Zhengrong Wang

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Abstract Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.

Carbon concentrations and isotopic ratios of eclogites from the Dabie and Sulu terranes in China

Abstract Both concentration and isotope composition of bulk carbon in apatite and host eclogites from the Dabie–Sulu ultrahigh pressure (UHP) terranes in China have been determined along with their oxygen isotope composition. The results show significant 13 C-depletion in the apatite (δ 13 C=−27.7‰ to −20.8‰) with the carbon concentrations of 0.59 to 1.65 wt.% CO 2 despite a large variation in δ 18 O (−6.5‰ to +9.5‰). The bulk carbon in 21 of the 24 eclogites has low δ 13 C values of −26.1‰ to −17.9‰ with low carbon content of 500 to 1000 ppm, whereas the other three samples show high δ 13 C values of −7.1‰ to −2.8‰ with high carbon contents of 2400 to 4300 ppm. Noncarbonate carbon was measured by treating the all eclogites with 5 N HCl solution, yielding uniformly low δ 13 C values of −27.9‰ to −24.2‰ and carbon contents of 200 to 300 ppm. Carbonate carbon is thus calculated by mass balance to have also low δ 13 C values of −25.6‰ to −15.1‰ for the 21 samples but high δ 13 C values of −4.3‰ to −1.2‰ for the remaining three samples. Secondary carbonate was identified in the three eclogites that are also depleted in 13 C primarily, but subjected to overprint of 13 C-rich CO 2 -bearing fluid subsequent to the UHP metamorphism. The isotopically light carbon in both eclogite and apatite is interpreted to represent the isotope composition of carbon in eclogite precursors before plate subduction, and thus has an origin of organic carbon from the Earths surface. The uniformly low δ 13 C values of apatite suggest that the CO 2 of metamorphic fluid in equilibrium with the host-eclogites would be derived from the oxidation of organic carbon rather than the decarbonation of underthrust carbonates during progressive metamorphism. Protoliths of the eclogites are inferred to be of igneous origin, which underwent more extensive interaction with organic matter than with meteoric-hydrothermal fluid on the subsurface of the continental crust. The break off of the subducted plate containing the isotopically light carbon and subsequent interactions with the surrounding mantle could produce the mafic and/or silicic magmas that are significantly depleted in 13 C relative to the primary mantle carbon. This may provide evidence for a linkage of the 13 C-depleted mantle carbon to a surficial source via plate subduction.

Oxygen isotope geochemistry of the second HSDP core

Oxygen isotope ratios were measured in olivine phenocrysts (~1 mm diameter), olivine microphenocrysts (generally ~100–200 µm diameter), glass, and/or matrix from 89 samples collected from depths down to 3079.7 m in the second, and main, HSDP core (HSDP-2). Olivine phenocrysts from 11 samples from Mauna Loa and 34 samples from the submarine section of Mauna Kea volcano have delta18O values that are similar to one another (5.11 ± 0.10‰, 1sigma, for Mauna Loa; 5.01 ± 0.07‰, for submarine Mauna Kea) and within the range of values typical of olivines from oceanic basalts (delta18O of ~5.0 to 5.2‰). In contrast, delta18O values of olivine phenocrysts from 20 samples taken from the subaerial section of Mauna Kea volcano (278 to 1037 mbsl) average 4.79 ± 0.13‰. Microphenocrysts in both the subaerial (n = 2) and submarine (n = 24) sections of Mauna Kea are on average ~0.2‰ lower in delta18O than phenocrysts within the same stratigraphic interval; those in submarine Mauna Kea lavas have an average delta18O of 4.83 ± 0.11‰. Microphenocrysts in submarine Mauna Kea lavas and phencrysts in Mauna Loa lavas are the only population of olivines considered in this study that are typically in oxygen isotope exchange equilibrium with coexisting glass or groundmass. These data confirm the previous observation that the stratigraphic boundary between Mauna Loa and Mauna Kea lavas defines a shift from “normal” to unusually low delta18O values. Significantly, they also document that the distinctive 18O-depleted character of subaerial Mauna Kea lavas is absent in phenocrysts of submarine Mauna Kea lavas. Several lines of evidence suggest that little if any of the observed variations in delta18O can be attributed to subsolidus alteration or equilibrium fractionations accompanying partial melting or crystallization. Instead, they reflect variable proportions of an 18O-depleted source component or contaminant from the lithosphere and/or volcanic edifice that is absent in or only a trace constituent of subaerial Mauna Loa lavas, a minor component of submarine Mauna Kea lavas, and a major component of subaerial Mauna Kea lavas. Relationships between the delta18O of phenocrysts, microphenocrysts, and glass or groundmass indicate that this component (when present) was added over the course of crystallization-differentiation. This process must have taken place in the lithosphere and most likely at depths of between ~5 and 15 km. We conclude that the low-delta18O component is either a contaminant from the volcanic edifice that was sampled in increasingly greater proportions as the volcano drifted off the center of the Hawaiian plume or a partial melt of low-delta18O, hydrothermally altered perdotites in the shallow Pacific lithosphere that increasingly contributed to Mauna Kea lavas near end of the volcanos shield building stage. The first of these alternatives is favored by the difference in delta18O between subaerial and submarine Mauna Kea lavas, whereas the second is favored by systematic differences in radiogenic and trace element composition between higher and lower delta18O lavas.

