Shichun Huang

Enriched components in the Hawaiian plume: Evidence from Kahoolawe Volcano, Hawaii

The geochemical differences between individual Hawaiian shields provide clues to the magma source components in the Hawaiian plume. Lavas from Koolau (Makapuu-stage) and Kahoolawe volcanoes define the enriched, i.e., relatively high 87Sr/86Sr and low 143Nd/144Nd, extreme for Hawaiian shield lavas. There are, however, important geochemical differences between these shields; Kahoolawe lavas lack the relatively high SiO2, low CaO, and high Sr/Nb and La/Nb that are characteristic of Makapuu-stage Koolau lavas, and they are offset from other Hawaiian shield lavas to high 87Sr/86Sr at a given 143Nd/144Nd. Consequently, a varying role for recycled plagioclase-rich gabbro is inferred, in particular, lower amounts of the low 87Sr/86Sr component in Kahoolawe lavas. Also, lavas from Loa-trend volcanoes, such as Kahoolawe, define trends ranging toward high 208Pb*/206Pb* and 87Sr/86Sr and low 143Nd/144Nd and 176Hf/177Hf. Such trends are consistent with variable amounts of recycled sediment sampled by Loa-trend volcanoes, with the largest proportion in Makapuu-stage Koolau lavas. Therefore the enriched component in the Hawaiian plume, the Koolau component, is recycled oceanic crust, which is heterogeneous because of varying proportions of sediment, basalt, and gabbro. Hawaiian shield-stage lavas range widely in 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, and 206Pb/204Pb, but they have similar ratios of Sr/Nd, Nd/Hf, and Hf/Pb, each varying by a factor of <3 among the Hawaiian shields. This observation has important consequences. Namely, the similar Hf/Pb ratios are inconsistent with a two-component (i.e., Kea and Koolau) mixing model for explaining the hyperbolic trend of 176Hf/177Hf versus 206Pb/204Pb defined by shield lavas. Such a model requires end-members with very different Hf/Pb (a factor of 15 to 40), but this is not observed; therefore a third component must be involved. On the basis of trends of 208Pb*/206Pb* versus 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf, we infer that Loa and Kea trend shield lavas contain variable amounts of the Loihi source component.

Major element variations in Hawaiian shield lavas: Source features and perspectives from global ocean island basalt (OIB) systematics

one end (with high 87 Sr/ 86 Sr, 187 Os/ 188 Os, SiO2, and Na2O/TiO2, and low 143 Nd/ 144 Nd, 206 Pb/ 204 Pb, TiO2, CaO and CaO/Al2O3) and by Kea and Loihi lavas at the other (with low 87 Sr/ 86 Sr, 187 Os/ 188 Os, SiO2, and Na2O/TiO2, and high 143 Nd/ 144 Nd, 206 Pb/ 204 Pb, TiO2, CaO and CaO/Al2O3). FeOtotal ,A l2O3 and Na2O concentrations do not correlate with radiogenic isotopes. The Hawaiian data set exhibits correlations that mirror the best correlations between major elements and radiogenic isotope in the global ocean island basalt (OIB) database. We suggest that the mechanism driving the correlations in Hawaii illustrates, in microcosm, a larger global process that generates major element variability in mantle plumes. Like the global arrays, the Hawaiian lavas withradiogenicPband SiO2-poorlavas are sourcedbyaSiO2-poormafic component (pyroxenite) admixed with peridotite, while Hawaiian lavas with unradiogenic Pb and high SiO2 are sourced by a SiO2-rich mafic component (eclogite). The variable SiO2 in the mafic component may result from different degrees of SiO2-extraction from the slab during subduction.

Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas

One explanation of the abrupt cooling episode known as the Younger Dryas (YD) is a cosmic impact or airburst at the YD boundary (YDB) that triggered cooling and resulted in other calamities, including the disappearance of the Clovis culture and the extinction of many large mammal species. We tested the YDB impact hypothesis by analyzing ice samples from the Greenland Ice Sheet Project 2 (GISP2) ice core across the Bølling-Allerød/YD boundary for major and trace elements. We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials. Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source. Circumstantial evidence such as very high, superchondritic Pt/Al ratios associated with the Pt anomaly and its timing, different from other major events recorded on the GISP2 ice core such as well-understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction, hints for an extraterrestrial source of Pt. Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis.

