• Title/Summary/Keyword: microlite

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Detailed Morphology and Texture of Microlites in Obsidian observed through Electron Microscopy (전자현미경을 이용한 흑요석 내 미세결정의 형태와 조직 관찰)

  • Jwa, Yong-Joo;Hwang, Ga-hyun
    • Journal of the Korean earth science society
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    • v.39 no.6
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    • pp.568-574
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    • 2018
  • Microlite is crystallized under the supercooled condition of volcanic magma. This study reports the morphology and texture of microlites included in the obsidians from the Baekdusan (Mt. Baekdu), Kyushu (Japan), Hokkaido (Japan) and Lipari Island (Italy) by using a scanning electron microscope (SEM). Morphology and texture observed from the back scattered electron (BSE) image could give more detailed information on the microlite crystallization which cannot be acquired from the conventional optical microscope. Ten types of microlites are newly described according to the 14 morphological types of microlites by Clark (1961): Lath, Crenulite, Bacillite, Margarite, Belonites, Trichites, Arculites, Furculite, Scopulites, Scopulitic growth. Detailed description of the ten microlites can be used to interpret the crystallization and paragenetic relations of crystals during the cooling of acidic volcanic magma.

Interpretion of Transition between Explosive and Effusive Eruptions from Microlite Textural Analyses in the Albong Lava Dome, Ulleung Island, Korea (울릉도 알봉 용암돔의 미정 조직분석으로부터 폭발성 및 분류성 분출 간의 전환 해석)

  • Hwang, Sang Koo;Kim, Ki Beom;Son, Young Woo;Hyeon, Hye Weon
    • Economic and Environmental Geology
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    • v.53 no.5
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    • pp.553-564
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    • 2020
  • Transition between explosive and effusive eruption in Ulleung Island is observed in the Nari Scoria Deposits and Albong Trachyandesite (lava dome) origined by dome-building eruption and may be related to factors such as magma influx, ascent rate and degassing. However, the interpretation of them has not been resolved yet because the interaction between these factors is not complex but also the resulting behaviour during eruption is unpredictable. This paper focuses on the explosive and effusive activity perceived during building the Albong lava dome in Nari caldera. Samples were collected along with time from the scoria deposits and lava dome, linked to eruption stage and style of activity. Textures of groundmass feldspar microlites from these samples are quantitatively analyzed, including measurements of areal number density, mean microlite size, crystal aspect ratio, groundmass crystallinity and crystal size. The microlite textures show that shallow pre- and syn-eruptive magmatic processes acted to govern the changing behaviour during the eruption. Transition between explosive and effusive eruption was driven by the dynamics of magma ascent in the conduit, with degassing and crystallisation acting via feedback mechanisms, resulting in a cycle of effusive and explosive eruption.

Eruption Types and Textures of Pyroclastics from the Jugam Scoria Deposits, Ulleung Island, Korea (울릉도 죽암분석층에서 나온 화성쇄설물들의 조직과 분화유형)

  • Hwang, Sang Koo;Ahn, Ung San;Lee, So Jin;Oh, Kyung Sik
    • Economic and Environmental Geology
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    • v.52 no.5
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    • pp.459-469
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    • 2019
  • We present a quantitative evaluation of density, vesicularity and microtextures for coarse lapilli collected from the Jugam Scoria Deposits, northeastern Ulleung Island. Lapilli from the deposits have modal vesicularities of 61% in the lower part and 67% in the upper part, and vesicle populations dominated by non-interconnected subround vesicles. Clasts of modal vesicularity have margin-parallel zonation, with subaerially quenched rims interpreted to preserve "syn-fragmentation" magmatic textures in microlite-free sideromelane rims, grading "post-fragmentation" tachylitic interiors with vesicle and microlite textures that progressively coarsen from rim to interior. Degassing scenarios are linked to syn-fragmentation vesicle textures to demonstrate that the magmas degassed in dominantly closed systems. And diffusion-controlled cooling rates of trachyandesitic pyroclasts in contact with atmosphere are linked to post-fragmentation evolution of vesicle and microlite textures to infer about transportation and dispersal of the pyroclasts in low shooting jets. These textural analyses show that the Jugam eruptions were strictly applied to the strombolian type, analogous to the hawaiian type among any classical subaerial eruption type.

Morphology and Texture of Microlites in the Baekdusan and Kyushu Obsidians (백두산과 규슈 흑요석 내 미세결정의 형태와 조직)

  • Hwang, Ga-hyun;Jwa, Yong-Joo
    • Journal of the Korean earth science society
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    • v.38 no.7
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    • pp.546-551
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    • 2017
  • We examine the morphology and texture of microlites included in the obsidians from the Baekdusan (Mt. Baekdu) and Kyushu (SW Japan) by using an electron microscope. According to the morphological types of microlites, the microlites in the Baekdusan obsidians are classified as Arculites, Asteroidal and Crenulite, whereas those in the Kyushu obsidians as various types of Arculites, Bacillite, Belonites, Crenulite, Furculite, Lath, Margarite, and Scopulites. Specifically, the Arculites in both obsidians show very distinctive crystal relationships, though they are mainly composed of magnetite and pyroxene. The Baekdusan Arculites exhibit the poikilitic texture of pyroxene crystals enclosing multiple magnetite crystals, whereas the Kyushu ones demonstrate the granular and/or intergranular texture of interlocking between magnetite and pyroxene crystals. This distinction can be used to discriminate the provenance of the obsidian artefacts from the prehistoric sites in the Korean Peninsula.

