Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ($${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{spl}/\mathrm{melt}}$$ D Fe 2 O 3 spl / melt ) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ D Fe 2 O 3 opx / melt = 0.63 ± 0.10 and $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{cpx}/\mathrm{melt}}$$ D Fe 2 O 3 cpx / melt = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{spl}/\mathrm{melt}}$$ D Fe 2 O 3 spl / melt function alone allows ~ 40% of the variation in MORB compositions. If we allow $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ D Fe 2 O 3 opx / melt and $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ D Fe 2 O 3 opx / melt to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

U-depleted versus U-enriched signatures in complementary crust-mantle sources of volcanic rocks from Asia

<p>The Nb/U~47 and Th/U~4 ratios are considered as indicative for the OIB source referred by some authors to lower mantle plumes that in fact have no specific geochemical signatures but HIMU component. The Th/U ratio may vary because of the different garnet–melt and/or clinopyroxene–melt partition coefficients of U and Th. Anomalously high or low Th/U values in rocks can also be related to the input or removal of U, the migration of which is controlled by its mobility under oxidizing conditions owing to the formation of water-soluble uranyl  compounds with hexavalent U. These variations definitely distinguish non-plume magmatic sources. The Th/U ratio decreases to 2.5 in the MORB source and increases to 6 in the continental lower crust one. We describe anomalous behavior of uranium in sources of Cenozoic basalts and basaltic andesites from Primorye, Lesser Khingan, Tunka Valley, as well as similar Cretaceous-Paleogene rocks from Tien Shan. Significant deviations of the Th/U and Nb/U ratios from the OIB values are characteristics mostly of garnet-free sources. The U-depleted and U-enriched signatures are used as sensitive indicators for deciphering crust–mantle transitional processes.</p><p>This work is supported by the RSF grant 18-77-10027.</p>

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

Low recycling efficiency of boron into the deep mantle

<p>Boron is a distinctly crustal element in that it is strongly enriched in the surface reservoirs, such as continental crust, seawater, sediments, serpentinites and altered oceanic crust, relative to the mantle. These B-enriched reservoirs are also isotopically very distinct from the regular depleted upper mantle (d<sup>11</sup>B = -7.1 ±0.9 ‰ [10.1016/j.gca.2017.03.028]). This has encouraged the idea that boron could be an ideal tracer for subducted surface materials in the deep mantle in the form of isotopically anomalous recycled components in ocean island basalts (OIB) and enriched MORB. Yet, the potential of a geochemical tracer of this type is weakened by its extraction from the slab at the onset of subduction by dewatering and metamorphic dehydration, because this process depletes the recycled components in fluid-mobile elements. As such, this “subduction barrier” diminishes the deep recycling efficiency of incompatible, fluid-mobile tracers like B.</p><p>This study focuses on the B abundances and B isotopic compositions of glasses and melt inclusions that show low Cl/K ratios and are thought to represent the uncontaminated mantle signal from the HIMU (Tuvalu and Mangaia), EM1 (Pitcairn) and EM2 (Samoa) sources. Strikingly, all samples are depleted in boron by a factor of approximately 1.5 to 4 relative to non-fluid-mobile elements of similar incompatibility (e.g. LREE, P, Be). This negative boron anomaly is ubiquitous in OIB and is consistent with the results of previous studies [10.1016/0016-7037(95)00402-5; 10.1016/j.epsl.2018.12.005]. It also mirrors their characteristic negative Pb anomaly. These anomalies show that the mantle sources of OIB are depleted in B (and Pb) relative to non-fluid-mobile elements of similar incompatibility and relative to the MORB-source mantle. This is best explained by the presence in the OIB sources of recycled components that are enriched in all incompatible elements except for the fluid-mobile B (and Pb). The fluid mobile elements must have been preferentially extracted in the subduction barrier and returned to the surface on the short path via arc magmas. Arc magmas consistently show a general enrichment in isotopically heavy boron [10.1007/978-3-319-64666-4_9] with positive B anomalies.</p><p>Despite of the low recycling efficiency of boron into the convecting mantle, OIB still have B isotope signatures that are distinct from those of MORB. Previous studies have reported OIB signatures slightly lighter than MORB and the primitive mantle [10.1016/j.epsl.2018.12.005]. However, our study exclusively finds isotopically heavy B with a range in d<sup>11</sup>B from MORB-like values (-8.6 ±2.0 ‰) up to -2.5 ±1.5‰ for EM1 and HIMU lavas. The total OIB range is small but significant, and is consistent with the deep recycling of material that is strongly depleted in boron, but isotopically distinct (with isotopically heavy B in the case of our EM1 and HIMU samples). The B depletion combined with the B isotopic anomaly in OIB shows that B is efficiently (but not quantitatively) removed from the slab during subduction, and that isotopically distinct mantle domains are thus produced. The subduction barrier for boron increases its strength as a tracer in arcs, but it diminishes its potential as a tracer of deep mantle recycling.</p>

Compositions and formation conditions of primitive magmas of the Karymsky volcanic center, Kamchatka: evidence from melt inclusions and trace-element thermobarometry

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H2O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H2O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of DQFM = 1.5 ± 0.4. The obtained estimates of H2O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H2O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably to decrease the existing temperature estimates for the mantle wedge above subduction zones.