147Sm-143Nd systematics of Earth are inconsistent with a superchondritic Sm/Nd ratio

The relationship between the compositions of the Earth and chondritic meteorites is at the center of many important debates. A basic assumption in most models for the Earth’s composition is that the refractory elements are present in chondritic proportions relative to each other. This assumption is now challenged by recent 142Nd/144Nd ratio studies suggesting that the bulk silicate Earth (BSE) might have an Sm/Nd ratio 6% higher than chondrites (i.e., the BSE is superchondritic). This has led to the proposal that the present-day 143Nd/144Nd ratio of BSE is similar to that of some deep mantle plumes rather than chondrites. Our reexamination of the long-lived 147Sm-143Nd isotope systematics of the depleted mantle and the continental crust shows that the BSE, reconstructed using the depleted mantle and continental crust, has 143Nd/144Nd and Sm/Nd ratios close to chondritic values. The small difference in the ratio of 142Nd/144Nd between ordinary chondrites and the Earth must be due to a process different from mantle-crust differentiation, such as incomplete mixing of distinct nucleosynthetic components in the solar nebula.

Ice-VII inclusions in Diamonds: Evidence for Aqueous Fluid in Earth’s Deep Mantle

Encapsulating Earths deep water filter Small inclusions in diamonds brought up from the mantle provide valuable clues to the mineralogy and chemistry of parts of Earth that we cannot otherwise sample. Tschauner et al. found inclusions of the high-pressure form of water called ice-VII in diamonds sourced from between 410 and 660 km depth, the part of the mantle known as the transition zone. The transition zone is a region where the stable minerals have high water storage capacity. The inclusions suggest that local aqueous pockets form at the transition zone boundary owing to the release of chemically bound water as rock cycles in and out of this region. Science, this issue p. 1136 The presence of ice-VII in diamond inclusions requires regions of the mantle with a free aqueous phase. Water-rich regions in Earth’s deeper mantle are suspected to play a key role in the global water budget and the mobility of heat-generating elements. We show that ice-VII occurs as inclusions in natural diamond and serves as an indicator for such water-rich regions. Ice-VII, the residue of aqueous fluid present during growth of diamond, crystallizes upon ascent of the host diamonds but remains at pressures as high as 24 gigapascals; it is now recognized as a mineral by the International Mineralogical Association. In particular, ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and around the 660-kilometer boundary.

Reply to Boslough: Is Greenland Pt anomaly global or local?

Besides providing additional arguments against the Pt depositing event (1) as a cause of the Younger Dryas cooling, Boslough’s letter (2) raises an important question about the scale of this event. Indeed, a localized deposition of Pt by the Cape York meteorite shower is an attractive hypothesis considered by us initially (3), but abandoned because of (i) a large difference in the Pt/Ir ratios between the Cape York iron and the Pt anomaly in the GISP2 ice core, and (ii) a long ingrowth time of the anomaly (∼20 y), significantly exceeding the expected lifetime (∼5 y) of fine dust in the atmosphere. Such a long ingrowth time is unlikely to result from a later disturbance of ice because both chemical (sulfate) and particle (volcanic ash) spikes induced by volcanic eruptions before and after the Pt anomaly are typically contained within a thin ice layer deposited over 1–2 y. Therefore, the alternative assumed by us is either an abnormally high dust suspension time in the stratosphere or multiple injections of Pt-rich materials to the atmosphere, or both. In either case, a global anomaly is expected.

Mantle geochemistry: Insights from ocean island basalts

The geochemical study of the Earth’s mantle provides important constraints on our understanding of the formation and evolution of Earth, its internal structure, and the mantle dynamics. The bulk Earth composition is inferred by comparing terrestrial mantle rocks with chondrites, which leads to the chondritic Earth model. That is, Earth has the same relative proportions of refractory elements as that in chondrites, but it is depleted in volatiles. Ocean island basalts (OIB) may be produced by mantle plumes with possible deep origins; consequently, they provide unique opportunity to study the deep Earth. Isotopic variations within OIB can be described using a limited number of mantle endmembers, such as EM1, EM2 and HIMU, and they have been used to decipher important mantle processes. Introduction of crustal material into the deep mantle via subduction and delamination is important in generating mantle heterogeneity; however, there is active debate on how they were sampled by mantle melting, i.e., the role of olivine-poor lithologies in the OIB petrogenesis. The origin and location of high 3He/4He mantle remain controversial, ranging from unprocessed (or less processed) primitive material in the lower mantle to highly processed materials with shallow origins, including ancient melting residues, mafic cumulates under arcs, and recycled hydrous minerals. Possible core-mantle interaction was hypothesized to introduce distinctive geochemical signatures such as radiogenic 186Os and Fe and Ni enrichment in the OIB. Small but important variations in some short-lived nuclides, including 142Nd, 182W and several Xe isotopes, have been reported in ancient and modern terrestrial rocks, implying that the Earth’s mantle must have been differentiated within the first 100 Myr of its formation, and the mantle is not efficiently homogenized by mantle convection.

