• The Journal of the Petrological Society of Korea
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• v.19 no.3
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• pp.227-244
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• 2010

Abundant spinel lherzolite xenoliths showing distinctively different textural types such as protogranular, porphyroclastic, and mylonitic texture are trapped in the basaltic rocks from southeastern part of Jeju Island. These xenoliths show the textural spectrum from coarse-grained protogranular through porphyroclastic with bimodal grain size to fine-grained and foliated mylonitic texture. They tend to decrease in grain sizes and show more linear grain boundaries and more frequent triple junctions from protogranular through porphyroclastic to mylonitic. Spinel has different occurrence mode according the textural type. Spinel is always associated with orthopyroxene in protogranular texture, whereas it is scattered and independent of orthopyroxene in mylonitic texture. Additionally, porphyroblast from porphyroclastic and mylonitic textures has internal deformation features such as kink band, undulatory extinction and curved lamella, whereas neoblast is strain-free. These textural features indicate increasing degree of static/dynamic recrystallization from protogranular through porphyroclastic to mylonitic texture. The mg#[$=100{\times}Mg/(Mg+Fe_t)$] of olivine, orthopyroxene and clinopyroxene is relatively constant (ol: 88-91; opx: 89-92; cpx: 89-92) regardless of textural differences. The mg# of constituent minerals, NiO content (0.3~0.4 wt%) and MnO content (0.1~0.2 wt%) of olivine are similar to those of mantle xenoliths worldwide, also indicating that studied spinel lherzolite xenoliths were mantle residues having experienced 20~25% partial melting. The geochemical and textural characteristics have close relations showing that LREE and incompatible trace elements content of orthopyroxene and clinopyroxene increases from protogranular through porphyroclastic to mylonitic. These observations suggest that the studied mantle xenoliths experienced metasomatism by LREE enriched melt or fluid after partial melting, indicating a close relation between deformation and metasomatism. The metasomatism was possibly confined to narrow shear zones from where porphyroclastic and mylonitic textured xenoliths originated. These shear zones might favorably drive the percolation of LREE-enriched melts/fluids responsible for the metasomatism in the lithospheric mantle below the Jeju Island.

• The Journal of the Petrological Society of Korea
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• v.25 no.1
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• pp.39-50
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• 2016

Negative crystal shaped $CO_2$-rich fluid inclusions, trapped as primary inclusions in neoblasts and as secondary inclusions in porphyroblasts, were studied in spinel peridotite xenoliths from Jeju Island. Based on microthermometric experiments, the solid phase melts at $-57.1^{\circ}C$(${\pm}0.9^{\circ}C$) with no other observable melting events, indicating that the trapped fluid is mostly $CO_2$. The homogenization temperatures show a much wider range from $-39^{\circ}C$(${\rho}=1.12g/cm^{3)}$) to $23^{\circ}C$(${\rho}=0.82g/cm^{3)}$), suggesting that most of the inclusions (originally trapped at mantle conditions) re-equilibrated to lower density values. Nevertheless, the highest density $CO_2$ in our fluid inclusions is consistent with entrapment of fluids at upper mantle pressures (and depths). The calculated trapping pressure from $CO_2$-rich fluid inclusions that appear to be free from re-equilibrium, e.g., showing the lowest homogenization temperatures, is ${\approx}0.9GPa$. Based on the petrographic evidences, the fluid entrapment can be regarded as a late stage event in the evolution of the shallow lithospheric mantle.

• The Journal of the Petrological Society of Korea
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• v.13 no.1
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• pp.34-45
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• 2004

This paper describes the textural relations of mantle xenoliths and fluid inclusions in mantle-derived rocks found in alkaline basalts from Jeju Island which contain abundant ultramafic, felsic, and cumulate xenoliths. Most of the ultramafic xenoliths are spinel-lherzolites, composed of olivine, orthopyroxene, clinopyroxene and spinel. The felsic xenoliths considered as partially molten buchites consist of quartz and plagioclase with black veinlets, which are the product of ultrahigh-temperature metamorphism of lower crustal materials. The cumulate xenoliths, clinopyroxene-rich or clinopyroxene megacrysts, are also present. Textural examination of these xenoliths reveals that the xenoliths are typically coarse grained with metamorphic characteristics, testifying to a complex history of evolution of the lower crust/upper mantle source region. The ultramafic xenoliths contain protogranular, porphyroclastic and equigranular textures with annealing features, indicating the presence of shear regime in upper mantle of the Island. The preferential associations of spinel and olivine with large orthopyroxenes suggest a previous high temperature equilibrium in the high-Al field and the original rock-type was a Al-rich orthopyroxene-bearing peridotite without garnet. Three types of fluid inclusions trapped in mantle-derived xenoliths include CO$_2$-rich fluid (Type I), multiphase silicate melt (glass ${\pm}$ devitrified crystals ${\pm}$ one or more daughter crystals ＋ one or more vapor bubbles) (Type II), and sulfide (melt) inclusions (Type III). C$_2$-rich inclusions are the most abundant volatile species in mantle xenoliths, supporting the presence of a separate CO$_2$-rich phase. These CO$_2$-rich inclusions are spatially associated with silicate and sulfide melts, suggesting immiscibility between them. Most multiphase silicate melt inclusions contain considerable amount of silicic glass. reflecting the formation of silicic melts in the lower crust/upper mantle. Combining fluid and melt inclusion data with conventional petrological and geochemical information will help to constrain the fluid regime, fluid-melt-mineral interaction processes in the mantle of the Korean Peninsula and pressure-temperature history of the host xenoliths in future studies.