Mineral Geochemistry of the Albite-Spodumene Pegmatite in the Boam Deposit, Uljin (울진 보암광산의 조장석-스포듀민 페그마타이트의 광물 지화학 조성 연구)

  • Park, Gyuseung;Park, Jung-Woo;Heo, Chul-Ho
    • Korean Journal of Mineralogy and Petrology
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    • v.35 no.3
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    • pp.283-298
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    • 2022
  • In this study, we investigated the mineral geochemistry of the albite-spodumene pegmatite, associated exogreisen, and wall rock from the Boam Li deposit, Wangpiri, Uljin, Gyeongsangbuk-do, South Korea. The paragenesis of the Boam Li deposit consists of two stages; the magmatic and endogreisen stages. In the magmatic stage, pegmatite dikes mainly composed of spodumene, albite, quartz, and K-feldspar intruded into the Janggun limestone formation. In the following endogreisen stage, the secondary fine-grained albite along with muscovite, apatite, beryl, CGM(columbite group mineral), microlite, and cassiterite were precipitated and partly replaced the magmatic stage minerals. Exogreisen composed of tourmaline, quartz, and muscovite develops along the contact between the pegmatite dike and wall rock. The Cs contents of beryl and muscovite and Ta/(Nb+Ta) ratio of CGM are higher in the endogreisen stage than the magmatic stage, suggesting the involvement of the more evolved melts in the greisenization than in the magmatic stage. Florine-rich and Cl-poor apatite infer that the parental magma is likely derived from metasedimentary rock (S-type granite). P2O5 contents of albite in the endogreisen stage are below the detection limit of EDS while those of albite in the magmatic stage are 0.28 wt.% on average. The lower P2O5 contents of the former albite can be attributed to apatite and microlite precipitation during the endogreisen stage. Calcium introduced from the adjacent Janggun formation may have induced apatite crystallization. The interaction between the pegmatite and Janggun limestone is consistent with the gradual increase in Ca and other divalent cations and decrease in Al from the core to the rim of tourmaline in the exogreisen.

Mineralogical Study on Microlites in the Baekdusan Obsidian and the Gadeokdo Obsidian Artefacts (백두산 흑요석과 가덕도 흑요석제 석기에 포함된 미세결정에 대한 광물학적 연구)

  • Jin, Mi Eun;Moon, Sung Woo;Ryu, Choon Kil;Jwa, Yong-Joo
    • Journal of the Mineralogical Society of Korea
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    • v.27 no.4
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    • pp.243-249
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    • 2014
  • We conducted investigations into the morphological characterstics and chemical compositions of the microlites included in the obsidians from the North Korean side Baekdusan (NK) and in the obsidian artefacts from the Busan Gadeokdo prehistoric site (BG). The morphology of the microlites from the NK can be divided into three groups, and trichite-acicular and trichite-asteroidal types are predominant. On the other hand the microlites from the BG show various morphological features such as acicular, margarite, lath, and asteroidal types. Silicate microlites from the NK are mostly of Ca-pyroxene compositions (hedenbergite to augite), whereas those from the BG are mostly of biotite (annite) with small amount of clinopyroxene (ferrosilite). It is well understood that the microlites from the NK are different from those from the BG in terms of the morphology, mineral content and mineral composition. Thus the obsidian aretefacts from the Busan Gadeokdo prehistoric site seem to have no genetical relationship to the obsidians from the Baekdusan.

Crystal Populations Within a Porphyritic Dike in Ulleung Island: Are All Clinopyroxenes in the Stage I Dodong Basalt Sectored? (울릉도 반정질 암맥의 결정군집: 화산단계 I 도동현무암의 모든 단사휘석은 섹터누대를 가지는가?)

  • Munkhbayar, Enkhjin;Park, Jongkyu;Jang, Yun-Deuk
    • Korean Journal of Mineralogy and Petrology
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    • v.34 no.4
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    • pp.277-291
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    • 2021
  • A mineral phase contained in a rock sample can be categorized into several crystal populations with distinct origins and crystal growth histories. A detailed textural and mineralogical investigation on a clinopyroxene megacryst-bearing porphyritic dike with glassy chilled margins (tachylyte) in the southeastern Ulleung Island was conducted to decipher its crystal populations. During this study, we have measured glass and mineral major element compositions using electron microprobe analyzer. Tachylyte has a homogeneous trachyandesitic composition without any significant alteration characteristics, suggesting that its composition may represent the original melt composition of the dike. Clinopyroxene and plagioclase larger than 0.5 mm are antecrysts equilibrated with a more primitive melt composition than trachyandesitic tachylyte. Meanwhile, clinopyroxene and plagioclase microlites (<0.5mm) are regarded as primocrysts crystallized in-situ. According to our results, all clinopyroxenes in the Stage I Dodong Basalt are sectored into basal and prismatic parts, whose compositional ranges systematically vary: [Mg+Si+Fe]basal ↔ [Al+Ti+Na]prism. Therefore, we suggest that the effect of the elemental partitioning caused by the clinopyroxene sector zonation in Stage I volcanism should be considered in the future works.