Nobumichi Shimizu

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Abstract Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio ∼2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (∼1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.

Relative depletion of niobium in some arc magmas and the continental crust : partitioning of K, Nb, La, and Ce during melt/rock reaction in the upper mantle

Abstract Depletion of Nb relative to K and La is characteristic of lavas in subduction-related magmatic arcs, as distinct from mid-ocean ridge basalts. Nb depletion is also characteristic of the continental crust. This and other geochemical similarities between the continental crust and high-Mg# andesite magmas found in arcs suggests that the continental crust may have formed by accretion of andesites. Previous studies have shown that the major element characteristics of high-Mg# andesites may be produced by melt/rock reaction in the upper mantle. In this paper, new data on partitioning of K, Nb, La and Ce between garnet, orthopyroxene and clinopyroxene in mantle xenoliths, and on partitioning of Nb and La between orthopyroxene and liquid, show that garnet and orthopyroxene have Nb crystal/liquid distribution coefficients which are much larger than those of K and La. Similar fractionations of Nb from K and La are expected in spinel and olivine. For this reason, reactions between migrating melt and large masses of mantle peridotite can produce substantial depletion of Nb in derivative liquids. Modeling shows that reaction between ascending, mantle-derived melts and mantle peridotite is a viable mechanism for producing the trace element characteristics of high-Mg# andesite magmas and the continental crust. Alternatively, small-degree melts of metabasalt and/or metasediment in the subducting slab may leave rutile in their residue, and will thus have large Nb depletions relative to K and La [1]. Slab melts are too rich in light rare earth elements and other incompatible elements, and too poor in compatible elements, to be parental to arc magmas. However, ascending slab melts may be modified by reaction with the mantle. Our new data permit modeling of the trace element effects of reaction between small-degree melts of the slab and mantle peridotite. Modeling shows that this type of reaction is also a viable mechanism for producing the trace element characteristics of high-Mg# andesites and the continental crust. These findings, in combination with previous results, suggest that melt/rock reaction in the upper mantle has been an important process in forming the continental crust and mantle lithosphere.

Rapid reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions

The solubility of H 2 O in silicate melt drops substantially with decreasing pressure, so that a magma initially containing several weight percent H 2 O in a crustal magma reservoir is left with only a few thousand parts per million following ascent and eruption at the Earth’s surface. This rapid release of volatiles makes determining the pre-eruptive H 2 O contents of magmas very difficult. Olivine-hosted melt inclusions are thought to retain their H 2 O because they are protected from decompression by the strength of the host crystal, and pre-eruptive concentrations obtained from melt inclusions have been used to both estimate the amount of H 2 O in the upper mantle and investigate its role in the melt generation process. The greatest uncertainty involved in constraining upper mantle conditions from melt inclusions is the potential for rapid diffusive loss or gain of H + (protons) through the host olivine. Here we present results from hydration and dehydration experiments that demonstrate that, contrary to the widely held view, H 2 O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H + through the host olivine are possible on very short time scales. We also show that the Fe 3+ /ΣFe of an olivine-hosted melt inclusion maintains equilibrium with the external environment via diffusion of point defects through the host olivine. Our results demonstrate that, while pre-eruptive H 2 O and Fe 3+ /ΣFe can be reliably estimated, olivine-hosted melt inclusions do not necessarily retain a record of the H 2 O and O 2 fugacity conditions at which they formed. High-H 2 O melt inclusions are particularly susceptible to diffusive dehydration, and therefore are not reliable proxies for the state of the upper mantle.

Diffusivity of oxygen in jadeite and diopside melts at high pressures

Abstract The diffusivity of oxygen was determined in melts of Jadeite (NaAlSi2O6) and diopside (CaMgSi2O6) compositions using diffusion couples with 18O as a tracer. In the Jadeite melt, the diffusivity of oxygen increases from 6.87 −0.25 +0.28 × 10 −10 cm 2 / sec at 5 Kb to 1.32 ± 0.08 × 10 −9 cm 2 / sec at 20 Kb at constant temperature (1400°C), whereas in the diopside melt at 1650°C, the diffusivity decreases from 7.30 −0.18 0.29 × 10 −7 cm 2 / sec at 10 Kb to 5.28 −0.55 +0.60 × 10 −7 cm 2 / sec at 17 Kb. These results demonstrate that the diffusivity is inversely correlated with the viscosity of the melt. For the jadeite melt, in particular, the inverse correlation is very well approximated by the Eyring equation using the diameter of oxygen ions as a unit distance of translation, suggesting that the viscous flow is rate-limited by the diffusion of individual oxygen ions. In the diopside melt, the activation volume is slightly greater than the molar volume of oxygen ion, indicating that the individual oxygen ion is the diffusion unit. The negative activation volume obtained for the jadeite melt is interpreted as the volume decrease associated with a diffusive jump of an oxygen ion due to local collapse of the network structure.

