Pavel Kepezhinskas

Petrogenesis of slab-derived trondhjemite–tonalite–dacite/adakite magmas

The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al 2 O 3 at the 70% SiO 2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young ( −1 ) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

Trace element and SrNdPb isotopic constraints on a three-component model of Kamchatka Arc petrogenesis

The Kamchatka arc (Russia) is located in the northwestern Pacific Ocean and is divided into three segments by major sub-latitudinal fault zones (crustal discontinuities). The southern (SS) and central (CS) segments are associated with the subduction of old Pacific lithosphere, whereas the northern, inactive segment (NS) was formed during westward subduction of young (< 15 Ma) Komandorsky Basin oceanic crust. Further segmentation of the arc is outlined by the development of the Central Kamchatka Depression (CKD) intra-arc rift, which is oriented parallel to the arc and is splitting the CS into the active Eastern Volcanic Front (EVF) and the largely inactive, rear-arc Sredinny Range. The NS volcanics (15-5 Ma) include calc-alkaline lavas, shoshonites, adakites, and Nb-enriched arc basalts. Isotopically all magma types share high 143Nd/144Nd ratios of 0.512976-0.513173 coupled with variable 87Sr/86Sr (0.702610-0.70356). NS lavas plot within or slightly above the Pacific MORB field on the Pb isotopic diagrams. The EVF volcanoes have more radiogenic 143Nd/144Nd (0.51282-0.513139) and 208Pb/204Pb (38.011–38.1310) than the NS lavas. CKD lavas display MORB-like Nd isotope ratios at slightly elevated 87Sr/86Sr values accompanied by a slightly less radiogenic Pb composition. Kamchatka lavas are thought to be derived from a MORB-like depleted source modified by slab-derived siliceous melts (adakites) and fluids (NS), or fluids alone (CS and SS). The NS and EVF lavas may have been contaminated by small fractions of a sedimentary component that isotopically resembles North Pacific sediment. Petrogenesis in the Kamchatka arc is best explained by a three-component model with depleted mantle wedge component modified by two slab components. Slab-derived hydrous melts produced incompatible element characteristics associated with northern segment lavas, while hydrous slab fluids caused melting in the depleted mantle below the southern and central segments of the Kamchatka arc. Trace element characteristics of Kamchatka lavas appear to be controlled by slab fluids or melts, while radiogenic isotope ratios which are uniform throughout the arc reflect depleted composition of sub-arc mantle wedge.

Progressive enrichment of island arc mantle by melt-peridotite interaction inferred from Kamchatka xenoliths

Abstract The Pliocene (7 Ma) Nb-enriched arc basalts of the Valovayam Volcanic Field (VVF) in the northern segment of Kamchatka arc (Russia) host abundant xenoliths of spinel peridotites and pyroxenites. Textural and microstructural evidence for the high-temperature, multistage creep-related deformations in spinel peridotites supports a sub-arc mantle derivation. Pyroxenites show re-equilibrated mosaic textures, indicating recrystallization during cooling under the ambient thermal conditions. Three textural groups of clinopyroxenes exhibit progressive enrichment in Na, Al, Sr, La, and Ce accompanied by increase in Sr/Y, La/Yb, and Zr/Sm. Trace elements in various mineral phases and from felsic veins obtained through ion microprobe analysis suggest that the xenoliths have interacted with a siliceous (dacitic) melt completely unlike the host basalt. The suite of xenoliths grade from examples that display little evidence of metasomatic reaction to those containing an assemblage of minerals that have been reproduced experimentally from the reaction of a felsic melt with ultramafic rock, e.g., pargasitic amphibole, albite-rich plagioclase, Al-rich augite, and garnet. The dacitic veins within spinel lherzolite display a strong enrichment in Sr and depletion in Y and the heavy rare earth elements (e.g., Yb). The dacites are comparable to adakites (melts derived from subducted metabasalt), and not typical arc melts. We believe that these potential slab melts were introduced into the mantle beneath this portion of Kamchatka subsequent to partial melting of a relatively young (and hot) subducted crust. Island arc metasomatism by peridotite-slab melt interaction is an important mantle hybridization process responsible for arc-related alkaline magma generation from a veined sub-arc mantle.

Evidence suggests slab melting in arc magmas

Most recent geology textbooks state that subduction-related volcanism is due to the melting of the down-going lithosphere. However, for the last 30 years, few in the field have seriously believed that the subducting slab is the source of arc basalts. The accepted hypothesis involves melting of the mantle wedge above the slab via hydrous fluids produced during the transition of the subducting basalt from amphibolite to eclogite. The parental basalts differentiate primarily through crystal fractionation, magma mixing, and differentiation at the Mohorovicic discontinuity into andesites and dacites as they ascend; the basalts are too dense to rise through the lower continental crust. This explains the relative abundance of differentiated rocks in arcs.

