A chart for judging the reliability of point counting results

Plas, L. van der; Tobi, A. C.

doi:10.2475/ajs.263.1.87

Cited by 463 publications

(66 citation statements)

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“…For each section 1600 points were counted. The standard deviation was determined according to the rules and nomogram of Van der Plas and Tobi (1965). These observations are reproduced raphically in Figs.…”

Section: The Three Concepts Applied To a Spenjic Soilmentioning

confidence: 96%

Micromorphological Quantification of Clay Illuviation

Miedema

Slager

1972

Journal of Soil Science

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Three concepts are introduced to describe the extent of clay illuviation phenomena quantitatively for pedogenic interpretations: (I) the degree of clay illuviation per thin section or (sub)horizon, subdivided into 5 classes, ranging from negligible (< 0.3 per cent by vol.) to very strong (> 7 per cent by vol.) clay illuviation; (2) the degree of reworking of the illuviated clay, subdivided into 3 classes, ranging from weak (< 30 per cent reworking) to strong (> 70 per cent);(3) the profile clay illuviation index: the s u m of products of clay illuviation percentage per (sub)horizon and horizon thickness (in cm). The index ranges from very low (< 50 per cent cm) to very high (> 700 per cent an). A distinction was made between the profile index based on the in situ illuviation features only and that based on all the illuviation features.

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Section: The Three Concepts Applied To a Spenjic Soilmentioning

confidence: 96%

Micromorphological Quantification of Clay Illuviation

Miedema

Slager

1972

Journal of Soil Science

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“…When converting 2-D data to a 3-D projection, however, the error depends on the stereological model used and is thus harder to estimate (i.e., particle shapes have to be assumed, Cashman, 1988). Absolute sampling errors related to non-representativeness were calculated for FOAMS and ImageJ analyses ( Table 2) using populationsample size relations (Van der Plas and Tobi, 1965). Error on equivalent diameter of clasts and vesicles is 5%, associated with "misrepresentation of 1 pixel in area" within a particle (Sahagian and Proussevitch, 1998;Shea et al, 2010).…”

Section: Uncertainties and Errors In Textural And Bulk Analysesmentioning

confidence: 99%

Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

et al. 2016

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The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behavior hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaitén, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H 2 O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ∼3-4 MPa are associated with fragmentation of foam and dense magma at ∼200-360 m depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of meters. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially Saubin et al.Chaitén Tuffisites Record Conduit Processes heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic magma and creates a complex textural collage within dense rhyolitic lava, in which neighboring fused clasts may have experienced vastly different degassing histories.

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“…Point counting was performed on thin sections of igneous rocks using a Prior J0415G point counter using the method described in Middleton et al (1985) and counting 1000 points per thin section. The 95 % confidence interval (2σ) calculated for any mineral phase is at maximum 1.6 % using the formula of van der Plas and Tobi (1965). Equal-area projections and contouring were produced using the open-source software Stereonet 9.2.0 applying the algorithms described in Allmendinger et al (2012) and Cardozo and Allmendinger (2013).…”

Section: Sampling Structural Analyses Microfabrics and Xrd Analysesmentioning

confidence: 99%

New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

Martha

Dörr

Gerdes

et al. 2016

Int J Earth Sci (Geol Rundsch)

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comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206 Pb/ 238 U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206 Pb/ 238 U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U-Pb ages and published K-Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206 Pb/ 238 U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during Abstract The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousiatype rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U-Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and Electronic supplementary material The online version of this article (1 3 the Oligocene to Early Miocene and change in stress field (NE-SW compression).

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A chart for judging the reliability of point counting results

Cited by 463 publications

References 0 publications

Micromorphological Quantification of Clay Illuviation

Micromorphological Quantification of Clay Illuviation

Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

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