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Author

John M. Hora

Other affiliations: University of Wisconsin-Madison
Bio: John M. Hora is an academic researcher from University of Göttingen. The author has contributed to research in topics: Geology & Mafic. The author has an hindex of 7, co-authored 8 publications receiving 605 citations. Previous affiliations of John M. Hora include University of Wisconsin-Madison.

Papers
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Journal ArticleDOI
TL;DR: Isotopic fingerprinting has been used to trace magmatic processes and the components that contribute to magmas as mentioned in this paper, and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes.
Abstract: Isotopic fingerprinting has long been used to trace magmatic processes and the components that contribute to magmas. Recent technological improvements have provided an opportunity to analyze isotopic compositions on the scale of individual crystals, and consequently to integrate isotopic and geochemical tracing with textural and petrographic observations. It has now become clear that mineral phases are commonly not in isotopic equilibrium with their host glass/groundmass. Isotopic ratios recorded from core to rim of a mineral grain reflect the progressive changes in the magma composition from which the mineral crystallized. The sense of these changes and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes such as magma mixing, contamination and recharge.

371 citations

Journal ArticleDOI
TL;DR: A more precise chronology of eruptions and associated volumes of this cen- tral Andean volcano, which was built upon 70-km-thick crust, provides a more com- plete view of how quickly volcanic edifi ces are built in this setting and how their mag- matic systems evolve during their lifetime.
Abstract: The 163 k.y. history as well as the chemi- cal and 46 km 3 volumetric evolution of Vol- can Parinacota are described in detail by new mapping, stratigraphy, and 57 40 Ar/ 39 Ar ages determined from groundmass or sani- dine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this cen- tral Andean volcano, which was built upon 70-km-thick crust, provides a more com- plete view of how quickly volcanic edifi ces are built in this setting and how their mag- matic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava fl ows (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 ~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 preced- ing 65 k.y. hiatus. Debris avalanche timing is refi ned here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifi ce is delineated by 16 groundmass and whole-rock 40 Ar/ 39 Ar ages, which include some of the youngest lava fl ows dated by this method. Increase in cone- building rate and a continued trend toward more mafi c compositions following collapse imply an inter-relationship between the pres- ence of the edifi ce and fl ux 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 signifi cantly retarded by thick crust, primitive, dry basalts might be.

113 citations

Journal ArticleDOI
01 Oct 2010-Geology
TL;DR: In this article, the authors used an independent chronometer, 238 U- 230 Th disequilibrium, to demonstrate that 40 Ar/39 Ar age disparity is not caused by differences in pre-eruption crystallization times.
Abstract: The 40 Ar/ 39 Ar 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 14 C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40 Ar/ 39 Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238 U- 230 Th disequilibrium, to demonstrate that 40 Ar/ 39 Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40 Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40 Ar/ 39 Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40 Ar 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.

56 citations

Journal ArticleDOI
TL;DR: 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 as discussed by the authors.

55 citations

Journal ArticleDOI
01 Jan 2005-Geology
TL;DR: The assumption that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement has been shown to be invalid in many instances as mentioned in this paper, which can be used to constrain petrogenetic pathways.
Abstract: The determination of accurate and precise isochron ages for igneous rocks requires that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement. Studies of young volcanic rocks at the mineral scale have shown this assumption to be invalid in many instances. Variations in initial isotope ratios can result in erroneous or imprecise ages. Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron. Independent age determinations and critical appraisal of petrography are needed to evaluate isotope data. If initial isotope ratio variability can be demonstrated, however, it can be used to constrain petrogenetic pathways.

36 citations


Cited by
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Journal ArticleDOI
01 Sep 2007-Geology
TL;DR: In this article, amphibole is used as a filter for water dissolved in mantle-derived arc magmas, and amphibole cumulates may act as a fertile source of intracrustal melts and fluids.
Abstract: Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus SiO2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this “cryptic amphibole fractionation.” If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective filter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fluids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle.

819 citations

Journal Article
TL;DR: In this article, a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles was created.
Abstract: We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

752 citations

Journal ArticleDOI
24 Mar 2017-Science
TL;DR: The evidence that shallow magma chambers are actually assembled quickly from much larger, crystal-rich transcrustal magmatic systems is reviewed, infering that igneous processes differ substantially in the lower and upper crust.
Abstract: Volcanoes are an expression of their underlying magmatic systems. Over the past three decades, the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust. A transcrustal perspective must balance slow (plate tectonic) rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions. Reconciling these observations is engendering active debate about the physical state, spatial distribution, and longevity of melt in the crust. Here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt-rich layers, focusing particularly on conditions that cause them to destabilize, ascend, and accumulate in voluminous but ephemeral shallow magma chambers.

673 citations

Book ChapterDOI
01 Jan 2012
TL;DR: In this article, a cyclic sedimentation series, calibrated to the geomagnetic polarity and biostratigraphic scales, has been used to improve the resolution of the Paleogene time scale.
Abstract: All Paleocene stages (i.e., Danian, Selandian and Thanetian) have formally ratified definitions, and so have the Ypresian and Lutetian Stages in the Eocene, and the Rupelian Stage in the Oligocene. The Bartonian, Priabonian and Chattian Stages are not yet formally defined. After the global catastrophe and biotic crisis at the CretaceousePaleogene boundary, stratigraphically important marine microfossils started new evolutionary trends, and on land the now flourishing mammals offer a potential for stratigraphic zonation. During the Paleogene the global climate, being warm until the late Eocene, shows a significant cooling trend culminating in a major cooling event in the beginning of the Oligocene, preparing the conditions for modern life and climate. Orbitally tuned cyclic sedimentation series, calibrated to the geomagnetic polarity and biostratigraphic scales, have considerably improved the resolution of the Paleogene time scale. The Paleogene Period - ResearchGate. Available from: http://www.researchgate.net/publication/236611297_The_Paleogene_Period [accessed Mar 23, 2015].

615 citations

Journal ArticleDOI
TL;DR: The history of the southern central Andes, including the world's second largest plateau and adjacent intermontane basins and ranges of the Eastern Cordillera and the northern Sierras Pampeanas of Argentina and Bolivia, impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the development of orographic barriers, and their influence on erosion and landscape evolution at various timescales as discussed by the authors.
Abstract: The history of the southern central Andes, including the world’s second largest plateau and adjacent intermontane basins and ranges of the Eastern Cordillera and the northern Sierras Pampeanas of Argentina and Bolivia, impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the development of orographic barriers, and their influence on erosion and landscape evolution at various timescales. Protracted aridity in the orogen interior has facilitated the creation and maintenance of the Puna-Altiplano plateau. Contraction and range uplift, filling of basins, and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior, which caused the eastward migration of deformation into the foreland and successive aridification. The uplift of the Andean orogen has also had a far-reaching influence on atmospheric and moisture-transport patterns in South America. This is documented by the onset of humid climate conditions on the eastern side of the Andes in late Miocene time, which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen, and changes in tectonic processes in the Andean orogenic wedge.

372 citations