Jinlong Ma

Contrasting Cenozoic lithospheric evolution and architecture in the Western and Eastern Sino-Korean Craton : Constraints from geochemistry of basalts and mantle xenoliths

Cenozoic basalts from both sides of the Daxin’anling‐Taihang gravity lineament that separates the Sino‐Korean craton (SKC) into western and eastern parts have been studied. In the western SKC, magmas evolved from xenolith‐bearing alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and an increase in Yb content. Sr‐Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin, although lithospheric mantle may have also contributed to these basalts. This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern SKC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the Late Cretaceous–Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the SKC, with initial extension in the eastern part owing to the Late Mesozoic paleo‐Pacific subduction and subsequent extension in the western SKC induced by the Early Tertiary Indian‐Eurasian collision. An implication is that the lithospheric mantle in the western SKC is relatively old compared with that beneath the eastern SKC, which may be a mixture of old lithospheric relicts and newly accreted mantle. This predicted lithospheric architecture is consistent with Sr‐Nd isotopic data and recent Re‐Os age determinations from mantle xenoliths included in Cenozoic basalts.

SHRIMP U-Pb zircon geochronology of Yangfang aegiriteaugite syenite in Wuyi Mountains of South China and its tectonic implications

In addition to the wide spread peraluminous granites, some alkaline or alkali-rich intrusive rocks were recently observed in the Wuyi Mountains. The Yangfang aegiriteaugite syenites in the Wuyi Mountains are strongly enriched in light rare earth elements (LREE), K, Ba, Ga, Zr and Y. What is more, differentiations of REE are obvious. In addition, the Yangfang aegiriteaugite syenites have also some characteristics similar to A-type granite. Zircons are selected from the Yangfang aegiriteaugite syenites for Sensitive High-Resolution Ion Microprobe (SHRIMP-II) U-Pb geochronology study. 15 analyses give concordant206Pb/238U ages in a narrow range of 233–249 Ma, which correspond to a single age population with a weighted mean206Pb/238U age of 242 ± 4 Ma, which records the crystallization time of the aegiriteaugite syenites. It is suggested that: (1) there were early Triassic peralkaline intrusive rocks in South China; (2) the early Indosinian extension event was probably active in local area, South China. This discovery provides a new insight for farther understanding of “Indosinian orogeny” in South China.

Magnetism of a red soil core derived from basalt, northern Hainan Island, China: Volcanic ash versus pedogenesis

Similar to loess‐paleosol sequences in northwestern China, terrestrial sedimentary sequences (red soils) in southern China also provide sensitive Quaternary records of subtropical/tropical paleoclimate and paleoenvironment. Compared with red clay sequences originated from eolian dust, red soils derived from bedrock have received little attention. In this study, a long core of red soil derived from weathered basalt in northern Hainan Island, China, was systematically investigated by using detailed magnetic measurements and rare earth element analyses. The results show that an extremely strong magnetic zone with a maximum magnetic susceptibility (>10 × 10−5 m3 kg−1) is interbedded in the middle of the core profile. This layer contains a significant amount of superparamagnetic magnetite/maghemite particles that primarily originated from volcanic ash, with secondary contributions from pedogenesis. The former has an average grain size of ~19 nm with a normal distribution of volume, and the latter has a much wider grain size distribution. The presence of volcanic ash within the red soil indicates that these Quaternary basalts were not formed by continuous volcanic eruptions. Moreover, the magnetic enhancement patterns differ between the upper and lower zones. The upper zone is more magnetically enhanced and experienced higher precipitation and temperature than the lower zone. Discrimination of superparamagnetic particles originating from pedogenic processes and volcanic ash thus provides a sound theoretical base for accurate interpretation of magnetism in red soils in this region.

Matrix effects and mass bias caused by inorganic acids on boron isotope determination by multi-collector ICP-MS

The influence of inorganic acids (HCl, HNO3, and HF) on boron isotope measurement by using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been investigated. The acid concentration is in the range of 0–0.2 M. Generally, acids can enhance B signal intensities and reduce isotopic mass bias compared to that of the same B concentration in a H2O matrix. The signal enhancement in each acid matrix differs slightly, while B isotopic mass bias is significantly different among them, with the highest 11B/10B ratio in the HF matrix and the lowest in the HCl matrix. In HCl and HNO3 matrices, boron isotopic mass bias reduces when the acid concentration goes up. However, such a scenario is not observed in the HF matrix. Furthermore, the 11B/10B ratio in the HF matrix is the same as that in the H2O matrix within the studied acid concentration (up to 0.2 M). This implies that changes in mass bias and the B signal cannot be related to the same process in ICP-MS. We suggest that B signal enhancement in inorganic acids can mainly be attributed to Coulomb fission during aerosol transport towards plasma, while boron ion redistributions in the plasma caused by matrix element (e.g. Cl, N) ionization lead to changes in isotopic mass bias. As acids can cause considerable matrix effects and mass bias for boron, acidity match between samples and standard solutions is imperative for accurate and precise B isotope measurement by MC-ICP-MS.

Rubidium purification via a single chemical column and its isotope measurement on geological standard materials by MC-ICP-MS

A chromatographic procedure for Rb, K, Ba and Sr one-by-one separation from geological materials has been developed by using a single column with packing Sr-spec resin, followed by high-precision Rb isotope measurement by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The main matrix elements (Ti, Al, Mg, Fe, Mn, Na and Ca) were removed with 4.4 mL of 3 M HNO3 first; Rb and K were then sequentially eluted in different volumes with the same acid. After that, Ba was eluted with 8 M HNO3, and finally Sr was eluted with Milli-Q water. This procedure enables us to collect pure Rb, K, Ba and Sr one by one in a single column with recovery close to 100%. The purified Rb was measured by using a standard-sample bracketing method via MC-ICP-MS. The short-term precision for δ87Rb was better than ±0.03‰ (2SD, for 15 hours), and the long-term (more than 12 months) external precision was better than ±0.06‰ (2SD) based on repeated analysis of Rb standard solution NIST SRM 984. Matrix effects on Rb isotope measurement by MC-ICP-MS were tested by doping pure Rb standard with various amounts of matrix elements and were found to be insignificant when Na/Rb < 2, Ca/Rb < 4 and K/Rb < 25. We measured a set of geological reference standards by this method and found significant Rb isotopic variation. Our study suggests that Rb isotope geochemistry may be a promising tracer of various cosmochemical and geological processes.