Hydrous Phase Relations and Trace Element Partitioning Behaviour in Calcareous Sediments at Subduction-Zone Conditions

Abstract

We report the results of experiments on two natural marine sediments with different carbonate contents (calcareous clay: CO 2 ¼ 6Á1 wt %; marl: CO 2 ¼ 16Á2 wt %) at subduction-zone conditions (3 GPa, 750-1200 C). Water (7-15 wt %) was added to the starting materials to simulate the effects of external water addition from within the subducting slab. The onset of melting is at 760 C in water-rich experiments; melt becomes abundant by 800 C. In contrast, the onset of melting in published, water-poor experiments occurs at variable temperatures with the production of significant melt fractions being restricted to more than 900 C (phengite-out). The different solidus temperatures (T solidus ) can be ascribed to variable fluid X H2O [H 2 O/(CO 2 þ H 2 O)], which, in turn, depends on bulk K 2 O, H 2 O and CO 2 . Partial melts in equilibrium with residual garnet, carbonate, quartz/coesite, epidote, rutile, kyanite, phengite, and clinopyroxene are granitic in composition, with substantial dissolved volatiles. Supersolidus runs always contain both silicate melt and solute-rich fluid, indicating that experimental conditions lie below the second critical endpoint in the granite-H 2 O-CO 2 system. Carbonatite melt coexists with silicate melt and solute-rich fluid above 1100 C in the marl. The persistence of carbonate to high temperature, in equilibrium with CO 2 -rich hydrous melts, provides a mechanism to both supply CO 2 to arc magmas and recycle carbon into the deep Earth. The trace element compositions of the experimental glasses constrain the potential contribution of calcareous sediment to arc magmas. The presence of residual epidote and carbonate confers different trace element characteristics when compared with the trace element signal of Ca-poor marine sediments (e.g. pelagic clays). Notably, epidote retains Th and light rare earth elements, such that some melts derived from calcareous sediments have elevated Ba/Th and U/Th, and low La/Sm PUM , thereby resembling fluids conventionally ascribed to altered oceanic crust. Our results emphasize the importance of residual mineralogy, rather than source lithology, in controlling the trace element characteristics of slab-derived fluids.

Ce/Mo and Molybdenum Isotope Systematics in Subduction‐Related Orogenic Potassic Magmas of Central‐Southern Italy

“…Accordingly, in the experiments of Skora et al (2015), residual rutile was observed both for Ca-poor and Ca-rich sediments during melting. Accordingly, in the experiments of Skora et al (2015), residual rutile was observed both for Ca-poor and Ca-rich sediments during melting.…”

“…This argues against rutile as the key phase responsible for the difference in Ce/Mo between Tuscan and Vesuvius magmas. Accordingly, in the experiments of Skora et al (2015), residual rutile was observed both for Ca-poor and Ca-rich sediments during melting. Therefore, other residual phases retaining Mo must have been present during sediment melting.…”

“…Assuming a sediment melting degree of some 50% (e.g., Skora et al, 2015Skora et al, , 2017, we calculated the D Mo and D Ce needed to attain Ce/Mo values in the sediment melt components able to account for the Ce/Mo of the studied samples (Figure 6c). In the case of the Vesuvius magmas, in order to achieve the low inferred Ce/Mo melt , we assume Mo as perfectly incompatible in the residual mineralogy (D Mo = 0) and even in this end-ember case Ce needs to be compatible (symbols are shown for D Ce = 1, 3, 7 in Figure 6c).…”

Contrasting sediment melt and fluid signatures for magma components in the Aeolian Arc: Implications for numerical modeling of subduction systems

“…The arc signature depends on the composition and proportion of the different source components (subducting oceanic crust, the overlying sediment, and mantle) and the thermal regime of the subduction system will control the mineral phase stability and the solidus of the subducted material. Variations in these conditions define the stability of key potential mineral phases (e.g., rutile for Ti, Nb, and HFSE, garnet for HREE, phengite for LILE, zircon for Zr and Hf, allanite, monazite and apatite for Th, U, and REE) resulting in a slab‐signal that will manifest differently in every arc depending mainly on subduction temperature and secondary on the potential of extracting fluids or melts from the subducting slab [e.g., Hermann , ; Hermann et al ., ; Klimm et al ., ; Hermann and Rubatto , ; Skora and Blundy , ; Avanzinelli et al ., ; Martindale et al ., ; Skora et al ., ].…”

Melting carbonated epidote eclogites: carbonatites from subducting slabs

“…The latter two studies contain H 2 O in the starting material and its effect on the composition of the liquids has been discussed above. In carbonated pelites (i.e., H 2 O-bearing), carbonatitic melts have been found to exsolve from the silicate liquid above 1100°C at 3-5 GPa (Thomsen and Schmidt 2008;Skora et al, 2015). Hydrated, carbonated peridotites again show carbonatites on the solidus at 1070°C at 2-3 GPa (Tumiati et al 2013).…”