• The Journal of the Petrological Society of Korea
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• v.20 no.1
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• pp.61-75
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• 2011

Mantle-derived xenoliths, corresponding to spinel harzburgite and lherzolite in alkali basalts from Jeju Island, are metasomatized to various extents. They contain distinctive secondary orthopyroxene, forming corona or poikilitic textures. It clearly indicate that this secondary orthopyroxene has been produced at the expense of olivine along the grain boundaries and margins, suggesting silica-enriched metasomatic melt infiltrated through grain boundaries. Based on the geotectonic characteristics of Jeju Island and textural characteristics and major elements composition of mantle xenoliths, it is suggested that the silica-enriched melt/fluid could have derived from the ancient subducted slab, possibly in the mantle wedge, implying that the high $SiO_2$ activity in the lithospheric upper mantle beneath Jeju Island at that time.

• The Journal of the Petrological Society of Korea
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• v.4 no.2
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• pp.104-123
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• 1995

Mantle xenoliths in alkali basalt from Boun, Gansung area, and Baegryung island in S. Korea are spinel lherzolites composed of olivine, orthopyroxene, clinopyroxene, and spinel. The xenoliths generally show triple junctions among grams, kink-banding in olivine and pyroxenes, and protogranular and eqigranular textures having m orlentatron of specific direction. Anhedral brown spinels are disseminated in the intergranular spaces of minerals. Mineral compositions are very homogeneous without compositional zonation from rim to core in grains regardless different locahties. Olivine shows Fo. component of 89.0-90.2 and low CaO of 0.03-0.12wt%, orthopyroxene is enstatite with En component of 89.0 - 90.0 and $Al_2O_3$ of 4-5wt%, and clinopyroxene is diopside having En. component of 47.2-49.1 and $Al_2O_3$ of 7.42-7.64wt% from Boun and 4.70-4.91wt% from Baegryung showing local variation. Spinel shows the distinctive negative trend with increasing of A1 and decreasing of Cr, and Mg value and Cr number are 75.1-81.9 and 8.5-12.6, respectively. To estlmate T and P for these mantle xenoliths pyroxene-geothermometers (Wood and Banno, 1973; Wells, 1977; Mercier, 1980; Sachtleben and Seck, 1981; Bertrand and Mercier, 1985; Brey and Kohler, 1990) and Al-solubility geobarometer (Mercier, 1980; Lane and Ganguly, 1980) are used. Temperatures of Mercier (1980) and Sachtleben and Seck (1981) are compatible and equilibrium temperatures of xenoliths, average value of these two, aiie from $970^{\circ}C$ to $1020^{\circ}C$, and equihbrium pressures derived from Mercier (1980) are in the range of 12-19 Kb (42-63 Km). These temperatures and pressures seem to be reasonble wlth the consideration of Al-isopleths in MAS system (Lane and Ganguly, 1980) and Fe effect on Al-solubility in orthopyroxene (Lee and Ganguly, 1988). Equllibrium of temperatures and pressures of xenoliths in P-T space belong to ocenanic geothem among the Mercier's mantle geotherms (1980) and are completely different from continental geotherms of S. Africa (Lesotho) and S. India having different geologcal ages. anera1 compositions of spmel-lherzohtes in S. Korea and eastern China are primitwe and paleogeothems of both are very s~mllar, but degrees of depletion of upper mantle could be locally different from each other since eastern China has various depleted xenoliths due to different degrees of partial melting.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2000.05a
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• pp.19-19
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• 2000