Experimental maturation of feathers: implications for reconstructions of fossil feather colour

Fossil feathers often preserve evidence of melanosomes—micrometre-scale melanin-bearing organelles that have been used to infer original colours and patterns of the plumage of dinosaurs. Such reconstructions acknowledge that evidence from other colour-producing mechanisms is presently elusive and assume that melanosome geometry is not altered during fossilization. Here, we provide the first test of this assumption, using high pressure–high temperature autoclave experiments on modern feathers to simulate the effects of burial on feather colour. Our experiments show that melanosomes are retained despite loss of visual evidence of colour and complete degradation of other colour-producing structures (e.g. quasi-ordered arrays in barbs and the keratin cortex in barbules). Significantly, however, melanosome geometry and spatial distribution are altered by the effects of pressure and temperature. These results demonstrate that reconstructions of original plumage coloration in fossils where preserved features of melanosomes are affected by diagenesis should be treated with caution. Reconstructions of fossil feather colour require assessment of the extent of preservation of various colour-producing mechanisms, and, critically, the extent of alteration of melanosome geometry.

Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains

High δ13C values, up to +5.7‰ (PDB), were measured in marbles associated with ultrahigh‐pressure eclogites in the Dabie Mountains, East China. Such a carbon isotope anomaly is interpreted to represent the carbon isotope composition of premetamorphic limestone (i.e., the protolith of the marbles), because there is no known process to enrich carbonate in 13C during regional metamorphism. The high δ13C also corresponds to high δ18O of up to +22.9‰ (SMOW), suggesting that premetamorphic δ13C values would be about +5 to +6‰ for their protolith (limestones). The carbon isotope anomaly in the protolith of the marbles implies a local change in the redox state of water during deposition of the limestones. Preservation of the unusually high positive δ13C values in the marbles supports the previous conclusion that there was no significant isotopic exchange with the mantle during subduction of supracrustal rocks to mantle depths of greater than 100 km.

The fossil record of insect color illuminated by maturation experiments

Structural coloration underpins communication strategies in many extant insects but its evolution is poorly understood. This stems, in part, from limited data on how color alters during fossilization. We resolve this by using elevated pressures and temperatures to simulate the effects of burial on structurally colored cuticles of modern beetles. Our experiments show that the color generated by multilayer reflectors changes due to alteration of the refractive index and periodicity of the cuticle layers. Three-dimensional photonic crystals are equally resistant to degradation and thus their absence in fossil insects is not a function of limited preservation potential but implies that these color-producing nanostructures evolved recently. Structural colors alter directly to black above a threshold temperature in experiments, identifying burial temperature as the primary control on their preservation in fossils. Color-producing nanostructures can, however, survive in experimentally treated and fossil cuticles that now are black. An extensive cryptic record is thus available in fossil insects to illuminate the evolution of structural color.

