High-field-strength element depletions in arc basalts due to mantle–magma interaction

Oxidized primary arc magmas: Constraints from Cu/Zr systematics in global arc volcanics

Arc volcanics are more oxidized than mid-ocean ridge basalts (MORB), but it is debated whether this is a mantle feature or a result of magmatic evolution. Copper, a sulfur-loving element, has been used to trace the behavior of redox-sensitive sulfur during mantle melting and infer similar redox states of sub-arc and sub-ridge mantle. Previous studies, however, neglected elevated sulfur contents in the sub-arc mantle, leading to underestimation of oxygen fugacities, and did not recognize systematic Cu variations in arc volcanics. Here, we show that the Cu/Zr ratio is a sensitive indicator that responds to sulfur content, oxygen fugacity, and extent of melting of the mantle. Because of higher mantle S contents, Cu systematics of arc magmas require one log unit higher oxygen fugacities of sub-arc than sub-ridge mantle. Low Cu contents of thick-crusted arc volcanics result from low extents of melting of sulfur-rich mantle, obviating the need for deep crustal sulfide fractionation, with substantial implications for the origin of porphyry-Cu deposits.

Sediment-Peridotite Reaction Controls Fore-Arc Metasomatism and Arc Magma Geochemical Signatures

Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.

Abyssal Peridotite as a Component of Forearc Mantle: Inference from a New Mantle Xenolith Suite of Bankawa in the Southwest Japan Arc

Lithology and petrologic nature of the forearc mantle have been left unclear due to the very limited sampling to date. Here, we present petrological data on a forearc peridotite suite obtained as xenoliths in an alkali basalt dike (7.5 Ma) from the Bankawa area in the Southwest Japan arc for our better understanding of the forearc mantle. The host alkali basalt is of asthenosphere origin, and passed through a slab window with slight chemical modification by the slab-derived component. The Bankawa peridotite suite is comprised of lherzolites, which contain various amounts of secondary phlogopite and were metasomatized to various degrees. The least metasomatized lherzolite exhibits Fo91 of olivine, Cr/(Cr + Al) = 0.3 of chromian spinel, and depletion of middle to light rare-earth elements in clinopyroxene, and is overall similar to an abyssal lherzolite. It had originally formed at the proto-Pacific Ocean and then was trapped at a eastern margin of Eurasian continent by initiation of subduction. The forearc mantle peridotite formed as a residue of proto-arc magma formation is depleted harzburgite as represented by the peridotites obtained from the forearc seafloor, but can be less depleted abyssal peridotite if being devoid of partial melting or reaction with magmas after entrapment.

Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland

Zircon megacrysts are locally abundant in 1-40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U-Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Arc-like magmas generated by mélange-peridotite interaction in the mantle wedge

The mechanisms of transfer of crustal material from the subducting slab to the overlying mantle wedge are still debated. Mélange rocks, formed by mixing of sediments, oceanic crust, and ultramafics along the slab-mantle interface, are predicted to ascend as diapirs from the slab-top and transfer their compositional signatures to the source region of arc magmas. However, the compositions of melts that result from the interaction of mélanges with a peridotite wedge remain unknown. Here we present experimental evidence that melting of peridotite hybridized by mélanges produces melts that carry the major and trace element abundances observed in natural arc magmas. We propose that differences in nature and relative contributions of mélanges hybridizing the mantle produce a range of primary arc magmas, from tholeiitic to calc-alkaline. Thus, assimilation of mélanges into the wedge may play a key role in transferring subduction signatures from the slab to the source of arc magmas.

Mélange rocks are predicted to form at the slab-mantle interface in most subduction zones, but their role in arc magmatism is still debated. Here, the authors show that melting of peridotite hybridized by mélange rocks produces melts that carry the major and trace element abundances of natural arc magmas.

Geochemical and mineralogical evidence that Rodinian assembly was unique

The mineralogy and geochemistry associated with Rodinian assembly (~1.3-0.9 Ga) are significantly different from those of other supercontinents. Compared to other supercontinents, relatively more Nb-bearing minerals, Y-bearing minerals, and zircons formed during Rodinian assembly, with corresponding enrichments of Nb, Y, and Zr concentrations in igneous rocks. By contrast, minerals bearing many other elements (e.g., Ni, Co, Au, Se, and platinum group elements) are significantly less abundant, without corresponding depletion of Ni and Co concentrations in igneous rocks. Here we suggest that the Nb, Y, and Zr enrichments in igneous rocks and relatively more occurrences of corresponding Nb-bearing minerals, Y-bearing minerals, and zircons result from significant non-arc magmatism during the mid-Proterozoic, while fewer occurrences of many other minerals suggest enhanced erosion of Rodinian volcanic arcs and orogens. The prolonged, extrovert assembly of Rodinia from thickened mid-Proterozoic continental crust via two-sided subduction can account for both the prevalence of non-arc magmatism and the enhanced erosion.

Lithospheric-scale extension during Grampian orogenesis in Scotland

This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).