We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

http://people.earth.yale.edu/sites/default/files/herzberg10.pdf

Thermal history of the Earth and its petrological expression

Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their trace-element and isotopic characteristics Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere In melting experiments on natural amphibole-rich veins at 15 gigapascals, we found that partial melts of metasomatic veins can reproduce key major- and trace-element features of oceanic and continental alkaline magmas Moreover, experiments with hornblendite plus lherzolite showed that reaction of melts of amphibole-rich veins with surrounding lherzolite can explain observed compositional trends from nephelinites to alkali olivine basalts We conclude that melting of metasomatized lithosphere is a viable alternative to models of alkaline basalt formation by melting of recycled oceanic crust with or without sediment

http://science.sciencemag.org/content/sci/320/5878/916.full.pdf

Metasomatized lithosphere and the origin of alkaline lavas.

Large igneous provinces (LIPs) are known for their rapid production of enormous volumes of magma (up to several million cubic kilometres in less than a million years), for marked thinning of the lithosphere, often ending with a continental break-up, and for their links to global environmental catastrophes. Despite the importance of LIPs, controversy surrounds even the basic idea that they form through melting in the heads of thermal mantle plumes. The Permo-Triassic Siberian Traps--the type example and the largest continental LIP--is located on thick cratonic lithosphere and was synchronous with the largest known mass-extinction event. However, there is no evidence of pre-magmatic uplift or of a large lithospheric stretching, as predicted above a plume head. Moreover, estimates of magmatic CO(2) degassing from the Siberian Traps are considered insufficient to trigger climatic crises, leading to the hypothesis that the release of thermogenic gases from the sediment pile caused the mass extinction. Here we present petrological evidence for a large amount (15 wt%) of dense recycled oceanic crust in the head of the plume and develop a thermomechanical model that predicts no pre-magmatic uplift and requires no lithospheric extension. The model implies extensive plume melting and heterogeneous erosion of the thick cratonic lithosphere over the course of a few hundred thousand years. The model suggests that massive degassing of CO(2) and HCl, mostly from the recycled crust in the plume head, could alone trigger a mass extinction and predicts it happening before the main volcanic phase, in agreement with stratigraphic and geochronological data for the Siberian Traps and other LIPs.

/pdf/linking-mantle-plumes-large-igneous-provinces-and-422ofretq3.pdf

Linking mantle plumes, large igneous provinces and environmental catastrophes

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile-enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ~1450–1500°C in the Atlantic and 1500–1600°C in the Pacific, substantially greater than ~1350°C for ambient mantle. Lavas from the Galapagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

/pdf/petrology-of-some-oceanic-island-basalts-primelt2-xls-4ejvkk7e5f.pdf

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

/pdf/the-amount-of-recycled-crust-in-sources-of-mantle-derived-4ohxfxntm0.pdf

Vladimir V. Matvienkov

Papers

The Amount of Recycled Crust in Sources of Mantle-Derived Melts