Texture constraints on crystal size distribution methodology: An application to the Laki fissure eruption (original) (raw)
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Grain-size distribution of volcaniclastic rocks 1: A new technique based on functional stereology
Journal of Volcanology and Geothermal Research, 2012
The power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits. Grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits. However, the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record. Here we show that functional stereology can be used to obtain actual 3D volume fractions of clast populations from 2D cross-sectional images. Tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method. We show that the true grain size distribution is finer grained than its representation on a random 2D section. Our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes. We anticipate that the original grain-size distribution of welded ignimbrites can also be characterized by this method. Our method using functional stereology can be universally applied to any type of consolidated, weakly to non-deformed clastic material, regardless of grain size or age and therefore has a wide application in geology.
The origin of basaltic lava flow textures
2006
This study addresses the links between the surface morphologies, internal structure, and microtexture of basaltic lava flows. The AD 1783-84 Laki eruption in south Iceland produced a 600 km 2 basaltic lava flow-field dominated by rubbly pahoehoe surface morphology. Field observations and aerial photograph interpretation show that the lava surface gradually changed from spiny to slabby and then to rubbly along single flows through repetitive crust disruption at the active front. The rubble was compressed into ridges when lava advance was obstructed and during large lava surges that coincided with the opening of new eruptive fissures. Fluid lava was transported in an extensive network of tubes that formed within the flows. Petrological study of Laki near-vent tephra and lava surface samples shows that, during the eruption, the magma lost-1 wt. % of water during ascent, which induced melt undercooling and triggered groundmass crystallization. This caused an-10% anorthite gap across plagioclase phenocrysts and drove considerable microlite formation (up to 30 vol.%) in the early stages of flow. It is estimated that fluid lava was transported from the vent to the most distal active front, 60 km from the vent, with cooling rates of < 0.5°Clkm. Young lava flow-fields in the Reykjanes Peninsula (Iceland), and some flows from the flood basalts of the Columbia River Province (USA), have surface and internal structures intermediate between rubbly pahoehoe and 'a'a. The increasing size and decreasing number density of plagioclase microlites with increasing depth in these flows, as in the Laki lavas, indicate that solidification rates decreased sharply inwards. Differences between sections are attributed to variations in lava bulk composition and the transport mode, duration of emplacement, and interaction with surface water of the fluid lava. Pahoehoe and 'a'a have low and high plagioclase number densities respectively, with an inverse correlation with the average size and aspect ratio of plagioc1ases. Rubbly pahoehoe lavas have intermediate characteristics. This correlation between lava surface morphologies and plagioclase textural characteristics provides a tool that may be useful for inferring eruption and emplacement processes from textural measurements of flow interiors in ancient basaltic lava flow-fields. A rna mere et mon pere qui m'ont encouragee tout au long de ce travail. A rna sreur pour nos eclats de rire (te souviens-tu de mon volcan de sable?). A Antoine pour nos balades en France et en Angleterre. A tous mes fantastiques amis fran9ais (et belges) pour leur soutien outre-manche (Anne &
Journal of Physics: Conference Series, 2019
Mount Lasem is an extinct volcano in Central Java. Based on field work, five continuous central lava eruption sequences were identified. The rocks sampled from those lava units were classified as andesite which exhibit sign of magma mixing based on textural features from optical microscopy observation. With consideration of the importance of plagioclase in magma cooling reconstruction, a quantitative approach using crystal size distribution (CSD) of plagioclase phenocryst (long axis >=0.5 mm) and microphenocryst (long axis 0.1 – <0.5 mm) from five lava units was conducted. This method, which has never been applied to the eruption products of this volcano, revealed variation of magma resident time during the continuous central eruption phase of Mount Lasem. By using G=10-10 mm/s (growth rate) for plagioclase phenocryst and microphenocryst, residence times of the magma for both populations were predicted at 63 – 120 years and 8 – 18 years respectively. This results show that the...
Quantitative analysis of X-ray microtomography images of geomaterials: Application to volcanic rocks
Geosphere, 2010
X-ray computed microtomography is an excellent tool for the three-dimensional analysis of rock microstructure. Digital images are acquired, visualized, and processed to identify and measure several discrete features and constituents of rock samples, by means of mathematical algorithms and computational methods. In this paper, we present digital images of volcanic rocks collected with X-ray computed microtomography techniques and studied by means of a software library, called Pore3D, custom-implemented at the Elettra Synchrotron Light Laboratory of Trieste (Italy). Using the Pore3D software, we analyzed the fabrics and we quantifi ed the characteristics of the main constituents (vesicles, crystals, and glassy matrix) of four different types of pyroclasts: frothy pumice, tube pumice, scoria , and "crystalline" scoria. We identifi ed the distinctive features of these different types of volcanic rocks. The frothy pumices show vesicles that coalesce in isotropic aggregates, especially toward the sample interior, while the scoriae have a low porosity and an abundance of isolated vesicles. In the "crystalline" scoria sample most of the vesicle separation is due to the presence of crystals of different types, while the tube pumice shows an anisotropic distribution of vesicles and crystals at the microscale, as also observed at the scale of the hand sample. Quantitative analysis and textural information may supply an additional tool to investigate the eruptive processes and the origin of volcanic rocks.