The nature of metasomatism in the sub-arc mantle wedge: evidence from Re-Os isotopes in Kamchatka peridotite xenoliths

Abstract We have performed Re–Os isotope measurements on a suite of 21 Kamchatka mantle xenoliths including 19 harzburgites and two lherzolites, from the northern arc front (Valovayam Volcano), the southern arc front (Avachinsky Volcano), and behind the arc front in the south (Bakening Volcano). Os and Re concentrations vary from 0.02 to 8.2 and 0.003 to 0.437 ppb, respectively, and 187Re/188Os varies from 0.004 to 3.811. 187Os/188Os ratios range from 0.1226 to 0.1566. Regional variations in Re–Os isotope signatures are apparent, with peridotites from Avachinsky exhibiting the least radiogenic Os isotope signatures and lowest Re/Os ratios, and those from Bakening the most radiogenic Os and highest Re/Os. Peridotites from Valovayam span the distinct compositional fields defined by the Avachinsky and Bakening peridotites. All of the Kamchatka peridotites are, however, characterized by radiogenic 187Os/188Os compared to non-arc continental peridotites with comparable Re abundances or Re/Os ratios. The relatively radiogenic Os isotope signatures in the Kamchatka peridotites cannot easily be explained by contamination of the xenoliths by their host lavas, as this process would result in Re/Os ratios higher than observed in the xenoliths. In situ radiogenic ingrowth of high Re/Os mantle followed by recent Re depletion also cannot explain the observed radiogenic Os signatures in the Kamchatka peridotites, as the time required for radiogenic ingrowth would be significantly greater than the age of the lithospheric terranes that make up the respective regions of Kamchatka. The radiogenic Os isotope signatures in the Kamchatka peridotites are instead attributed to metasomatism of the Kamchatka sub-arc mantle wedge by radiogenic slab-derived fluids and melts. The regional variations in Re–Os isotope signatures are consistent with previous petrographic and geochemical studies of the Kamchatka mantle xenoliths that reveal multistage metasomatic histories resulting from interaction of the mantle wedge with a variety of slab-derived fluids and melts, including silicic slab-melt metasomatism associated with subduction of relatively hot, young (∼15–25 Ma) oceanic crust in the northern arc front, hydrous slab-fluid metasomatism associated with subduction of colder, old (∼100 Ma) oceanic crust in the southern arc front, and carbonate-rich slab-melt metasomatism in the southern segment behind the arc front, where the slab is deeper. Positive correlations between 187Os/188Os, La/Sm, and Ru/Ir in Avachinsky harzburgites support a model in which high fO2, Cl-rich, hydrous slab fluids transport LREE, Ru, and radiogenic Os into the mantle wedge beneath the southern arc front. Re is either not transported, or is not retained in the mantle during fluid–mantle interaction. Relatively higher Re and more radiogenic Os (but low Os abundances) in the Valovayam and Bakening peridotites indicate that both scavenging of mantle Os as well as exchange with radiogenic slab-derived Os, and incorporation of Re, occurs during interaction of the mantle wedge with oxidized, adakitic, and carbonate-rich slab melts. Similar ranges of Re–Os isotope signatures in peridotites from Avachinsky, Japan and Lihir, and from Valovayam and the Cascades, respectively, suggest that the age (temperature) and depth of subducting oceanic crust influences the Re–Os composition of metasomatized sub-arc mantle.

Insights into the volcanic arc mantle wedge from magnesian lavas from the Kamchatka arc

Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchatka peninsula south of its junction with the Aleutian arc. Most volcanism occurs within the Central Kamchatka Depression (CKD), a large graben oriented parallel to the trench, and along the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc-alkaline to tholeiitic. Fractionation of a mineral assemblage including olivine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible crustal assimilation, produces the calc-alkaline trend. A suite of near-primitive high-Mg basalts provides geochemical records of mantle sources and processes unobscured by differentiation. Rare earth element (REE) patterns range from slightly depleted ((Ce/Yb) n =0.8-1.5) to slightly enriched ((Ce/Yb) n =1.5-3.5). Rocks with the depleted REE patterns occur at the volcanic front in regions where a volcano or volcanic chain exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the volcanic front where behind-the-front volcanism is absent. Modeling of trace element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted source, inferred to represent refractory source material remaining after a previous generation of melt extraction within the arc. Mantle source material apparently convects into the mantle wedge from the rear, producing relatively enriched magmas when it melts for the first time. Relatively depleted magmas are produced if a second period of melting ensues as the mantle reaches the volcanic front.

