Heat capacity and phase equilibria of hollandite polymorph of KAlSi 3 O 8

The low-temperature heat capacity ( C p ) of KAlSi 3 O 8 with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi 3 O 8 hollandite is 166.2±0.2 J mol −1  K −1 , including an 18.7 J mol −1  K −1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K 2 Si 4 O 9 with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K 2 O–Al 2 O 3 –SiO 2 . The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K 2 O–Al 2 O 3 –SiO 2 confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi 3 O 8 hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al 2 SiO 5 ) = corundum (Al 2 O 3 )  + stishovite (SiO 2 ), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi 3 O 8 ·H 2 O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi 3 O 8 hollandite +  H 2 O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi 3 O 8 hollandite + corundum + H 2 O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46910/1/269_2006_Article_63.pd

Volatile (H2O, CO2, Cl, S) budget of the Central American subduction zone

After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H2O, 7.8–11.4 (9.3) Tg/Ma/m CO2, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H2O, 0.02–0.45 (0.17) Tg/Ma/m CO2, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown