GEOCHEMICAL JOURNAL Vol. 50(2016) No. 4

Identification of underlying geochemical processes based on samples of different types such as stream sediments, soils, and water is important for a range of applications including mineral exploration, land use planning, and environmental assessment of both natural and anthropogenic factors. However, almost all geochemical compositions of these samples are subject two limitations: outliers and the data closure effect. In the present study, bivariate relationships between selected major elements are examined to illustrate their spurious correlation by using centered log ratio (clr) transformation. In addition, robust factor analysis (FA) and compositional data analysis are used to prevent the effect of outliers and to reduce the influence of data closure in the identification of geochemical processes. First, a k-means algorithm is applied to partition geochemical data into three clusters to enhance the interpretation of the geochemical data. Then, robust FA is applied to log ratio-transformed geochemical data. The first five factors are extracted on the basis of the scree plot of eigenvalues. The results indicate that robust FA applied to log ratio-transformed data can be used to effectively identify geochemical processes and to determine the extent of anthropogenic and natural influences such as mineralization, weathering and diagenesis, heavy metal accumulation or contamination, or a combination of these factors. Several geochemical processes are indicated by the first five factors, explained as follows: (a) F1 reflects granitic rocks and natural or industrial contamination by Cu, Ni, Sb, As, Cd, and Cr; (b) F2 reflects W polymetallic mineralization; (c) F3 reflects Au anomalies and heavy metal contamination by Zn, Cd, Mn, and Pb; (d) F4 reflects Mo and Au anomalies; and (e) F5 reflects Ag-W-Be-La mineralization and heavy metal contamination by Hg and Sb.

The Jinju (H5) meteorite fell as four fragments in the Jinju area, Gyeonsangnam-do, South Korea, on March 9, 2014. The major element concentrations and noble gas isotopic compositions were determined for Jinju-1 and Jinju-2, stones of the first and second discoveries, respectively, by using X-ray fluorescence (XRF) analysis and noble gas mass spectrometry combined with total melting and stepwise heating noble gas extraction. The major element composition agrees well with that of equilibrated H chondrites. Very low contamination from terrestrial atmospheric noble gases was detected, which may be attributed to the rapid recovery of the stones within two days after the fall of the Jinju meteorite. A short cosmic ray exposure age of 2.4 My was obtained. The (U,Th)-He, and K-Ar ages are concordant at ca. 4.0 Gy, which suggests that no severe heating event occurred to cause a loss of He after the resetting at 4.0 Gy. Overabundances of trapped Ar and Kr, which were released at 800°C, were found in addition to “normal” Q gas of Ar, Kr and Xe at ≥1400°C. The low-temperature component could be explained by Ar and Kr supplied by an adjacent impact region that were trapped when the minerals of the Jinju meteorite were formed during cooling. The heavy shock event that occurred at 4.0 Gy might have produced the observed numerous vugs and vapor growth crystals in this meteorite (Choi et al., 2015) and could have supplied the Ar and Kr to thermally weak minerals.

The formation of lake terrace and salar-type lake-bed borate deposit at Nie`er Co Lake (hereafter Nie`er Co), northwestern Tibet, remain debated. We performed a detailed study of the hydrogeochemistry, geochemical evolution, and enrichment of Nie`er Co waters using the chemical compositions and stable isotopes (δD and δ<sup>18</sup>O) of hot springs, stream water, ice, seep water, and lake water. Ion concentrations and the total dissolved solids (TDS) were in the order: lake water ≥ stream water > seep water > ice. We classified waters into three types: T1 (hot springs), T2 (seep water, stream water, and ice), and T3 (lake water). Hydrogeochemistry evolved from Na-HCO<sub>3</sub> in headwaters to Ca-Mg-SO<sub>4</sub>/Cl in the lake, passing through an intermediate Ca-Mg-HCO<sub>3</sub> transitional stage in-between. The main processes controlling water chemistry were found to be carbonate dissolution and continuous evapoconcentration. This was confirmed using the Hardie and Eugster model, which demonstrated that the evolution of lake water during evaporation should reach a final composition of Na-Mg-SO<sub>4</sub>-Cl. Using the δD and δ<sup>18</sup>O of snow, stream water, ice, and seep water, we determined the Nie`er Co meteoric water line (NMWL) to be δD = 8.18δ¹⁸O + 30.27 (γ = 0.97). Stream water, ice, and seep water originate from the melting of mountain snow or ice. Lake water undergoes strong evaporation, and hot springs originate from local meteoric water and belong to a low-medium temperature hydrothermal system. Three main steps control the formation of the Nie`er Co borate deposits: (a) meteoric waters filter through deep, B-rich volcanic strata; (b) water flows upwards into hot springs, carrying B and other minerals into the lake recharge waters; and (c) lake water is concentrated owing to the arid and cold climate, allowing kurnakovite and other borate minerals to precipitate. The integrated hydrogeochemical approach developed in this study furthers our understanding of Qinghai-Xizang (Tibet) plateau borate deposits.