;Ultramafic xenoliths found in alkali basalts from Jeju-do, Korea are mostly spinel Iherzolites composed of olivine, orthopyroxene, clinopyroxene and spinel. A subordinate amount of spinel harzburgites and pyroxenites are also found. Temperatures for these xenoliths were estimated from the compositions of coexisting pyroxenes (Wood '||'&'||' Banno 1973; Wells 1977; Bertrand '||'&'||' Mercier 1985; Brey '||'&'||' Kohler 1990), the AI-solubility in orthopyroxene coexisting with olivine and spinel (Sachtleben '||'&'||' Seck 1981; Webb '||'&'||' Wood 1986), and from Fe/Mg partitioning between olivine and spinel (Ballhaus et al. 1991). Temperature estimates from the thermometers by Wells (1977) and Brey and Kohler (1990) are compatible. Average equilibrium temperatures by these two methods for spinel peridotites range from 890 to 1030$^{\circ}$C. Pressures for spinel peridotites were estimated from the geobarometer by Kohler and Brey (1991) derived from the equilibrium Ca content of olivine coexisting with clinopyroxene, and fall within the range of 12.9 to 26.3 kbar. The combination of the thermometer by Brey and Kohler (1990) and the geobarometer by Kohler and Brey (1991) yields P- T estimates for Jeju-do spinel peridotites that fall in experimentally determined spinel lherzolite field in CFMASCr system (O'Neill 1981). These P-T data sets have been used to construct the Quaternary Jeju-do geotherm, which is significantly different from the conventional conductive geotherm. The xenolith-derived geotherm has a higher T gradient at low P (13 kbar) than at high P, which may be due to perturbation of the conductive heat flow by magma underplating or overplating at the crust-mantle boundary. Temperature estimates and statistics on the xenoliths indicate that the crust/mantle boundary in Jeju-do lies at about 11 kbar (~39 km). Spinellherzolite is inferred as a main constituent rock of the uppermost lithospheric mantle beneath Jeju-do. Pyroxenites were intercalated in peridotites in similar depth and temperature as re-equilibrated veins or lens.

• Journal of the Mineralogical Society of Korea
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• v.18 no.1
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• pp.61-72
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• 2005

Modal and chemical compositions of clinopyroxnes in spinel peridotites from the Baegryeong Island and the Boeun, enclosed in Miocene alkali basalt, are important for understanding the pre-eruptive temperature condition and chemical processes such as mantle depletion and enrichment. All spinel peridotites show transitional texture between protogranular and porphyroclastic textures. Temperature ranges of spinel peridotites from the Baegryeong Island and the Boeun at 15 kb are 773∼1188℃ and 705∼1106℃, respectively. The spinel peridotites from the Baegryeong Island and the Boeun have undergone the 1∼10% and 1∼4% fractional melting, which were determined by using primitive mantle-normalized Y and Yb of clinopyroxenes. LREE enrichment patterns of clinopyroxene indicate that these rocks from both areas have undergone cryptic mantle metasomatism without new minerals.

• Journal of the Mineralogical Society of Korea
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• v.18 no.1
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• pp.33-43
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• 2005

Three types of clinopyroxenes in alkali basaltic rocks from Jeju Island can be identified on the basis of geochemical and textural data. Type Ⅰ is Cr-rich diopside in spinel peridotites from the upper mantle. Type Ⅱ is augite in fine-grained pyroxenites which are possibly either magmatic vein or metamorphic segregations owing to anatexis of the upper mantle. The augite of Type Ⅱ contains high Ca and Mg and relatively low Ti. Type Ⅲ is thought to be either cumulates or cognate phenocrysts and can be subdivided into Ⅲa, Ⅲb, and Ⅲc based on their occurrence mode. Clinopyroxenes of Type Ⅰ have the highest Mg# and Si and the lowest Ti, whereas those of Type Ⅲhave lower Mg#와 Si and higher Ti. These geochemical characteristics indicate that (Ti+Al/sup Ⅵ/)/Si and Al/sup Ⅵ//Al/sup Ⅵ/ increase from Type Ⅰ to Type Ⅲ. It is possibly interpreted that Type Ⅰ is of the highest pressure origin and Type Ⅲ of the lowest. Fractionation of high-pressure clinopyroxenes would result in evolved undersaturated alkali-enriched liquids, probably producing the alkali-enriched host basaltic rocks in Jeju Island.

