Ji-Feng Ying

Review of the Lithium Isotope System as a Geochemical Tracer

Lithium isotopes have many advantageous characteristics as geochemical tracers for a wide range of geological processes, covering fluid/melt-mineral interactions from the Earths surface to the mantle. The latest developments in Li isotope geochemistry, in terms of measuring techniques, major geochemical reservoirs, the contrasting behaviors during near-surface weathering, and crust/mantle cycling of materials in subduction zones, as well as the signature of convective mantle, and controlling factors in Li isotopic fractionation are briefly reviewed in this article. With better quantification of partition coefficients (mineral-melt/fluid) and fractionation factors (such as temperature and diffusion coefficients), the Li isotope system will become a powerful geochemical tracer in future studies.

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region, east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (87Sr/86Sr = 0.7046–0.7087; 143Nd/144Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.

Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton: Implication for replacement process of lithospheric mantle

Mesozoic (125 Ma) Fangcheng basalts from Shandong Province contain clearly zoned olivines that are rare in terrestrial samples and provide first evidence for the replacement of lithospheric mantle from high-Mg peridotites to low-Mg peridotites through peridotite-melt reaction. Zoned olivines have compositions in the core (Mg# = 87.2–90.7) similar to those olivines from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the North China craton and in the rim (Mg# = 76.8–83.9) close to olivine phenocrysts of the host basalts (75.7–79.0). These compositional features as well as rounded crystal shapes and smaller grain sizes (300–800 μm) demonstrate that these zoned olivines are mantle xenocrysts, i.e. disaggregates of mantle peridotites. Their core compositions can represent those of olivines of mantle peridotites. The zoned texture of olivines was formed through rapid reaction between the olivine xenocryst and the host basalt. This olivine-basaltic melt reaction could have been ubiquitous in the Mesozoic lithospheric mantle beneath the North China craton, i.e. an important type of the replacement of lithospheric mantle. The reaction resulted in the transformation of the Paleozoic refractory (high-Mg) peridotites to the late Mesozoic fertile (low-Mg) and radiogenic isotope-enriched peridotites, leading to the loss of old lithospheric mantle.

Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton

Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ7Li in Archean lithospheric mantle. The δ7Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from −27 to +21, in marked deviation from the δ7Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1–2 ppm). The Li abundances and δ7Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ7Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ7Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

Zoned olivine xenocrysts in a late Mesozoic gabbro from the southern Taihang Mountains: implications for old lithospheric mantle beneath the central North China Craton

Zoned olivine grains are abundant in the late Mesozoic Shatuo gabbro (southern Taihang Mountains, central North China Craton). Olivine cores are rich in MgO and NiO, rims are rich in FeO and MnO, and both cores and rims have very low CaO contents. The cores invariably have a high Mg no. (92–94), similar to olivine xenocrysts from Palaeozoic kimberlites in eastern China. The compositional features of these olivines imply that they are xenocrysts rather than phenocrysts, namely, disaggregates of mantle peridotites at the time of intrusion. The compositional similarity of olivine cores to xenocrysts from Palaeozoic kimberlites suggests that the lithospheric mantle beneath the central North China Craton is ancient and refractory in nature, and quite different from eastern China, where the mantle is mainly composed of newly accreted materials resulting from large-scale lithospheric removal and replacement. The contrasting features of the lithospheric mantle beneath the eastern and central North China Craton imply that the large-scale lithospheric removal in Phanerozoic times was mainly confined to the eastern North China Craton.

Breakdown of orthopyroxene contributing to melt pockets in mantle peridotite xenoliths from the Western Qinling, central China: constraints from in situ LA-ICP-MS mineral analyses

Major and trace element compositions of constituent minerals, partly decomposed rims of orthopyroxenes (DRO), ‘closed’ melt pockets (CMP) and open melt pockets (OMP) in some Western Qinling peridotite xenoliths were obtained by LA-ICP-MS. Systematic core-to-rim compositional variations of garnet, clinopyroxene and orthopyroxene demonstrate that these minerals underwent variable degrees of subsolidus breakdown or partial melting. Both DROs and CMPs consist of similar mineral assemblages and are characterized by high TiO2, CaO + Na2O and low MgO contents; they are enriched in LREE and LILE compositions, have positive anomalies in Pb, Sr and particularly Ti, negative Th and U, and variable Zr and Hf anomalies. These chemical features are distinct and reflect reactions involving the orthopyroxenes. Compared to the CMPs, the OMPs, which are composed of a complex assemblage of minerals, display lower FeO and MgO contents, larger ranges in SiO2 and Na2O, higher TiO2, Al2O3, CaO and trace element concentrations, slightly negative Zr and Hf anomalies, and apparently negative Ti anomalies. Modeling calculations of partial fusion of orthopyroxenes and clinopyroxenes suggest that the CMPs most likely originated from the breakdown of orthopyroxenes with variably minor contribution of external melts from the melting of clinopyroxenes, whereas the OMPs were probably formed from the modification of the CMPs through the interaction with large amount of external melts.

