Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology

http://szseminar.asu.edu/readings/EPSL_Schmidt_1998.pdf

Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation

Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.

https://science.sciencemag.org/content/sci/268/5212/858.full.pdf

Serpentine Stability to Mantle Depths and Subduction-Related Magmatism

Subduction zones: subduction zones

It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process.

Growth of early continental crust controlled by melting of amphibolite in subduction zones

Deep fluids in subduction zones

The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids

The importance of serpentinite mylonites for subduction and exhumation of oceanic crust

The Earth’s mantle contains non-radiogenic noble gas isotopes that imply transfer of noble gases from the atmosphere into the mantle through subduction. Hydrated serpentinite rocks within subducting oceanic lithosphere are recognized as key carriers of water and chlorine, but the pathways for noble gas subduction have been poorly constrained. Here we analyse the concentration of noble gas isotopes and halogens in rocks from the Ligurian Alps, Italy and the Betic Cordillera, Spain. These rocks and the fluid inclusions trapped within them preserve a record of serpentinite dehydration during progressively deeper stages of subduction. We find that the noble gas and halogen signature of serpentinites reflects that of sea water and sediment pore fluids. The fluids released become progressively depleted in noble gases and chlorine is subducted to greater depths than bromine or iodine. After complete breakdown of the serpentinites at mantle depths of around 70 km, the dehydrated rock residues still retain a seawater-derived noble gas signature and significant chlorine. We suggest that these samples are proxies for serpentinite dehydration in cold slabs at mantle depths of 200 km or greater. We conclude that atmospheric noble gases are readily incorporated into hydrous minerals formed close to the sea floor, and incompletely removed by subduction zone metamorphism. This implies that noble gases can be subducted to great depths in the Earth’s mantle. Noble gases from the atmosphere are carried into Earth’s mantle through subduction. Geochemical analyses of rocks that record slab dehydration during progressively deeper stages of subduction indicate that noble gases incorporated into hydrous minerals close to the sea floor are efficiently transported to mantle depths of at least 200 km.

High abundances of noble gas and chlorine delivered to the mantle by serpentinite subduction

The Erro-Tobbio peridotite (western Alps) is a slice of subcontinental mantle that underwent pre-Jurassic passive rifting, followed by sea-floor hydration and Cretaceous (Alpine) subduction. Analysis of its extension- and subduction-related structures and metamorphism suggest the dip below the Adria plate of both the extensional detachments and the subduction plane. High-pressure boudinage structures of rigid metarodingite within ductile antigorite serpentinite prove that antigorite survived subduction and remained stable at eclogite facies conditions. Subduction of serpentinized peridotites from ophiolite slabs is here considered the most effective mechanism of bringing water to great depth within the mantle.

Marco Scambelluri

Papers

Deep fluids in subduction zones

The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids

The importance of serpentinite mylonites for subduction and exhumation of oceanic crust

High abundances of noble gas and chlorine delivered to the mantle by serpentinite subduction

Subduction of water into the mantle: History of an Alpine peridotite