Fang-Zhen Teng

Iron Isotope Fractionation During Magmatic Differentiation in Kilauea Iki Lava Lake

Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (56Fe/54Fe) of late-stagemeltveins are 0.2 permil (‰) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2‰ lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.

Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis

The magnesium (Mg) isotopic compositions of 40 seawater samples from the Gulf of Mexico and of one seawater sample from the southwest Hawaii area were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to investigate the homogeneity of Mg isotopes in seawater. The results indicate that the Mg isotopic composition of seawater from the Gulf of Mexico is homogeneous, both vertically and horizontally, with average values for δ(26)Mg = -0.832 ± 0.068 and δ(25)Mg = -0.432 ± 0.053 (n = 40, 2SD)--identical to those of seawater from Hawaii (average δ(26)Mg = -0.829 ± 0.037 and δ(25)Mg = -0.427 ± 0.033) and to the average literature values of seawater worldwide (δ(26)Mg = -0.83 ± 0.11 and δ(25)Mg = -0.43 ± 0.06, n = 49, 2SD). Collectively, global seawater has a homogeneous Mg isotopic composition with δ(26)Mg = -0.83 ± 0.09 and δ(25) Mg = -0.43 ± 0.06 (2SD, n = 90). The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precipitation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Considering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excellent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis.

Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Abstract To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ7Li varying from -3.1 to +6.6. The δ7Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from -3.1 to +2.5 and overlap with post- Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ7Li and elements indicative of degrees of granite differentiation (SiO2, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to reflect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may reflect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100.1100 ppm), quartz (30.140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ7Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ7Li showing an inverse correlation with Li concentration. This correlation reflects the mixing of isotopically heavy Li in quartz and lighter Li in fluid inclusions, as documented by fluid inclusion compositions (δ7Li = +8.1 to +13.4 and Li of 280 to 3960 ppm). Extrapolation of this trend to an estimated intrinsic Li concentration in quartz of <30 ppm, yields an inferred δ7Li for fluid inclusion-free quartz of >+21. The large difference in δ7Li between quartz and other minerals may reflect 7Li preference for less highly coordinated sites, which have higher bond-energies (i.e., the two- or fourfold site in quartz vs. higher coordination number sites in other minerals). Comparison of the Li isotopic composition of fluid inclusions with that of the wall zone of the Tin Mountain pegmatite suggests ~4‰ isotopic fractionation during fluid exsolution, which agrees with the results derived from studies of hydrothermal alteration of basalts.

High-precision copper and iron isotope analysis of igneous rock standards by MC-ICP-MS

Stable isotopic systematics of Cu and Fe are two important tracers for geological and biological processes. Generally, separation of Cu and Fe from a matrix was achieved by two independent, completely different methods. In this study, we report a method for one-step anion-exchange separation of Cu and Fe from a matrix for igneous rocks using strong anion resin AG-MP-1M. Cu and Fe isotopic ratios were measured by multi-collector inductively coupled plasma mass-spectrometry (Neptune plus) using a sample–standard bracketing method. External normalization using Zn to correct for instrumental bias was also adopted for Cu isotopic measurement of some samples. In addition, all parameters that could affect the accuracy and precision of isotopic measurements were examined. Long-term external reproducibility better than ±0.05‰ (2SD) for δ65Cu and ±0.049‰ (2SD) for δ56Fe was routinely obtained. Cu and Fe isotopic compositions of commercially accessible igneous rock standards including basalt, diabase, amphibolite, andesite and granodiorite were measured using this method. δ65Cu values of igneous rock standards vary from −0.01 to +0.39‰ (n = 11) with an overall range (0.40‰) that exceeds about 8 times that of the current analytical precision. The improved precisions of stable Cu isotopic analysis thus demonstrate that igneous rocks are not homogeneous in Cu isotopic composition. The procedure for one-step separation of Cu and Fe and high-precision analysis of Cu and Fe isotopic ratios have an important advantage for economical and efficient study of stable Cu and Fe isotopic systematics in geological and biological fields.

Comparison of factors affecting the accuracy of high-precision magnesium isotope analysis by multi-collector inductively coupled plasma mass spectrometry

RATIONALE High accuracy is the prerequisite for high-precision isotopic analysis. METHODS Here we evaluate how the presence of matrix elements, and mismatch between samples and standards in Mg concentration and acid molarity affect the accuracy of stable Mg isotopic analysis on Nu Plasma and IsoProbe multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) instruments. RESULTS Our results show that these factors can lead to large (>1‰) deviation in the high-precision analysis of Mg isotopes. The degree and direction of these accuracy offsets can vary for different instruments, instrumental settings and different laboratories. CONCLUSIONS Detailed tests and tight controls on these effects are thus needed for high-precision high-accuracy stable Mg isotopic analysis.