• The Journal of the Petrological Society of Korea
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• v.13 no.1
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• pp.33-45
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• 2004

• 한국지구물리탐사학회:학술대회논문집
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• 2009.10a
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• pp.127-127
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• 2009

• Korean Journal of Mineralogy and Petrology
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• v.36 no.4
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• pp.365-376
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• 2023

Songaksan, formed about 3800 year ago, is one of the tuff rings in the Jeju Island. Mantle xenoliths, spinel peridotites, are enclosed in the Songaksan Trachybasalt. The spinel peridotites are less than 2 cm in size and are composed of olivine, orthopyroxene, clinopyroxene, and spinel. The uniform compositions of the minerals from core to rim indicate that equilibrium was reached in the spinel peridotites before these were enclosed in the host magma. The spinel peridotites originated at depths between 55 and 60 km with equilibrium temperatures ranging from 915 to 968℃. The spinel peridotites from Songaksan reveal porphyroclastic texture with a lot of neoblast minerals. Olivines display strong kink banding, indicating that the upper mantle of Songaksan has been deformed. The spinel peridotites from Songaksan have undergone about 5~7% fractional melting, and cryptic metasomatism by an silicate melt. The period of entrainment and transport of the spinel peridotites in the host magma is about 15 days.

• The Journal of the Petrological Society of Korea
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• v.21 no.1
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• pp.13-30
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• 2012

Phlogopite and kaersutite, showing distinctively different textural characteristics compared to the common phenocrysts, are observed in alkali basalt from Jeju Island. They occur as large crystals (2-10 mm) in host basalts, whereas fine-grained phlogopite and kaersutite occur in ultramafic mantle xenoliths and mafic gabbroic xenoliths, respectively, as an interstitial and microvein phases, or in corona textures (<1 mm). This textural characteristics of fine-grained grains clearly indicates secondary in origin. Phlogopite contains high $TiO_2$(4.1-6.9 wt%) and F(2.8-4.6 wt%) and relatively high mg#[=100Mg/(Mg+$Fe^t$) in mols, where $Fe^t$ is total iron](88-80), whereas kaersutite has high $TiO_2$(5.6-6.11 wt%) and much lower mg#s(68-64). Our textural observations and the geochemical character of these hydrous minerals suggest that they were unrelated to each other and mica formation happened early in the upper mantle before the mantle xenoliths had been trapped. In contrast, kaersutite formation has happened later, probably during the late stage of crystallization as intracrustal processes. The presence of phlogopite and kaersutitic amphibole is a direct evidence for K-, Ti-, F- and $H_2O$-bearing fluid/melt percolation in the lithosphere beneath Jeju Island, indicating that they are product of interaction between host rock/peridotite/fluid-melt. Thus, the upper mantle/lower crust beneath Jeju Island are metasomatized to various extents, characterized by a change in major metasomatic hydrous minerals from phlogopite to amphibole with decreasing depth.

• Journal of the Korean earth science society
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• v.34 no.2
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• pp.109-119
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• 2013

Duibaejae basalts from Goseong, Gangwon-do, are divided into the lower basalt and the upper basalt depending on the properties, such as occurrence, mineral compositions, and major and trace compositions of the basalts. The lower basalts have characteristics of agglomerate rocks as well as contain, crustal and mantle xenoliths, and olivine, pyroxene, and plagioclase xenocrysts. The upper basalts with columnar joints contain relatively more mantle xenolith and olivine xenocryst than the lower basalts. The major and trace element compositions suggest that the composition of the upper basalts is close to primary magma composition. Enrichment and depletion patterns of the trace and the rare-earth elements of the lower basalts are similar to those of the upper basalts, whereas the lower basalts are more LREE enriched than the upper basalts. The source magmas of the lower and upper basalts from Duibaejae volcanic edifice were generated from about 0.8-1.2% and 3.7-4.0% batch melting of garnet peridotite, respectively. The abundance of granite xenolith, and plagioclase and quartz xenocrysts with reaction rim indicates that the lower basalts, compared with upper basalts, might have been assimilated with the crustal materials during ascending to surface.