OXYGEN ISOTOPE CONSTRAINTS ON THE STRUCTURE AND EVOLUTION OF THE HAWAIIAN PLUME

The oxygen isotope stratigraphy of Ko‘olau volcano, Hawaii, is constructed by analyzing olivine phenocrysts from the KSDP drill core and submarine land-slide deposits. Along with those of subaerial (Makapu‘u) Ko‘olau olivines (Eiler and others, 1996a), they span the full range of the δ18OVSMOW variation previously observed in “Loa-trend” Hawaiian volcanoes (Lō‘ihi, Mauna Loa, Hualalai, and Ko‘olau), vary systematically with the stratigraphic position, and correlate with other geochemical properties of their host lavas (Tanaka and others, 2002; Haskins and Garcia, 2004; Huang and Frey, 2005; Salters and others, 2006; Fekiacova and others, 2007). These observations can be explained if the “Loa-trend” volcanoes (including Ko‘olau) are constructed of magmas made by mixing peridotite melt with variable proportions of eclogite melt derived from a mafic constituent of the Hawaiian plume having a composition resembling recent mid-ocean-ridge basalts. We present a model of this magma mixing process that simultaneously explains the correlations among oxygen isotopes, major elements, trace elements and radiogenic isotopes. Although a number of models of this kind, differing in several parameters, describe the data equally well, all statistically acceptable models require an “enriched” component with a MORB-like HREE pattern and enriched oxygen isotope composition (δ18OVSMOW = 7.8-9.7‰), consistent with this component being an upper crustal (layer 1 or 2) basalt or gabbro with a low-temperature alteration history, possibly containing a small amount of sediment. Abundances of some minor elements—Ni and Ti—are not well described by this model; we show that these shortcomings are derived from the compositional assumption and operational difficulties, that is, TiO2 content is too high in our assumed eclogite end-member, and the inversion of NiO content in the melt by the olivine addition calculation is imprecise due to the sensitivity of DNiOolivine/melt to the melt composition and to crystallization process. Previous studies have advocated that Hawaiian lavas were derived from partial melts of an olivine-free pyroxenite formed by reaction of eclogite-derived melt with peridotite (for example, Sobolev and others, 2005). Our study shows that the peridotite-derived and eclogite-derived melt-mixing model can explain the geochemistry of Hawaiian lavas as well, including high-Ni Ko‘olau olivines. We find that an olivine-free mantle source for Hawaiian lavas is unnecessary, although melt-rock interaction could be important in modifying melt composition. Inverting for mixing proportions and degree of melting, we estimate that the amount of recycled crust in the Hawaiian plume is <24 weight percent. Comparison of the late shield-stage “Loa-trend” (particularly Ko‘olau lavas) and “Kea-trend” (particularly Mauna Kea lavas) suggests that the geochemical diversity of Hawaiian lavas is produced by a thermally and chemically-zoned plume.

Correction to ‘Experimental maturation of feathers: implications for reconstructions of fossil feather colour’

[ Biol. Lett . 9 , 20130184. (Published online 27 March 2013) ([doi:10.1098/rsbl.2013.0184][2])][2] The methods section of the main text should state that the results reported are those of maturation experiments that were carried out for 1 h (not 24 h); feather morphology did not survive for 24 h

The effect of solution chemistry on nucleation of nesquehonite

Several series of nesquehonite nucleation experiments (62 experiments in total) were conducted in aqueous solutions having Mg2+/CO32− activity ratios (here referring to log(aMg2+/aCO32−)) ranging between −0.96 to 2.89, but different saturation states (Ω ranging between 2.49–6.90) and solution pH. The goal was to understand the effect of solution chemistry on the nucleation of nesquehonite. Our experimental results show that induction-time estimates from our precipitation experiments with similar Mg2+/CO32− activity ratios are consistent with classical nucleation theory (CNT), while the surface energy derived from CNT varies with Mg2+/CO32− activity ratios. The induction times of nesquehonite nucleation are scattered noticeably when the saturation state of solution is low (Ω < 4), and the nuclei surface energy, derived from the relationship between induction time and saturation state of solution, increases with increasing Mg2+/CO32− activity ratios. These observations can be explained by the different absorption behaviors of Mg2+ and CO32− and/or reduced Gibbs free energies through better screening of the electric double layer. A surface energy model involving solution composition is developed that combines surface complexation with electrostatic models. This new model takes into account how surface charge may affect surface energy. This model implies that the highest surface energy may occur around the point of zero charge (p.z.c), where the nucleation is fastest (or conversely, where the induction time is shortest) under low saturation states, but not under high saturation states. An accelerated attachment kinetic of monomers is also expected at the p.z.c. where high energy surface requires surface absorbed ions to have higher reactivity. This study provides deeper insight into mechanisms of nesquehonite nucleation in nature, and guidelines for accelerating the precipitation rates of nesquehonite.