Abstract
The use of mineral interfaces, in sand-sized rock fragments, to infer the influence exerted by mechanical durability on the generation of siliciclastic sediments, has been determined for plutoniclastic sand. Conversely, for volcaniclastic sand, it has received much less attention, and, to our knowledge, this is the first attempt to make use of the volcaniclastic interfacial modal mineralogy of epiclastic sandy fragments, to infer mechanical durability control at a modern beach environment. Volcaniclastic sand was collected along five beaches developed on five islands, of the southern Tyrrhenian Sea (Alicudi, Filicudi, Salina, Panarea and Stromboli) from the Aeolian Archipelago, and one sample was collected near the Stromboli Island volcanic crater. Each sample was sieved and thin sectioned for petrographic analysis. The modal mineralogy of the very coarse, coarse and medium sand fractions was determined by point-counting of the interfacial boundaries discriminating 36 types of interfaces categories, both no-isomineralic and/or no iso-structural (e.g., phenocrystal/glassy groundmass or phenocrystal/microlitic groundmass boundaries) and iso-mineralic interfaces, inside volcanic lithic grains with lathwork and porphyric textures. A total of 47,386 interfacial boundaries have been counted and, the most representative series of interfaces, from the highest to the lowest preservation, can be grouped as: a) ultrastable interfaces, categorized as Pl (Plagioclase)/Glgr (Glassy groundmass) > > Px (Pyroxene)/Glgr > > Ol (Olivine)/Glgr > > Op (Opaque)/Glgr > > Hbl (Hornblende)/Glgr> > Bt (Biotite)/Glgr > > Idd (Iddingsite)/Glgr > > Rt (Rutile) / Glgr; b) stable interfaces, categorized as Pl/Migr (Microlitic groundmass) > > Op/Migr > > Px/Migr > > Ol/Migr; c) moderately stable interfaces, categorized as Op/Px > > Op/Hbl > > Px/P > > Ol/Pl> > Bt/Op; and d) unstable interfaces, categorized as Pl/Pl > > Px/Px > > Ol/Ol > > Op/Op > > Hbl/Hbl > > Bt/Bt. Grains, eroded from the volcanic bedrock, if affected solely by abrasion, developed a rounded and smoothed form, with prevailing no-isostructural interfaces such as Plagioclase/Glassy groundmass, Pyroxene/Glassy groundmass and Olivine/Glassy groundmass interfaces. Grains that during transport suffered fracturing and percussion have a sharp and angular form: these combined transport mechanisms produce mainly volcanic sandy grains with iso-structural interfaces, such as Pl/Pl, Px/Px, Hbl/Hbl, and, to a lesser extent, Bt/Op and Bt/Glgr interfaces.
Highlights
IntroductionThe use of mineral interfaces, in sand-sized rock fragments, has been determined for plutoniclastic sand in first-order stream sand (e.g., Heins 1993, 1995), glacial sand (e.g., Caracciolo et al 2012), and in parent felsic plutonic and gneissic bedrock (e.g., Weltje et al 2018). Heins (1993, 1995) demonstrated that the textural parameters of parent rock represent the main control on rock-fragment abundance in modern felsic plutoniclastic stream sands derived from granodioritic plutons of the Cordilleras of the United States and Mexico
Heins (1993, 1995) demonstrated that the textural parameters of parent rock represent the main control on rock-fragment abundance in modern felsic plutoniclastic stream sands derived from granodioritic plutons of the Cordilleras of the United States and Mexico. These studies demonstrated that the textural parameters of the parent granodioritic source lithotypes represent the main control on rock-fragment abundance in modern plutoniclastic sand (Heins 1993), and that the types, and durability, of mineral interfaces, preserved inside the felsic sandy phanerites, are closely related to climate and topography of the source area (Heins 1995)
The petrographic analysis of the thin sections was used to determine the exact nature of the crystal/crystal interfaces, of the crystals/glassy groundmass interfaces, and of the crystals/microlitic groundmass explained in Table 1, crucial because the interface is the boundary where breakdown process occurs (e.g., Cather and Folk 1991; Heins 1993, 1995; Palomares and Arribas 1993; Caracciolo et al 2012; Weltje et al 2018)
Summary
The use of mineral interfaces, in sand-sized rock fragments, has been determined for plutoniclastic sand in first-order stream sand (e.g., Heins 1993, 1995), glacial sand (e.g., Caracciolo et al 2012), and in parent felsic plutonic and gneissic bedrock (e.g., Weltje et al 2018). Heins (1993, 1995) demonstrated that the textural parameters of parent rock represent the main control on rock-fragment abundance in modern felsic plutoniclastic stream sands derived from granodioritic plutons of the Cordilleras of the United States and Mexico. The interface analysis and the size-composition evolution of the rock fragment assemblage of the Swiss Alps plutoniclastic sand moraines (Caracciolo et al 2012) indicate that, in terms of stability, QQ > PlPl>QPl > PlK (where Q, Pl, and K stand for quartz, plagioclase, and Kfeldspar, respectively). These are the sandy-textured interfaces with the highest mechanical preservation potential, consistent with the assessment of Heins (1995) regarding the higher preservation potential of noisomineralic PlK and QK interfaces compared to isomineralic KK bonds. These are the sandy-textured interfaces with the highest mechanical preservation potential, consistent with the assessment of Heins (1995) regarding the higher preservation potential of noisomineralic PlK and QK interfaces compared to isomineralic KK bonds. Weltje et al (2018) parameterized crystal-interface frequencies in granitoids bedrock (granodiorite, granite, monzogranite and orthogneiss) outlining that a quantitative description of parent-rock texture has to be taken into account for regional sediment-generation studies
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