[1] The amount of H2O subducted to postarc depths dictates such disparate factors as the generation of arc and back-arc magmas, the rheology of the mantle wedge and slab, and the global circulation of H2O. Perple_X was used to calculate phase diagrams and rock physical properties for pressures of 0.5–4.0 GPa and temperatures of 300–900°C for a range of bulk compositions appropriate to subduction zones. These data were merged with global subduction zone rock fluxes to generate a model for global H2O flux to postarc depths. For metasomatized igneous rocks, subducted H2O scales with bulk rock K2O in hot slabs. Metasomatized ultramafic rocks behave similarly in cold slabs, but in hot slabs carry no H2O to magma generation depths because they lack K2O. Chert and carbonate are responsible for minimal H2O subduction, whereas clay-rich and terrigenous sediments stabilize several hydrous phases at low temperature, resulting in significant postarc slab H2O flux in cold and hot slabs. Continental crust also subducts much H2O in cold slabs because of the stability of lawsonite and phengite; in hot slabs it is phengite that carries the bulk of this H2O to postarc depth. All told, the postarc flux of H2O in cold slabs is dominated by terrigenous sediment and the igneous lower crust and mantle and is proportional to bulk rock H2O. In contrast, in hot slabs the major contributors of postarc slab H2O are metasomatized volcanic rocks and subducted continental crust, with the amount of postarc slab H2O scaling with K2O. The Andes and Java-Sumatra-Andaman slabs are the principal suppliers of pelagic and terrigenous sediment hosted H2O to postarc depths, respectively. The Chile and Solomon arcs contribute the greatest H2O flux from subducted continental and oceanic forearc, respectively. The Andean arc has the greatest H2O flux provided through subduction of hydrated ocean crust and mantle. No correlation was observed between postarc slab H2O flux and slab seismicity.

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H2O subduction beyond arcs

Differentiation of the continental crust by relamination

Metamorphic chemical geodynamics of subduction zones

The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with 20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

http://www.igg.cas.cn/xwzx/yjcg/201306/W020130604610508726981.pdf

Partial melting of lower crust at 10–15 kbar: constraints on adakite and TTG formation

Calcic, igneous amphiboles are of special interest as their compositional diversity and common occurrence provide ample potential to investigate magmatic processes. But not all amphibole-based barometers lead to geologically useful information: recent and new tests reaffirm prior studies (e.g., Erdman et al. 2014), indicating that amphibole barometers are generally unable to distinguish between experiments conducted at 1 atm and at higher pressures, except under highly restrictive conditions. And the fault might not lie with experimental failure. Instead, the problem may relate to an intrinsic sensitivity of amphiboles to temperature ( T ) and liquid composition, rather than pressure. The exceptional conditions are those identified by Anderson and Smith (1995): current amphibole barometers are more likely to be useful when T Amp = Fe Amp /(Fe Amp +Mg Amp ) Such analysis reveals that amphiboles are vastly less complex than may be inferred from published catalogs of end-member components. Most amphiboles, for example, consist largely of just three components: pargasite [NaCa 2 (Fm 4 Al)Si 6 Al 2 O 22 (OH) 2 ], kaersutite [NaCa 2 (Fm 4 Ti)Si 6 Al 2 O 23 (OH)], and tremolite + ferro-actinolite [Ca 2 Fm 5 Si 8 O 22 (OH) 2 , where Fm = Fe+Mn+Mg]. And nearly all remaining compositional variation can be described with just four others: alumino-tschermakite [Ca 2 (Fm 3 Al 2 )Si 6 Al 2 O 22 (OH) 2 ], a Na-K-gedrite-like component [(Na,K)Fm 6 AlSi 6 Al 2 O 22 (OH) 2 ], a ferri-ferrotschermakite-like component [Ca 2 (Fm 3 Fe 2 3+ )Si 6 Al 2 O 22 (OH) 2 ], and an as yet unrecognized component with 3 to 4 Al atoms per formula unit (apfu), 1 apfu each of Na and Ca, and 4 Ti(Fe 3+ ,Al)Si 5 Al 3 O 23 (OH). None of these components, however, are significantly pressure ( P ) sensitive, leaving the Al-in-amphibole approach, with all its challenges, the best existing hope for an amphibole barometer. A new empirical barometer based on D Al successfully differentiates experimental amphiboles crystallized at 1 to 8 kbar, at least when multiple P estimates, from multiple amphibole compositions, are averaged. Without such averaging however, amphibole barometry is a less certain proposition, providing ±2 kbar precision on individual estimates for calibration data, and ±4 kbar at best for test data; independent checks on P are thus needed. Amphibole compositions, however, provide for very effective thermometers, here based on D Ti , D Na , and amphibole compositions alone, with precisions of ±30 oC. These new models, and tests for equilibrium, are collectively applied to Augustine volcano and the 2010 eruption at Merapi. Both localities reveal a significant cooling and crystallization interval (>190–270 oC) at pressures of 0.75 to 2.2 kbar at Augustine and Merapi, respectively, perhaps the likely depths from which pre-eruption magmas are stored. Such considerable intervals of cooling at shallow depths indicate that mafic magma recharge is not a proximal cause of eruption. Rather, eruption triggering is perhaps best explained by the classic “second boiling” concept, where post-recharge cooling and crystallization drive a magmatic system toward vapor saturation and positive buoyancy.