No Measurable Calcium Isotopic Fractionation During Crystallization of Kilauea Iki Lava Lake

In order to investigate possible Ca isotopic fractionation during basaltic magma differentiation, wemeasured Ca isotopic compositions of lavas recovered from Kilauea Iki lava lake at Hawaii. This set of lavas record the whole crystal fractionation history of basaltic magma, ranging from olivine accumulation/fractionation to multiple phase crystallization, including plagioclase and clinopyroxene. Our results show no detectable Ca isotopic variation in all measured Kilauea lavas at a precision of ±0.07‰ for Ca/Ca (δCa = 0.80 ± 0.08, 2 SD, n = 19). Using such observation and published intermineral Ca isotopic fractionation factors, a Monte Carlo approach is used to estimate the mineral-melt Ca/Ca fractionation factors. We found that Ca isotopic fractionation between clinopyroxene and basaltic melt is small, with Δ Ca = 0.04 ± 0.03 at 1200 °C. To the best of our knowledge, this is the first estimated mineral-melt Ca isotopic fractionation factor reported. We use this estimated ΔCa and intermineral Ca isotopic fractionation factors to investigate Ca isotopic effects during mantle partial melting under 1–2 GPa. Our simulations show that the largest Ca/Ca effect, up to +0.3‰, is achieved in large degree melting residues during fractional and dynamic melting. In contrast, partial melts show negligible Ca/Ca isotopic effect, <0.07‰.

Bioavailability of Mineral-Bound Iron to a Snow Algal-Bacterial Coculture and Implications for Albedo-Altering Snow Algal Blooms

ABSTRACT Snow algae can form large-scale blooms across the snowpack surface and near-surface environments. These pigmented blooms can decrease snow albedo and increase local melt rates, and they may impact the global heat budget and water cycle. Yet, the underlying causes for the geospatial occurrence of these blooms remain unconstrained. One possible factor contributing to snow algal blooms is the presence of mineral dust as a micronutrient source. We investigated the bioavailability of iron (Fe)-bearing minerals, including forsterite (Fo90, Mg1.8Fe0.2SiO4), goethite, smectite, and pyrite as Fe sources for a Chloromonas brevispina-bacterial coculture through laboratory-based experimentation. Fo90 was capable of stimulating snow algal growth and increased the algal growth rate in otherwise Fe-depleted cocultures. Fo90-bearing systems also exhibited a decrease in the ratio of bacteria to algae compared to those of Fe-depleted conditions, suggesting a shift in microbial community structure. The C. brevispina coculture also increased the rate of Fo90 dissolution relative to that of an abiotic control. Analysis of 16S rRNA genes in the coculture identified Gammaproteobacteria, Betaproteobacteria, and Sphingobacteria, all of which are commonly found in snow and ice environments. Archaea were not detected. Collimonas and Pseudomonas, which are known to enhance mineral weathering rates, comprised two of the top eight (>1%) operational taxonomic units (OTUs). These data provide unequivocal evidence that mineral dust can support elevated snow algal growth under otherwise Fe-depleted growth conditions and that snow algal microbial communities can enhance mineral dissolution under these conditions. IMPORTANCE Fe, a key micronutrient for photosynthetic growth, is necessary to support the formation of high-density snow algal blooms. The laboratory experiments described herein allow for a systematic investigation of the interactions of snow algae, bacteria, and minerals and their ability to mobilize and uptake mineral-bound Fe. Results provide unequivocal and comprehensive evidence that mineral-bound Fe in Fe-bearing Fo90 was bioavailable to Chloromonas brevispina snow algae within an algal-bacterial coculture. This evidence includes (i) an observed increase in snow algal density and growth rate, (ii) decreased ratios of bacteria to algae in Fo90-containing cultures relative to those of cultures grown under similarly Fe-depleted conditions with no mineral-bound Fe present, and (iii) increased Fo90 dissolution rates in the presence of algal-bacterial cocultures relative to those of abiotic mineral controls. These results have important implications for the role of mineral dust in supplying micronutrients to the snow microbiome, which may help support dense snow algal blooms capable of lowering snow albedo and increasing snow melt rates on regional, and possibly global, scales.