Abundance and distribution of PGE and Au in the island-arc mantle: implications for sub-arc metasomatism

Ultramafic xenoliths from a veined mantle wedge beneath the Kamchatka arc have non-chondritic, fractionated chondrite-normalized platinum-group element (PGE) patterns. Depleted (e.g., low bulk-rock Al2O3 and CaO contents) mantle harzburgites show clear enrichment in the Pd group relative to the Ir group PGEs and, in most samples, Pt relative to Rh and Pd. These PGE signatures most likely reflect multi-stage melting which selectively concentrates Pt in Pt–Fe alloys while strongly depleting the sub-arc mantle wedge in incompatible elements. Elevated gold concentrations and enrichment of strongly incompatible enrichment (e.g., Ba and Th) in some harzburgites suggest a late-stage metasomatism by slab-derived, saline hydrous fluids. Positive Pt, Pd, and Au anomalies coupled with Ir depletions in heavily metasomatized pyroxenite xenoliths probably reflect the relative mobility of the Pd and Ir groups (especially Os) during sub-arc metasomatism which is consistent with Os systematics in arc mantle nodules. Positive correlations between Pt, Pd, and Au and various incompatible elements (Hf, U, Ta, and Sr) also suggest that both slab-derived hydrous fluids and siliceous melts were involved in the sub-arc mantle metasomatism beneath the Kamchatka arc.

On the Tectonic Significance of Arc Volcanism in Northern Kamchatka

The Vyvenka volcanic field records a period of Neogene, subduction-related volcanism in northern Kamchatka. Most models describing the tectonic evolution of the northwest Pacific do not account for this type of Neogene volcanism because the main locus of Pacific/Kula-North American convergence switched to the Aleutian Ridge during Eocene time. The Vyvenka volcanism, as well as oceanic spreading and crust formation within the Komandorsky Basin, demonstrate that this region remained tectonically and volcanically active in Neogene times. We report petrologic, geochemical, and K-Ar age data for the ~15 Ma Golovin and 6-8 Ma Valovayam volcanic rocks, two andésite suites within the Vyvenka volcanic field. The Golovin suite consists of medium- to high-K andesites with strong arc-like trace-element signatures, while the Valovayam suite consists of medium-K andesites with weaker arc-like trace-element signatures. The Valovayam andesites also contain some trace-element ratios indicative of melting of the subducted oceanic crust. These include high Sr/Y (30-50) and Zr/Sm greater than the chondritic value of 28. The Golovin andesites have overlapping Sr/Y (25-45) and lower Zr/Sm. The compositional differences between the Golovin and Valovayam andesites correlate with Neogene tectonic evolution of the Komandorsky region. In northern Kamchatka, subduction waned as spreading stopped in the Komandorsky Basin and newly generated oceanic crust entered the subduction zone. Thus, the trace-element signals of slab melts in the younger Valovayam rocks indicates melting of the young, hot Komandorsky Basin crust that entered the subduction zone and subsequent metasomatism of the mantle wedge. The weaker subduction signature of the Valovayam suite, which distinguishes it from the Golovin suite, records the decreasing vigor of subduction processes with time.

Nonchondritic Pt/Pd ratios in arc mantle xenoliths: Evidence for platinum enrichment in depleted island-arc mantle sources

Mantle-derived spinel harzburgites from the Kamchatka arc have fractionated Pd- group element patterns, and Pt is clearly enriched relative to Pd and Rh. This fractionation is also consistent with chondritic Ir-group–platinum-group element distribution and super- chondritic Pt/Pd, chondrite-normalized (Pt/Os)N and (Pt/Ir)N found in harzburgite xenoliths from the Tubaf seamount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc in Papua New Guinea. The nonchondritic Pt/Pd ratios and Pt enrichment in the island-arc mantle are best explained by extraction of melt in a back-arc volcanic-arc setting, which left refractory Pt-Fe alloys in the residual mantle. Pt-Fe alloys selectively concentrate Pt relative to Pd and, overall, increase the Pt/Pd in a subarc mantle wedge. Partial melting of this Pt-enriched mantle wedge above the subduction zone, due to hydrous flux from the subducting lithosphere, is potentially capable of producing Pt-rich primitive melts parental to the Alaskan-type ultramafic-mafic complexes.

Cold Moho boundary beneath island arc systems: An example from the North Kamchatka Arc

A suite of amphibole-bearing ultramafic and mafic xenoliths representing the crust-mantle boundary was sampled in the northern segment of the Kamchatka arc. These nodules exhibit low equilibration temperatures at elevated pressures suggesting relatively cold conditions at Moho depths, which are uncommon in modern island arcs. A model including successive stages of subduction initiation, subduction cessation resulting in cooling of the lithospheric wedge, followed by intra-arc rifting due to upwelling of astenospheric diapirs is proposed to explain the observed textural, petrologic and chemical characteristics of sampled rock assemblage.