohydrological consequences"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","75"],["dc.bibliographiccitation.issue","1-2"],["dc.bibliographiccitation.journal","Earth and Planetary Science Letters"],["dc.bibliographiccitation.lastpage","86"],["dc.bibliographiccitation.volume","285"],["dc.contributor.author","Hora, John Milan"],["dc.contributor.author","Singer, Bradley S."],["dc.contributor.author","Woerner, Gerhard"],["dc.contributor.author","Beard, Brian L."],["dc.contributor.author","Jicha, Brian R."],["dc.contributor.author","Johnson, Clark M."],["dc.date.accessioned","2018-11-07T08:27:39Z"],["dc.date.available","2018-11-07T08:27:39Z"],["dc.date.issued","2009"],["dc.description.abstract","The role of changing crustal interaction and plumbing geometry in modulating calc-alkaline vs. tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath Volcan Parinacota. Changes in petrologic affinity correlate with periods of cone-building, sector collapse, and rebuilding of the volcano over the last 52 ka, and are well explained by changes in magma recharge regime. With increasing recharge and magma output, lavas transition from low-Fe, strongly calc-alkaline, phenocryst-rich silicic compositions to medium-Fe, near-tholeiitic, mafic, and aphanitic characteristics. Strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously; these are characterized by distinctive (87)Sr/(86)Sr ratios, which suggest an initially compartmentalized system. Relatively high ((230)Th/(232)Th) activity ratios of similar to 0.72 in early-erupted calc-alkaline lavas are consistent with interaction with high-U upper crust. Low ((230)Th/(232)Th) activity ratios of similar to 0.55 and up to 33% Th-excess in younger near-tholeiitic lavas correlate with steep REE patterns, indicating lower-crustal interaction. Thorium-excesses at the time of eruption approach the maximum that can be generated via small-degree garnet-residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short, on the order of <2x10(4) yr. In contrast, lavas with greatest calc-alkalinity are also at or near secular equilibrium, suggesting stagnation times >3x10(5) yr in the upper crust. In addition to more traditional explanations tied to magma source, expression of low-Fe 'calc-alkaline' (CA) vs. medium-Fe 'near-tholeiitic' (TH) magma series at the scale of individual volcanoes is likely to be modulated by transitions from compartmentalized, stagnant, assimilation-prone 'dirty' systems (CA) to 'clean' systems (TH) that are characterized by rapid magma throughput and minimal opportunity for upper-crustal contamination. (C) 2009 Elsevier B.V. All rights reserved."],["dc.identifier.doi","10.1016/j.epsl.2009.05.042"],["dc.identifier.isi","000273062700007"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/16250"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Elsevier Science Bv"],["dc.relation.issn","0012-821X"],["dc.title","Shallow and deep crustal control on differentiation of calc-alkaline and tholeiitic magma"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","923"],["dc.bibliographiccitation.issue","10"],["dc.bibliographiccitation.journal","Geology"],["dc.bibliographiccitation.lastpage","926"],["dc.bibliographiccitation.volume","38"],["dc.contributor.author","Hora, John Milan"],["dc.contributor.author","Singer, Bradley S."],["dc.contributor.author","Jicha, Brian R."],["dc.contributor.author","Beard, Brian L."],["dc.contributor.author","Johnson, Clark M."],["dc.contributor.author","de Silva, Shan"],["dc.contributor.author","Salisbury, Morgan"],["dc.date.accessioned","2018-11-07T08:38:27Z"],["dc.date.available","2018-11-07T08:38:27Z"],["dc.date.issued","2010"],["dc.description.abstract","The Ar-40/Ar-39 radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between C-14 and longer-lived isotopic systems that are too imprecise for dating young samples. However, Ar-40/Ar-39 ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, U-238-Th-230 disequilibrium, to demonstrate that Ar-40/Ar-39 age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous Ar-40 in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in Ar-40/Ar-39 geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of Ar-40 due to rapid loss to an \"infinite reservoir\" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced."],["dc.identifier.doi","10.1130/G31064.1"],["dc.identifier.isi","000282348600016"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/18776"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Geological Soc Amer, Inc"],["dc.relation.issn","0091-7613"],["dc.title","Volcanic biotite-sanidine Ar-40/Ar-39 age discordances reflect Ar partitioning and pre-eruption closure in biotite"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","178"],["dc.bibliographiccitation.journal","Computers & Geosciences"],["dc.bibliographiccitation.lastpage","185"],["dc.bibliographiccitation.volume","56"],["dc.contributor.author","Hora, John