Destruction of the North China Craton Induced by Ridge Subductions

The destruction of the North China Craton (NCC) mainly occurred in the Cretaceous and has been attributed to a “top-down” rapid delamination, “bottom-up” long-term thermal/chemical erosions, or hydration by subduction-released fluids. On the basis of the distribution of one Jurassic and two Early Cretaceous adakite belts and the drifting history of the paleo-Pacific Plate, we propose that three ridge subduction events dominated the large-scale decratonization in the NCC. Both physical erosion and magmatism induced by ridge subduction contributed to the destruction of the NCC; the last ridge subduction, at Ma, was the key driving force in the final destruction. We present mineralogical, geochemical, and isotopic data in support of the ridge subduction model: flat subduction of a spreading ridge resulted in stronger physical erosion on the thick lithosphere mantle of the NCC. Consequently, slab melting occurred during ridge subduction, forming adakites with mantle Mg isotope compositions, followed by A-type granites as a result of asthenosphere upwelling. Delaminated lower continental crust was also partially melted after reacting with hydrous magmas, as indicated by eclogite xenoliths, resulting in a zircon age spectrum similar to that of the NCC and some adakitic samples with chemical characteristics similar to those of the Dabie adakites. The final decratonization was triggered by the last ridge subduction, with both physical erosion (flat subduction) and thermal erosion (adakitic and A-type magmatisms). Given that ridge subduction has occurred throughout Earth’s history, the associated decratonization processes are presumably a common phenomenon that modified the chemical compositions of the continental crust.

Tracing carbonate–silicate interaction during subduction using magnesium and oxygen isotopes

Subduction of carbonates and carbonated eclogites into the mantle plays an important role in transporting carbon into deep Earth. However, to what degree isotopic exchanges occur between carbonate and silicate during subduction remains unclear. Here we report Mg and O isotopic compositions for ultrahigh pressure metamorphic marbles and enclosed carbonated eclogites from China. These marbles include both calcite- and dolomite-rich examples and display similar O but distinct Mg isotopic signatures to their protoliths. Their δ(26)Mg values vary from -2.508 to -0.531‰, and negatively correlate with MgO/CaO ratios, unforeseen in sedimentary carbonates. Carbonated eclogites have extremely heavy δ(18)O (up to +21.1‰) and light δ(26)Mg values (down to -1.928‰ in garnet and -0.980‰ in pyroxene) compared with their protoliths. These unique Mg-O isotopic characteristics reflect differential isotopic exchange between eclogites and carbonates during subduction, making coupled Mg and O isotopic studies potential tools for tracing deep carbon recycling.

Limited magnesium isotope fractionation during metamorphic dehydration in metapelites from the Onawa contact aureole, Maine

Knowledge on the behavior of Mg isotopes during metamorphic dehydration is the prerequisite for applying Mg isotopes as tracers for crustal recycling. Here we report Mg isotopic compositions of metapelites from the Onawa contact aureole, Maine. Except one sample, all metapelites across the aureole, from the wall-rock regional metamorphic rocks to the partially melted rocks adjacent to the pluton, have similar Mg isotopic compositions (δ26Mg = −0.09 to +0.12‰). This observation indicates limited Mg isotope fractionation during metamorphic dehydration and fluid-rock interaction, due to the low Mg concentration in fluids relative to rocks. Our results suggest that Mg isotopic compositions of metapelites can record those of their protoliths and, hence, recycled clastic sedimentary materials may preserve their low-temperature Mg isotopic signatures through subduction zones. Therefore, Mg isotopes may serve as new tracers for crustal recycling, for example, tracing components experienced weathering cycles within granite sources.

Magnesium isotope geochemistry in arc volcanism

Significance Arc lavas from Martinique have nonmidocean ridge basalt Mg isotopic composition, which is consistent with the incorporation of subducted Mg. This is, to our knowledge, the first report of mantle-derived lavas with Mg isotopic composition heavier than oceanic basalts, heretofore shown to be isotopically homogenous. More importantly, our results provide insight into the strongly debated origins of Martinique arc lavas and suggest that contributions of Mg from fluids supplied by the subducted slab may play a significant control in the Mg isotopic systematics of arc lavas. Incorporation of subducted slab in arc volcanism plays an important role in producing the geochemical and isotopic variations in arc lavas. The mechanism and process by which the slab materials are incorporated, however, are still uncertain. Here, we report, to our knowledge, the first set of Mg isotopic data for a suite of arc lava samples from Martinique Island in the Lesser Antilles arc, which displays one of the most extreme geochemical and isotopic ranges, although the origin of this variability is still highly debated. We find the δ26Mg of the Martinique Island lavas varies from −0.25 to −0.10, in contrast to the narrow range that characterizes the mantle (−0.25 ± 0.04, 2 SD). These high δ26Mg values suggest the incorporation of isotopically heavy Mg from the subducted slab. The large contrast in MgO content between peridotite, basalt, and sediment makes direct mixing between sediment and peridotite, or assimilation by arc crust sediment, unlikely to be the main mechanism to modify Mg isotopes. Instead, the heavy Mg isotopic signature of the Martinique arc lavas requires that the overall composition of the mantle wedge is buffered and modified by the preferential addition of heavy Mg isotopes from fluids released from the altered subducted slab during fluid−mantle interaction. This, in turn, suggests transfer of a large amount of fluid-mobile elements from the subducting slab to the mantle wedge and makes Mg isotopes an excellent tracer of deep fluid migration.

Magnesium isotopic composition of the deep continental crust

Abstract To constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the middle and lower continental crust, 30 composite samples from high-grade metamorphic terranee and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranee ill the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO2 ranging from 45.7 to 72.5%, representative of the middle crust beneath eastern China. The δ26Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower continental crust (>30 km). They yield a large δ26Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ26Mg values of the middle and lower continental crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk continental crust is estimated to have an average δ26Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the continental crust reflects the combination of several processes, such as continental weathering, involvement of supracrustal materials in the deep crust, and fluid metasomatism.