Isotope fractionation in secondary ion mass spectrometry

We report a systematic ion microprobe study of the isotope fractionation effects produced during the sputtering of pure metal targets. The metals studied were B, Mg, Si, Cr, Ni, Cu, Ge, Mo, Ag, Sb, Re, Tl, and Pb; isotopic ratios were determined with typical precisions of 0.1%–0.3%. Detailed studies show that the data are free of possible instrumental biases, interference effects, etc. In all cases, the secondary ions produced by sputtering are enriched in the lighter isotope, and the degree of this enrichment is a function both of the spatial location and the energy of the extracted ions relative to the sputtering site. The maximum observed L/H enrichment factors vary from 6.5%/amu for B to 0.6%/amu for Pb, and are observed to show an approximate MH/ML [rather than a (MH/ML)1/2 ] dependence on mass. For masses greater than 50, the mass dependence of this fractionation can be closely described by an ionization model modified after Schroeer et al. [Surf. Sci. 34, 571 (1973)]; it is improbable that the obse...

Trace-element zoning in a metamorphic garnet

Trace-element zoning has been measured in an amphibolite garnet from the Tauern window, Austria, by using an ion microprobe. Humps in the zoning profiles of Na, Sc, V, Y, and the heavy rare-earth elements mark a period of open-system behavior. These humps correspond to a part of the major-element zoning profile that is interpreted as a P-T reversal. The source of the mass excesses of these elements remains ambiguous: they were derived either on a thin-section scale by the breakdown of trace-element-enriched refractory minerals or externally from unusual trace-element-enriched fluids. P-T paths determined from garnet zoning may require modification if open-system behavior is important during garnet growth.

Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt

Under the Sea Floor Microorganisms living in basaltic sea floor buried beneath sediments derive energy from inorganic components from the host rocks that interact with infiltrating seawater, which brings dissolved oxygen and other trace nutrients with it. Lever et al. (p. 1305) directly sampled the subseafloor community off the eastern flank of the Juan de Fuca Ridge in the Pacific Ocean and found evidence for ongoing microbial sulfate reduction and methanogenesis. Multiyear incubation experiments with samples of host rock confirmed the microbial activities measured in situ. Active methane- and sulfur-cycling microbial communities exist in deep basaltic ocean crust. Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earths oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ13C- and δ34S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ13C signatures locally attenuated by heterotrophic metabolism.

Open‐system melting in the upper mantle: Constraints from the Hayachine‐Miyamori ophiolite, northeastern Japan

The Hayachine and Miyamori hydrous ultramafic complexes, northeastern Japan, exhibit not only diverse lithological variations from very primordial spinel lherzolites (olivine forsterite mol % = 89 (Fo 89 ), spinel 100Cr/(Cr+Al) (Cr #) =10) to refractory harzburgites (Fo 93 , spinel Cr #=70) but also show diverse trace element characteristics in amphiboles and clinopyroxenes correlated with the major element behavior. Some of the most primitive lherzolites are from the Hayachine complex and contain a clinopyroxene with strongly light rare earth element (REE) depleted abundances. The slightly refractory aluminous spinel peridotites of the Miyamori complex forming kilometric blocks in chromian spinel peridotites, exhibit flat to weakly V-shaped REE patterns with lower heavy REE (HREE) abundances. The chromite-bearing peridotites of the Miyamori complex exhibit light REE (LREE) enriched patterns with further lower HREE abundances than the aluminous spinel peridotites. The most refractory clinopyroxene-free harzburgite (Fo 93 ) contains amphibole with the highest LREE/HREE ratio among the Miyamori-Hayachine peridotites. The observed covariance between major and trace elements together with geological and petrological data for the complex is reproduced with an open-system melting model, in which continuous supply of LREE-enriched melt, melting, and melt segregation are assumed to be coupled. Nonmodal melting with specified amounts of trapped melt is also taken into consideration. The result shows that REE and trace element data for less refractory peridotites can be reproduced fairly well by the model. Accounting for the REE patterns of more refractory peridotites, however, proved less tractable, indicative of the importance of a melting and melt separation process involving melt migration and reactions during the decompression of the mantle material.

World-wide occurrence of HFSE-depleted mantle

Abstract We report here the widespread occurrence of mantle peridotites in both suboceanic and subcontinental lithosphere that show trace element characteristics similar to those of island arc volcanics. These anhydrous peridotites, which reside at high levels in the mantle, cannot generate nor be in equilibrium with mid-ocean ridge or ocean island basalts, and have complex geochemical histories involving partial melting, melt extraction and metasomatism.

Trace element abundance patterns of garnet inclusions in peridotite-suite diamonds

Abstract Individual, sub-calcic, chrome-pyrope crystals from Finsch and Kimberley diamonds, Finsch and Bultfontein kimberlite heavy mineral concentrate, and from diamondiferous harzburgite-dunite xenoliths from the Udachnaya kimberlite pipe were analyzed for rare earth elements (REE), Sc, Ti, and Zr with the ion microprobe. The abundances and abundance ratios of these trace elements including LREE enrichment and low Ti, together with high and variable Cr contents, are inconsistent with a simple equilibrium relationship between peridotite-suite garnet and silicate and carbonate liquids. It is suggested that the trace element abundance patterns represent a signature of ancient mantle metasomatism which preceded the formation of peridotite-suite garnet and diamond.