Milan"],["dc.contributor.author","Kronz, Andreas"],["dc.contributor.author","Moeller-McNett, Stefan"],["dc.contributor.author","Woerner, Gerhard"],["dc.date.accessioned","2018-11-07T09:23:20Z"],["dc.date.available","2018-11-07T09:23:20Z"],["dc.date.issued","2013"],["dc.description.abstract","Application of geothermobarometry based on equilibrium exchange of chemical components between two mineral phases in natural samples frequently leads to the dilemma of either: (1) relying on relatively few measurements where there is a high likelihood of equilibrium, or (2) using many analysis pairs, where a significant proportion may not be useful and must be filtered out. The second approach leads to the challenges of (1) evaluation of equilibrium for large numbers of analysis pairs, (2) finding patterns in the dataset where multiple populations exist, and (3) visualizing relationships between calculated temperatures and compositional and textural parameters. Given the limitations of currently-used thermobarometry spreadsheets, we redesign them in a way that eliminates tedium by automating data importing, quality control and calculations, while making all results visible in a single view. Rather than using a traditional spreadsheet layout, we array the calculations in a grid. Each color-coded grid node contains the calculated temperature result corresponding to the intersection of two analyses given in the corresponding column and row. We provide Microsoft Excel templates for some commonly-used thermometers, that can be modified for use with any geothermometer or geobarometer involving two phases. Conditional formatting and ability to sort according to any chosen parameter simplifies pattern recognition, while tests for equilibrium can be incorporated into grid calculations. A case study of rhyodacite domes at Parinacota volcano, Chile, indicates a single population of Fe-Ti oxide temperatures, despite Mg-Mn compositional variability. Crystal zoning and differing thermal histories are, however, evident as a bimodal population of plagioclase-amphibole temperatures. Our approach aids in identification of suspect analyses and xenocrysts and visualization of links between temperature and phase composition. This facilitates interpretation of whether heat transfer was accompanied by bulk mass transfer, and to what degree diffusion has homogenized calculated temperature results in hybrid magmas. (c) 2013 Elsevier Ltd. All rights reserved."],["dc.description.sponsorship","German Science Foundation (DFG) [HO5049/1-1]"],["dc.identifier.doi","10.1016/j.cageo.2013.02.008"],["dc.identifier.isi","000320221600020"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/29553"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Pergamon-elsevier Science Ltd"],["dc.relation.issn","0098-3004"],["dc.title","An Excel-based tool for evaluating and visualizing geothermobarometry data"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","343"],["dc.bibliographiccitation.issue","3-4"],["dc.bibliographiccitation.journal","Geological Society of America Bulletin"],["dc.bibliographiccitation.lastpage","362"],["dc.bibliographiccitation.volume","119"],["dc.contributor.author","Hora, John Milan"],["dc.contributor.author","Singer, Bradley S."],["dc.contributor.author","Woerner, Gerhard"],["dc.date.accessioned","2018-11-07T11:04:27Z"],["dc.date.available","2018-11-07T11:04:27Z"],["dc.date.issued","2007"],["dc.description.abstract","The 163 k.y. history as well as the chemical and 46 km(3) volumetric evolution of Volcan Parinacota are described in detail by new mapping, stratigraphy, and 57 Ar-40/Ar-39 ages determined from groundmass or sanidine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this central Andean volcano, which was built upon 70-km-thick crust, provides a more complete view of how quickly volcanic edifices are built in this setting and how their magmatic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava flows (163-117 ka) followed by a rhyodacite dome plateau (47-40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche similar to 3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preceding 65 k.y. hiatus. Debris avalanche timing is refined here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifice is delineated by 16 groundmass and whole-rock Ar-40/Ar-39 ages, which include some of the youngest lava flows dated by this method. Increase in cone-building rate and a continued trend toward more mafic compositions following collapse imply an inter-relationship between the presence of the edifice and flux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well-dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not significantly retarded by thick crust, primitive, dry basalts might be."],["dc.identifier.doi","10.1130/B25954.1"],["dc.identifier.isi","000246465600007"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/51847"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Geological Soc Amer, Inc"],["dc.relation.issn","0016-7606"],["dc.title","Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: Ar-40/Ar-39 constraints from Volcan Parinacota, Chile"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]