• Korean Journal of Mineralogy and Petrology
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• v.36 no.4
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• pp.323-336
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• 2023

We investigated the nickel potential and genesis of ultramafic rocks in the Yugu area to secure nickel resources in South Korea. The Yugu ultramafic rocks, located in the southwest of the Gyeonggi Massif, are characterized by spinel peridotite and exhibit strong serpentinization along their boundaries. The serpentinization is observed as olivine transformed to antigorite and chrysotile, while pentlandite, the nickel sulfide mineral, altered into millerite and awaruite. Serpentine displays distinct foliation, aligning subparallel to the ultramafic rock boundaries and foliation of Yugu gneiss. This suggests that the uplift of ultramafic rocks resulted in hydrothermal infiltration likely sourced from the Yugu gneiss metamorphism. The Yugu ultramafic rocks are residues after 5~18% partial melting of abyssal peridotite. Enriched light rare earth elements and Eu imply secondary metasomatism. Geochemistry suggests a link between the formation of Yugu ultramafic rock and the Triassic collision of the North and South China continents. The nickel content is around 0.17~0.21%, mainly contained in olivine and serpentine. Hence, in addition to the mineral processing study on the sulfide minerals, focused studies on oxide minerals for enhanced nickel recovery within the Yugu ultramafic rock are strongly suggested.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2001.06a
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• pp.75-75
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• 2001

We report Sr, Nd and Pb isotope data of clinopyroxene separates from ultramafic xenoliths and their host basaltic rocks in Jeju Island, Baekryeong Island, Boeun and Ganseong, Korea. The isotopic data of the xenoliths and host basalts are distinctly different from those of Korean basement rocks. Except for two xenoliths from Ganseong, all samples in this study have isotopic ratios within the combined range of MORB-OIB data. All basaltic rocks have Nd-Sr-Pb isotope compositions different from those of xenoliths, indicating that the host basaltic magma did not derive from the lithospheric mantle where the xenoliths originated. The range of isotopic composition of xenoliths is much greater than that observed in host basalts, which reflects small-scale heterogeneity of the lithospheric mantle. The greater isotopic heterogeneity of the lithospheric mantle probably reflects its long-term stability. The spinel peridotite xenolith data of Jeju Island, Baekryeong Island and Boeun display mixing hyperbolas between DMM and EM II end members. Since Jeju basalts have EM II-like isotopic signature, the mixing relationship shown by the isotopic data of the Jeju xenoliths can be interpreted as the result of infiltration of metasomatic fluid or melt derived from basaltic magma into DMM-like lithospheric mantle. In contrast to other xenolith sites, the Ganseong xenoliths are dominantly clinopyroxene megacryst and pyroxenite. Clinopyroxene megacrysts have different isotopic ratios from their host basalt, reflecting its exotic origin. Two Ganseong xenoliths (wherlite and clinopyroxenite) have much enriched Sr and Nd isotopic ratios and Nd model ages of 2.5-2.9 Ga, and plot in an array away from the MORB-OIB field. The mantle xenoliths from Korean Peninsula have similar $\^$87/Sr/$\^$86/Sr,$\^$143/Nd/$\^$144/Nd and $\^$207/Pb/$\^$204/Pb ratios to, but higher $\^$208/Pb/$\^$204/Pb ratios than, those from eastern China, indicating that Korean xenoliths are derived from the lithospheric mantle with higher Th/U ratio compared with Chinese ones. The isotopic data of xenolith-bearing basalts of Baekryeong Island and Ganseong, along with Ulreung and Dok Islands, show a mixing trend betlveen DMM and EM I in Sr-Nd-Pb isotopic correlation diagrams, which is also observed in tile northeastern Chinese basalts. However, the Jeju volcanic rocks show an EM II signature that is observed in southeastern Chinese basalts. The isotopic variations in volcanic rocks from the northern and southern portions of the East Asia reflect a large-scale isotopic heterogeneity in their source mantle.

• The Journal of the Petrological Society of Korea
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• v.25 no.3
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• pp.261-281
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• 2016

Our knowledge of the lithosphere beneath the Korean Peninsula has been improved through petrologic and geochemical studies of upper mantle xenoliths hosted by Quaternary intraplate alkali basalts from Jeju Island. The xenoliths are mostly spinel lherzolites, accompanied by subordinate harzburgite and pyroxenites. The mantle xenoliths represent residual mantle material showing textural and geochemical evidence for at least a three-stage evolution, fractional partial melting, recrystallization, and metasomatism. Their composition primarily controlled by early fractional melt extraction and porphyroclastic and mylonitic fabrics formed in a shear-dominated environment, which was subsequently modified by residual slab-derived fluids (or melts). Modal metasomatic products occur as both anhydrous phase(orthopyroxene) and hydrous phase (phlogopite). Late-stage orthopyroxene is more common than phlogopite. However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of lithospheric mantle, resulting in the enriched mantle domains, such as EM I or EM II in the lithospheric mantle beneath East Asia.