Qing-Zhu Yin

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering.

Otolith Microchemistry Provides Information Complementary to Microsatellite DNA for a Migratory Fish

Abstract We investigated the ability of otolith microchemistry to discriminate natal habitats of the splittail Pogonichthys macrolepidotus, a migratory cyprinid endemic to the San Francisco Estuary, California. Splittails are broadly distributed in the brackish and freshwater portions of the lower estuary and make long-distance upstream migrations during winter to rivers and floodplains for spawning. We found that the ratios of Sr: Ca and 87Sr: 86Sr in the otoliths (ascertained by laser ablation inductively coupled mass spectrometry) of age-0 fish collected from natal habitats significantly varied among four primary spawning rivers. Based on these two constituents, quadratic discriminant function analysis correctly classified 71% of the fish to their natal rivers. Recent work with microsatellite DNA indicates that splittails from these same rivers represent two genetically distinct populations. Thus, integrating data obtained from otolith microchemistry and microsatellite DNA can provide complementary inf...

Micromagnetic coercivity distributions and interactions in chondrules with implications for paleointensities of the early solar system

[1] Chondrules in chondritic meteorites record the earliest stages of formation of the solar system, potentially providing information about the magnitude of early magnetic fields and early physical and chemical conditions. Using first-order reversal curves (FORCs), we map the coercivity distributions and interactions of 32 chondrules from the Allende, Karoonda, and Bjurbole meteorites. Distinctly different distributions and interactions exist for the three meteorites. The coercivity distributions are lognormal shaped, with Bjurbole distributions being bimodal or trimodal. The highest-coercivity mode in the Bjurbole chondrules is derived from tetrataenite, which interacts strongly with the lower-coercivity grains in a manner unlike that seen in terrestrial rocks. Such strong interactions have the potential to bias paleointensity estimates. Moreover, because a significant portion of the coercivity distributions for most of the chondrules is <10 mT, low-coercivity magnetic overprints are common. Therefore paleointensities based on the REM method, which rely on ratios of the natural remanent magnetization (NRM) to the saturation isothermal remanent magnetization (IRM) without magnetic cleaning, will probably be biased. The paleointensity bias is found to be about an order of magnitude for most chondrules with low-coercivity overprints. Paleointensity estimates based on a method we call REMc, which uses NRM/IRM ratios after magnetic cleaning, avoid this overprinting bias. Allende chondrules, which are the most pristine and possibly record the paleofield of the early solar system, have a mean REMc paleointensity of 10.4 mT. Karoonda and Bjurbole chondrules, which have experienced some thermal alteration, have REMc paleointensities of 4.6 and 3.2 mT, respectively.

Dating the First Stage of Planet Formation

The 53Mn-53Cr chronometer applied to bulk carbonaceous chondrites constrains the solar nebula volatile element fractionation, chondrule formation, and stage I planetary accretion timescale to within +0.91 to –1.17 Myr at 4568 Myr ago. The difference between the initial 53Cr/52Cr ratio of ordinary chondrites, defined by Chainpur (LL3.4) chondrules, and carbonaeous chondrites suggests that the former is coming from an isotopically evolved reservoir.

Signatures of the s-process in presolar silicon carbide grains: Barium through hafnium

Isotopic and elemental abundance signatures in the mass range Ba through Hf have been determined in a silicon carbide–rich sample of the Murchison carbonaceous chondrite, using inductively coupled plasma mass spectrometry (ICP-MS). Despite the problem of some isobaric interferences, useful results were obtained for a number of isotopes. Disagreements between astrophysical predictions and previous results for 137 Ba and 146 Nd obtained by thermal ionization mass spectrometry are confirmed. Our data for Dy are more in line with predictions, however. The s-process signatures for several other elements in the rare earth element (REE) mass range were observed for the first time and are also consistent with theoretical predictions. The elemental abundance pattern shows deficit relative to production ratios of the more volatile REE, notably Yb, and, to a lesser extent, Sm and Eu. This may allow estimating an average condensation temperature of trace elements into SiC. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon

Retrospective determination of natal habitats for an estuarine fish with otolith strontium isotope ratios