The chemical and isotopic compositions of gases discharging from shallow-water hydrothermal vents at Kueishantao (KST, northeast Taiwan) have been studied since 2000. In this paper, we present new data gathered from 2010 to 2014. The main component is CO₂ (162–882 mmol/mol), followed by N₂ (33–634 mmol/mol), CH₄ (18–190 mmol/mol), and H₂S (b.d.l.–172 mmol/mol). Helium isotope values indicate that more than 70% of the helium is mantle-derived. O₂ was mostly a contaminant from ambient seawater during sampling, or from air after sampling. By subtracting the contaminant fractions of N₂ and Ar, using the O₂ concentrations plus the N₂/O₂ or Ar/O₂ ratios in air, the residual N₂ and Ar were positively correlated to a significant extent, and exhibited N₂/Ar ratios close to air values. These corrected fractions of N₂ and Ar were derived from seawater percolated during the discharge stage. The δ¹³C values of CO₂ ranged from –8.2 to –5.5‰ (VPDB), whereas the CO₂ contents were largely controlled by its dissolution in the fluid phase. The estimated endmember contributions for CO₂ were: mantle (8–32%), sediment (14–27%), and marine limestone (54–72%). CH₄ was the main hydrocarbon in the KST gases, and exhibited a linear relationships with helium. The CH₄/³He ratios increased dramatically from 0.1–20 × 10⁶ during 2000 and 2003 to 3–5 × 10⁸ after 2010, either due to the increasing CH₄ contents or the decreasing input of magmatic helium. In the C₁/C₂₊ ratios versus δ¹³C(CH₄) (–26.8 to –24.5‰) diagram, the KST gas samples lie at the boundary between thermogenic and abiotic, suggesting that CH₄ may be derived from these two sources. Alternatively, according to the log(C₂/C₃) versus log(C₁/C₂) diagram and the C₁–C₅ distribution patterns, the C₂₊ hydrocarbons and less than 2% of the CH₄ may have originated from either a kinetically-controlled Fischer Tropsch-Type (FTT) reaction or a thermogenic process, whereas more than 98% of CH₄ was in equilibrium with CO₂. In addition, the linear-logC₂–C₅ patterns suggest that no significant secondary processes have occurred. This is the first study of KST gases to include carbon stable isotope ratio measurements, which provides important information about gases released from a shallow-water submarine hydrothermal field at a subduction zone.

To understand carbonate dissolution and/or preservation on the sea floor since the Last Glacial Maximum (LGM), we measured shell weights of planktonic foraminifera Globigerinoides ruber from four sediment cores retrieved from different water depths (~800 to 3300 m) across the northern Indian Ocean. G. ruber shell weight pattern shows an overall decrease starting from the LGM, with a spike, also termed as the ‘mystery interval’, during the early deglaciation (~17.5 to 14.5 ka). This shell weight maximum is a feature noted across the world oceans and considered to signify carbonate preservation, although it is missing from many sediment cores from the eastern equatorial Pacific, tropical Atlantic and subtropical Indian Ocean. The carbonate preservation spike during deglaciation in the northern Indian Ocean documented in this study suggests increased deep-water carbonate ion concentrations during the early deglaciation which in turn favored preservation. This study sheds new light on the preservation of carbonate and associated deep water circulation during deglaciation in the northern Indian Ocean.

Juina diamonds present a wide spectrum of mineral inclusions, covering both peridotitic and eclogitic composition. Among the most rare inclusions, carbonates are interpreted as an evidence of deep recycling of sedimentary carbon into the transition zone or the lower mantle. Yet, the δ¹³C values measured in three FIB-TEM foils by NanoSIMS 50 of an alluvial diamond with a carbonate inclusion range between –8.85 ± 1.32‰ and –2.31 ± 1.88‰ with a mean total value of –5.0 ± 2.3‰. These values are in the range of typical mantle carbon, as measured in diamonds of peridotitic paragenesis. Similar δ¹³C values from –8.5 to –4.4‰ are reported in literature for five other Juina diamonds with carbonate inclusions. We can postulate either that the diamond is peridotitic and carbonate precipitated from fluids and survived to the reduction to diamond or that formed from reduction of carbonatitic melts in the upper mantle, percolating through eclogite.