Large Lithium Isotopic Variations in Minerals from Peridotite Xenoliths from the Eastern North China Craton

To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and δ7Li of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) for six peridotite xenoliths from the eastern North China Craton. These xenoliths contain two lherzolites, two Cpx-rich lherzolites, and two wehrlites and are variably metasomatized. Lithium isotope zonation is observed in most peridotite minerals. The majority of Cpx grains display isotopically light cores with lower Li concentrations than the heavy rims. However, the Opx grains show a different style of zonation from Cpx, where higher Li concentrations in the cores are associated with much lighter δ7Li. Olivines in most peridotites have a restricted range of Li concentration and δ7Li within individual grains, whereas the olivines in a lherzolite show isotopically light cores (−10.3) with high Li concentrations (2.3 ppm) and heavy rims (5.5) with low Li concentrations (1.7 ppm). These Li isotopic variations in mineral phases may reflect the combined effects of diffusion-driven kinetic fractionation of Li isotopes during melt/fluid-peridotite interactions and slow cooling. Intersample heterogeneity of Li isotopes is also apparent. Olivine with forsterite (Fo) content of 91.3 in one lherzolite sample has “normal” mantle-like Li concentrations (1.1∼2.4 ppm) and light δ7Li (−10.3∼5.5), while Ol with Fo content of 89.7 in another lherzolite has slightly high Li concentrations (2.0∼3.0 ppm) but similar δ7Li (1.6∼6.4) relative to normal mantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier δ7Li (5.0∼22.0) than those in lherzolites and normal mantle. The δ7LiOl value correlates positively with Li concentration and negatively with Fo from lherzolites to Cpx-rich lherzolites, indicating a reaction between lherzolites and melts with isotopically heavy Li- and Fe-rich signatures. By contrast, olivines in wehrlites have extremely lower Fo contents (82.2∼83.2) and higher Li concentrations (2.4∼4.2 ppm) than those in normal mantle, while their δ7Li values are within the range of normal mantle, reflecting metasomatism of the peridotites by asthenospheric melt. Overall, the large intragrain and intersample variations in Li concentrations and isotopic compositions reflect kinetic isotope fractionation during multiple metasomatisms.

Metasomatized Lithospheric Mantle beneath the Western Qinling, Central China: Insight into Carbonatite Melts in the Mantle

Mantle xenoliths from the Western Qinling, central China, are dominated by lherzolites, which can be divided into four subgroups—namely, garnet-facies, coexisting spinel-garnet, spinel-facies, and carbonate-bearing ones. All these rocks display light rare earth element enrichment, positive Sr and Ba anomalies, carbonatite-like trace element patterns, and Sr-Nd-Pb isotopic mixing between depleted mantle and enriched mantle type II end members, consistent with the geochemical features resulting from carbonatite metasomatism. The garnet-facies lherzolites show high trace element concentrations but low LaN/YbN ratios, and they show high Sr and Pb isotopic ratios that are similar to those of carbonatites, suggesting that they were highly metasomatized. The spinel-facies group has the lowest trace element concentrations but higher LaN/YbN ratios than the garnet-facies group; their lowest Sr and Pb isotopic ratios are closer to those of the depleted mantle end member, implying low-degree metasomatism. Geochemical variation of the coexisting spinel-garnet sample lies between that of the garnet and spinel groups. The elevated and highly variable trace element concentrations and Sr-Pb isotopic values of the carbonate-bearing lherzolite group are most likely related to the modal content of carbonate minerals. Collectively, these geochemical features indicate a rising front of carbonatite metasomatism in the lithospheric mantle beneath the Western Qinling. Combining experimental and empirical data, the positive Pb, Y, and high-field strength element anomalies in the peridotites might be ascribed to the involvement of a subduction component in the carbonatite melts. On the basis of the data presented in this article, we propose a general model for carbonatite metasomatism in the lithospheric mantle to interpret the different signatures recorded in the garnet-facies peridotites (chemical imprint) and spinel-facies peridotites (occurrence of carbonate minerals), which has potential application to other regions that have undergone carbonatite metasomatism.

A brief review of isotopically light Li - a feature of the enriched mantle?

Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.

Magnesium Isotopic Evidence for Ancient Subducted Oceanic Crust in LOMU-Like Potassium-Rich Volcanic Rocks

To evaluate the role of subducted oceanic crust in the genesis of potassium-rich magmas, we report high-precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle and display considerable Mg isotopic variations, with δ26Mg ranging from −0.61 to −0.23. The covariation of δ26Mg with TiO2 in these rocks suggests that their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ26Mg and (Gd/Yb)N ratio as well as Sr-Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle-derived magmas.