5:15 PM - 6:30 PM
[SVC31-P02] Crystal Fracture in the Youngest Toba Tuff (74 ka) Pumices
Keywords:YTT Pumices, Crystal fracture, Feldspar, Bubble, Textural study
Pumice clasts from the enormous eruption of the Youngest Toba Tuff (YTT) contain crystals that are fragmented. That special texture is commonly seen as a product of large caldera-forming eruptions, thus understanding the origin of it can help to reveal the factors that lead to the eruptions. This study focuses on white pumices from non-welded YTT, especially feldspar (plagioclase and alkali feldspar) as the most abundant crystal and is expected to record the dynamic processes of YTT super eruption.
YTT white pumices consist of transparent (TWP: mainly containing larger vesicles, > 0.1 mm) and non-transparent pumices (NTWP: mainly containing smaller vesicles, < 0.1 mm). We studied 25 thin sections of pumice clasts with optical microscope. Textural analysis was carried out using microphotographs taken by SEM and further analysed using image processing software to extract the number of crystal fragments, pheno-bubble number density (PBND), and matrix-bubble number density (MBND). Chemical analysis for feldspar chemistry was conducted using EPMA.
First, we define a fragmented crystal (hereafter “crystal”) as a region containing broken pieces that preserve the initial form as one single crystal. We tentatively classify the size of feldspar into two groups based on crystal size namely microphenocryst (< 1mm), and phenocryst (> 1mm). In addition, we quantified fractured-crystals by classifying them into two classes in terms of number of fragments included in a “crystal”: type A includes 2 – 9 fragments, and type B does more than 10 fragments. Then, we examined the correlation among the average number of fragments in one thin section with total individual crystals in the same thin section, MBND, PBND, and chemical composition (anorthite content).
YTT feldspar is mostly characterized by crystal fractures (58.6%), especially up to 85.8% in phenocryst. Broken melt inclusions and melt inclusions containing vapor bubbles are observed in some of the crystals, especially associated with type B crystals. TWP is dominated by crystal type A with low average of crystal fragments per one thin section (2 – 10.5) and low MBND (104 mm-3). On the other hand, NTWP is dominated by crystal type B, which shows various values of average crystal fragments per one thin section (2 – 67) and has high MBND (105 mm-3 – 107 mm-3). However, the PBND is not so much different between the two types of pumices. Furthermore, crystal type B exhibits slightly higher anorthite content (An35-45) compared to less fragmented crystal type A (An25-35), and occasionally crystal type B occurs with some disequilibrium textures like patchy zoning, reverse zoning, and hollow texture.
Crystal fracture in YTT is thought to be formed by several mechanisms. YTT, as a crystal-rich magma system, experienced intensive collision of crystals. Phenocrysts, compared with microphenocrysts, are characterized by high number of crystal fragments. This could be explained by their longer times in magma chamber, which expose them to various dynamic processes as implied by the disequilibrium texture. These processes facilitate the formation of the cleavage planes that accommodate the crystals to be broken. From the correlation between MBND and the crystal fragmentation class, it is found that higher decompression rates make crystals fractured more, whereas lower decompression rates make less fractured crystals. In addition, magma decompression causes the volatile explosion of melt inclusions trapped in the crystals, which forces the crystal to be broken into many pieces, as was observed in crystal type B. Moreover, crystals with high anorthite content are characterized by high average number of fragments, suggesting that the crystal composition influences the strength of crystals which affects the crystal fracture formation.
YTT white pumices consist of transparent (TWP: mainly containing larger vesicles, > 0.1 mm) and non-transparent pumices (NTWP: mainly containing smaller vesicles, < 0.1 mm). We studied 25 thin sections of pumice clasts with optical microscope. Textural analysis was carried out using microphotographs taken by SEM and further analysed using image processing software to extract the number of crystal fragments, pheno-bubble number density (PBND), and matrix-bubble number density (MBND). Chemical analysis for feldspar chemistry was conducted using EPMA.
First, we define a fragmented crystal (hereafter “crystal”) as a region containing broken pieces that preserve the initial form as one single crystal. We tentatively classify the size of feldspar into two groups based on crystal size namely microphenocryst (< 1mm), and phenocryst (> 1mm). In addition, we quantified fractured-crystals by classifying them into two classes in terms of number of fragments included in a “crystal”: type A includes 2 – 9 fragments, and type B does more than 10 fragments. Then, we examined the correlation among the average number of fragments in one thin section with total individual crystals in the same thin section, MBND, PBND, and chemical composition (anorthite content).
YTT feldspar is mostly characterized by crystal fractures (58.6%), especially up to 85.8% in phenocryst. Broken melt inclusions and melt inclusions containing vapor bubbles are observed in some of the crystals, especially associated with type B crystals. TWP is dominated by crystal type A with low average of crystal fragments per one thin section (2 – 10.5) and low MBND (104 mm-3). On the other hand, NTWP is dominated by crystal type B, which shows various values of average crystal fragments per one thin section (2 – 67) and has high MBND (105 mm-3 – 107 mm-3). However, the PBND is not so much different between the two types of pumices. Furthermore, crystal type B exhibits slightly higher anorthite content (An35-45) compared to less fragmented crystal type A (An25-35), and occasionally crystal type B occurs with some disequilibrium textures like patchy zoning, reverse zoning, and hollow texture.
Crystal fracture in YTT is thought to be formed by several mechanisms. YTT, as a crystal-rich magma system, experienced intensive collision of crystals. Phenocrysts, compared with microphenocrysts, are characterized by high number of crystal fragments. This could be explained by their longer times in magma chamber, which expose them to various dynamic processes as implied by the disequilibrium texture. These processes facilitate the formation of the cleavage planes that accommodate the crystals to be broken. From the correlation between MBND and the crystal fragmentation class, it is found that higher decompression rates make crystals fractured more, whereas lower decompression rates make less fractured crystals. In addition, magma decompression causes the volatile explosion of melt inclusions trapped in the crystals, which forces the crystal to be broken into many pieces, as was observed in crystal type B. Moreover, crystals with high anorthite content are characterized by high average number of fragments, suggesting that the crystal composition influences the strength of crystals which affects the crystal fracture formation.