Maxim Portnyagin

Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction

Spatial geochemical variations of Quaternary lavas erupted along the northern segment of the Kamchatka arc are used to trace changes in magma generation across the subducting Pacific slab edge. The late Pleistocene–Holocene lavas of the northern end of the Central Kamchatka depression north of the Pacific slab edge show strong enrichment in high field strength elements and light rare earth elements, relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios, and oxygen isotope compositions similar to those of mid-oceanic-ridge basalts. These geochemical characteristics are distinct from the southern Central Kamchatka depression volcanoes located above the subducting Pacific slab. Extensive fluid-triggered mantle melting dominates magma genesis beneath the largest Kamchatka volcanoes in the south, whereas low-degree decompression melting of the Pacific asthenospheric mantle is the major magma generation process north of the Pacific slab edge. Quaternary detachment of the subducted Pacific plate fragment resulted in the influx of fertile mantle beneath Kamchatka. We propose that upwelling and southward flow of this hotter, more fertile mantle is the main reason for recent magmatism in northern Kamchatka and for the exceptional productivity of the Central Kamchatka depression volcanoes (Klyuchevskoy and Sheveluch), the most active arc volcanoes on Earth.

Komatiites reveal a hydrous Archaean deep-mantle reservoir

Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.

Geochemistry of Primitive Lavas of the Central Kamchatka Depression: Magma Generation at the Edge of the Pacific Plate

New and published major and trace element and isotope (O, Sr, Nd) compositions of the Late Quaternary rocks from the Central Kamchatka Depression (CKD) are used to demonstrate systematic changes in magma genesis along the northern segment of the Kamchatka Arc, above and north of the subducting Pacific slab edge. We envision a number of possible petrologic scenarios for magma generation beneath the CKD and formulate quantitative mass-balance models which lead to three major conclusions departing significantly from previous interpretations of the CKD rocks. First, this study demonstrates that eclogite melts contribute to the composition of virtually all CKD lavas and could be the major agent transferring material from the subducted slab to the mantle wedge, including fluid-mobile elements (e.g., K, Ba). Second, thermal state of the mantle wedge beneath the CKD has primary control on the major composition of primitive magmas, favoring production of low temperature andesitic and dacitic mantle melts toward the slab edge. Third, hydrous slab-fluids might not be required to generate CKD magmatism. Instead, strong enrichment in LILE, high δ 18 O and 87 Sr/ 86 Sr, in some CKD magmas could originate from assimilation of hydrothermally-altered mafic lithosphere. Several concurring factors could facilitate partial melting of the subducting slab beneath the all CKD volcanoes and favor variable modification of the eclogite melts during interaction with the mantle wedge. Large input from slab melting makes CKD magmatism unique in Kamchatka and may contribute to the CKD volcanoes being the most productive arc volcanoes on Earth.

Bowers Ridge (Bering Sea): An Oligocene–Early Miocene island arc

Bowers Ridge is an similar to 700 km long arcuate ridge behind the Central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian-Bering Sea region. Here we present the first geochemical and Ar-40/Ar-39 age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V Sonne cruise SO201-1b. The northern Bowers Ridge basement (26-32 Ma) consists of mafic to intermediate calc-alkaline rocks with adakite-like (Sr/Y = 33-53, La-N/Yb-N = 3.3-7.8), high field strength element (HFSE)-depleted (e.g., Nb-N/La-N = 0.07-0.31) trace element patterns and Sr-Nd-Pb isotope compositions within the Western Aleutian Arc array, implying magma generation above an obliquely subducting slab. The seamount samples (22-24 Ma) are HFSE-rich alkaline olivine basalts (La-N/Yb-N = 3.3-3.9, Nb-N/La-N = 1.0-1.4) with minor arc-type trace element signatures (Pb-N/Ce-N = 1.4-1.6, K-N/Nb-N = 1.7-1.9) but with Pacific mid-oceanic-ridge basalt (MORB)-like isotopic compositions, pointing to an origin by small-degree decompression melting from slightly subduction-modified mantle. The geochemistry of the recovered rocks can be explained by highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as a Cenozoic island arc.

Contrasting conditions of rift and off‐rift silicic magma origin on Iceland

Factors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H2O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, Oraefajokull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off-rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high-temperature hydrothermally altered crust (δ18O < 2‰) at shallow depths (≥1.8 km). These rhyolites are hot (935–1008°C), relatively dry (H2O < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874–902°C), wet (H2O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (Oraefajokull, Hekla) originate through differentiation deeper in the crust (≥4 km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal δ18O values (5.2–6‰). Although off-rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.

Experimental evidence for rapid re-equilibration of water between melt inclusions in olivine and host magma

ICP and XRF Spectroscopy methods were used to evaluate the metals (ppm) from soils for the mining dumps of Crucea- Botusana uranium deposit (Bistrita Mountains, Romania). The sequential extraction has emphasized the fact that U is associated with all the mineral fractions present in the soil samples. A great percentage of U can be found in the carbonate, organic and oxides fractions. The percentage of U detected in the exchangeable fraction is rather small. The fact that 21.77% of the total U can be found in the specifically absorbed and carbonate bound fraction, indicated the important role played by the carbonates in the retention of U; one the other hand this fraction is liable to release U if the pH should happen to change.Th appear in high-enough concentration in the soil is scarcely available because 70.29% is present in residual fraction, and about 21.78% in the organic and oxides fractions. This is certainly due to the fact that this naturally occurring radionuclide can be associated with relatively insoluble mineral phases like alumino-silicates and refractory oxides. Its association with the organic matter suggests that it can form soluble organic complexes that can facilitate its removal by the stream waters. In the case of Sr, the sequential extraction shows that it is very strongly fixed because the residual fraction concentrates the great amount of this element. What is interesting is the percentage of 2.65 % of Sr from the exchangeable fraction because it can be easily released and transported to the surrounding environment. Pb it is present in various relatively soluble pools (17.81% in carbonate boud and 34.85% in organically bound), which appears to be an efficient sink for this element. This fact may indicate a possible link between the biological activity and the Pb cycling into the soil. In addition, only 17.78% is present in the insoluble residual fraction. Although from our research it resulted that the radioactive metals does not concentrate in the exchangeable fraction (Th) or it concentrates very little in it (U and Sr), the isolation of the mineral fraction of soil rich in U, Th and Sr helps us in the future identification of the connections which control the cycle of the radioactive metals. These results have important implications for remediation strategies. The thorium and uranium from Crucea mining area are in labile, not strongly retained, fractions, thus making them amendable for remediation by phytoremediation.

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Abstract Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several <0.5 μm Pd–Sn, Pt–Ag, and Au–Ag phases are recorded within the matrix sulfides, commonly along phase boundaries and fractures. Major elements (S, Fe, Cu, Ni, Co), platinum group elements (PGE), and gold analyzed in the homogenized olivine-hosted sulfide melt inclusions, and phases identified in the matrix sulfides record the range of magmatic sulfide compositions. The most primitive sulfide liquids are notably enriched in Ni and Cu [(Ni+Cu)/Fe, at% > 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.

Experimental calibration and implications of olivine-melt vanadium oxybarometry for hydrous basaltic arc magmas

Abstract The strong dependence of vanadium partitioning between olivine and silicate melt ( DVO1-M

The origin of EM1 alkaline magmas during Cenozoic reorganization of subduction zone of Kamchatka

\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array}

Experimental evidence for rapid water exchange between melt inclusions in olivine and host magma

) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVO1-M