Hora, John Milan

[["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"]]