Contributions to Mineralogy and Petrology
Volcanic rocks commonly display complex textures acquired both in the magma reservoir and during ascent to the surface. While variations in mineral compositions, sizes and number densities are routinely analysed to reconstruct pre-eruptive magmatic histories, crystal shapes are often assumed to be constant, despite experimental evidence for the sensitivity of crystal habit to magmatic conditions. Here, we develop a new program (ShapeCalc) to calculate 3D shapes from 2D crystal intersection data and apply it to study variations of crystal shape with size for plagioclase microlites (l < 100 µm) in intermediate volcanic rocks. The smallest crystals tend to exhibit prismatic 3D shapes, whereas larger crystals (l > 5–10 µm) show progressively more tabular habits. Crystal growth modelling and experimental constraints indicate that this trend reflects shape evolution during plagioclase growth, with initial growth as prismatic rods and subsequent preferential overgrowth of the interme...
Determining volcanic eruption styles on Earth and Mars from crystallinity measurements
Nature Communications, 2014
Both Earth and Mars possess different styles of explosive basaltic volcanism. Distinguishing phreatomagmatic eruptions, driven by magma-water interaction, from 'magmatic' explosive eruptions (that is, strombolian and plinian eruptions) is important for determining the presence of near-surface water or ice at the time of volcanism. Here we show that eruption styles can be broadly identified by relative variations in groundmass or bulk crystallinity determined by X-ray diffraction. Terrestrial analogue results indicate that rapidly quenched phreatomagmatic ejecta display lower groundmass crystallinity (o35%) than slower cooling ejecta from strombolian or plinian eruptions (440%). Numerical modelling suggests Martian plinian eruptive plumes moderate cooling, allowing 20-30% syn-eruptive crystallization, and thus reduce the distinction between eruption styles on Mars. Analysis of Mars Curiosity rover CheMin X-ray diffraction results from Gale crater indicate that the crystallinity of Martian sediment (52-54%) is similar to pyroclastic rocks from Gusev crater, Mars, and consistent with widespread distribution of basaltic strombolian or plinian volcanic ejecta.
Multitemporal three dimensional imaging of volcanic products on the macro-and micro-scale
1] Textural characteristics of recently emplaced volcanic materials provide information on the degassing history, volatile content, and future explosive activity of volcanoes. Thermal infrared (TIR) remote sensing has been used to derive the micron-scale roughness (i.e., surface vesicularity) of lavas using a two-component (glass plus blackbody) spectral deconvolution model. We apply and test this approach on TIR data of pyroclastic flow (PF) deposits for the first time. Samples from two PF deposits (January 2005: block-rich and March 2000: ash-rich) were collected at Bezymianny Volcano (Russia) and analyzed using (1) TIR emission spectroscopy, (2) scanning electron microscope (SEM)-derived roughness (profiling), (3) SEM-derived surface vesicularity (imaging), and (4) thin section observations. Results from SEM roughness (0.9-2.8 mm) and SEM vesicularity (18-26%) showed a positive correlation. These were compared to the deconvolution results from the laboratory and spaceborne spectra, as well as to field-derived percentages of the block and ash. The spaceborne results were within 5% of the laboratory results and showed a positive correlation. However, a negative correlation between the SEM and spectral results was observed and was likely due to a combination of factors; an incorrect glass end-member, particle size effects, and subsequent weathering/reworking of the PF deposits. Despite these differences, this work shows that microscopic textural heterogeneities on PF deposits can be detected with TIR remote sensing using a technique similar to that used for lavas, but the results must be carefully interpreted. If applied correctly, it could be an important tool to map recent PF deposits and infer the causative eruption style/mechanism. Citation: Carter, A. J., M. S. Ramsey, A. J. Durant, I. P. Skilling, and A. Wolfe (2009), Micron-scale roughness of volcanic surfaces from thermal infrared spectroscopy and scanning electron microscopy,
2004
A knowledge of the distribution of crystal sizes and shapes can prove critical to an understanding of the emplacement and cooling history of igneous rocks. We outline here a relatively automated method, which uses standard image processing techniques to acquire size, shape and modal data for olivine phenocrysts and voids in a suite of picritic Hawaiian basalts. The technique requires nothing more than a petrographic microscope fitted with a digital camera, and a PC which will run off-the-shelf image processing software. The method could be adapted to a variety of petrographic problems and its use need not be restricted to the study of igneous rocks.