Amphibole thermometers and barometers for igneous systems and some implications for eruption mechanisms of felsic magmas at arc volcanoes

Xenoliths of subducted crustal origin hosted by Miocene ultrapotassic igneous rocks in the southern Pamir provide important new information regarding the geological processes accompanying tectonism during the Indo-Eurasian collision Four types have been studied: sanidine eclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite), felsic granulites (garnet, quartz, sanidine and kyanite), basaltic eclogites (omphacite and garnet), and a glimmerite (biotite, clinopyroxene and sanidine) Apatite, rutile and carbonate are the most abundant minor phases Hydrous phases (biotite and phengite in felsic granulites and basaltic eclogites, amphiboles in mafic and sanidine eclogites) and plagioclase form minor inclusions in garnet or kyanite Solid-phase thermobarometry reveals recrystallization at mainly ultrahigh temperatures of 1000–1100 C and nearultrahigh pressures of 2 5–2 8GPa Textures, parageneses and mineral compositions suggest derivation of the xenoliths from subducted basaltic, tonalitic and pelitic crust that experienced highpressure dehydration melting, K-rich metasomatism, and solid-state re-equilibration The timing of these processes is constrained by zircon ages from the xenoliths and Ar/Ar ages of the host volcanic rocks to 57–11Ma These xenoliths reveal that deeply subducted crust may undergo extensive dehydration-driven partial melting, density-driven differentiation and disaggregation, and sequestration within the mantle These processes may also contribute to the alkaline volcanism observed in continent-collision zones

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Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir

Apatite was analyzed by electron microprobe in 3 cumulate and 10 basaltic eucrites. Eucritic apatite is fluorine-rich with minor chlorine and hydroxyl (calculated by difference). We confirmed the hydroxyl content by measuring hydroxyl directly in apatites from three representative eucrites using secondary ionization mass spectroscopy. Overall, most eucritic apatites resemble fluorine-rich lunar mare apatites, but intriguing OH- and Cl-rich apatites suggest a role for water and/or hydrothermal fluids in the Vestan interior or on other related differentiated asteroids. Most late-stage apatite found in mesostasis has little hydroxyl or chlorine and is thought to have crystallized from a degassed magma; however, several apatites exhibit atypical compositions and/or textural characteristics. For example, the isotopically anomalous basaltic eucrite Pasamonte has apatite in the mesostasis with significant OH. Apatites in Juvinas also have significant OH and occur as veinlets crosscutting silicates. Euhedral apatites in the Moore County cumulate eucrite occur as inclusions in pyroxene and are also hydroxyl-rich (0.62 wt% OH). The OH was confirmed by SIMS analysis and this apatite clearly points to the presence of water, at least locally, in the Vestan interior. Portions of Elephant Moraine (EET) 90020 have large and abundant apatites, which may be the product of apatite accumulation in a zone of melt-rock reaction. Relatively chlorine-rich apatites occur in basaltic eucrite Graves Nunataks (GRA) 98098 (approximately 1 wt% Cl). Particularly striking is the compositional similarity between apatite in GRA 98098 and apatites in lunar KREEP, which may indicate the presence of residual magmas from an asteroid-wide magma ocean on Vesta.

The volatile content of Vesta: Clues from apatite in eucrites

Experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts

We have developed a set of four synthetic standards for the rare earth and high field strength elements designed for use in the determination of those elements in silicates. The base material is a glass nominally at the eutectic of the MgO-Al2O3-SiO2 ternary system. The nominal doping level was 2 wt% of the elements as oxides. To avoid problems associated with peak interference, the elements were doped in four separate glasses.

Alberto Patino-Douce

Papers

Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir

The volatile content of Vesta: Clues from apatite in eucrites

Experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts

New set of low concentration standards for la, ce, sm, yb, lu, y, sc, v, nb and ta in silicates

The Nature of Volatiles in Vesta: Clues from Apatite in Eucrites