We investigated the ability of strontium isotope ratios ( 87 Sr/ 86 Sr) in otolith cores to record the natal habitats of juvenile delta smelt Hypomesus transpacificus from the San Francisco Estuary, USA. Young delta smelt (<60 days old) were collected during the California Department of Fish and Game 20-mm Survey in May and June of 1999 at several potential natal areas: Napa River, Suisun Marsh, West Delta, North Delta, Central Delta, South Delta and East Delta. The core region of sagittal otoliths was assayed with laser ablation-multicollector inductively coupled plasma mass spectroscopy. The laser ablation technique provided precise estimates of 87 Sr : 86 Sr ratios with relative standard deviation of 0.003% (one sigma). Isotope ratios ranged from 0.7065 to 0.708 and were different among natal habitats. However, natal habitats within the delta region were not discernable among each other, and reflect the mixing of the two major rivers, Sacramento River and San Joaquin River within the delta. We will therefore be able to determine natal habitats for delta smelt by assaying the core region of the otoliths. The application of strontium isotope ratios ( 87 Sr/ 86 Sr) in fish otoliths will greatly improve conservation efforts for this protected species.

Carbon and other light element contents in the Earth’s core based on first-principles molecular dynamics

Carbon (C) is one of the candidate light elements proposed to account for the density deficit of the Earth’s core. In addition, C significantly affects siderophile and chalcophile element partitioning between metal and silicate and thus the distribution of these elements in the Earth’s core and mantle. Derivation of the accretion and core–mantle segregation history of the Earth requires, therefore, an accurate knowledge of the C abundance in the Earth’s core. Previous estimates of the C content of the core differ by a factor of ∼20 due to differences in assumptions and methods, and because the metal–silicate partition coefficient of C was previously unknown. Here we use two-phase first-principles molecular dynamics to derive this partition coefficient of C between liquid iron and silicate melt. We calculate a value of 9 ± 3 at 3,200 K and 40 GPa. Using this partition coefficient and the most recent estimates of bulk Earth or mantle C contents, we infer that the Earth’s core contains 0.1–0.7 wt% of C. Carbon thus plays a moderate role in the density deficit of the core and in the distribution of siderophile and chalcophile elements during core–mantle segregation processes. The partition coefficients of nitrogen (N), hydrogen, helium, phosphorus, magnesium, oxygen, and silicon are also inferred and found to be in close agreement with experiments and other geochemical constraints. Contents of these elements in the core derived from applying these partition coefficients match those derived by using the cosmochemical volatility curve and geochemical mass balance arguments. N is an exception, indicating its retention in a mantle phase instead of in the core.

Mass-Dependent and Mass-Independent Isotope Effects of Zinc in a Redox Reaction

We report the isotope fractionation of zinc (Zn) associated with a redox reaction between Zn(0) and Zn(II). Zn isotopes were found fractionated in pyrometallurgical biphase extraction between liquid zinc and molten chloride. The isotopic composition of Zn in the molten chloride phase was analyzed by multiple collector inductively coupled plasma mass spectrometry and reported as (m)Zn/64Zn (m = 66, 67, and 68) ratios. The observed isotope fractionation consists of the mass-dependent and mass-independent isotope effects. The contributions of the nuclear mass and the nuclear volume to the overall isotope effect were evaluated by employing first-principles quantum calculations and using reported isotope shifts in atomic spectra. The magnitude of the mass-dependent isotope effect was explained by the sum of the isotope effect via intramolecular vibrations and the correction to the Born-Oppenheimer electronic energy. The mass-independent isotope effect was correlated with the Gibbs free energy change in the redox reaction.

Mass-Independent Isotope Fractionation in the Chemical Exchange Reaction of Chromium (III) Using a Crown Ether

Chromium isotopes were fractionated in a liquid-liquid extraction system using dycycrohexano-18-crown-6 at various hydrochloric acid concentrations. The isotopic ratios of 50Cr/52Cr and 53Cr/52Cr were measured precisely by the multi-collector inductively coupled plasma spectrometry (MC-ICP-MS). The isotope enrichment factor for 50Cr-52Cr isotope pair was shown to vary between 1.5~2.3% and changed as a function of acidity. The magnitudes of mass-independent isotope fractionation could be attributed to the nuclear field shift effect.