“…A negative Ti anomaly in orthopyroxene is observed only in clinopyroxene-free harzburgite 19 . This finding is consistent with the absence of clinopyroxene porphyroclast, which constrains Stage 1 temperatures to above the clinopyroxene-out curve in the hydrous peridotite system (>1120 °C 20 ). Figure 3 Panels (a,b) show primitive-mantle (PM) normalized 18 trace element patterns of orthopyroxene (Opx) in Type A and Type B peridotites, with stages are denoted by subscript numbers in the figure.…”
Section: Mantle Wedge Peridotites In the Bohemiam Massifsupporting
confidence: 85%
“… 30 clinopyroxene (Cpx)-out curve is from ref. 20 other phase boundaries are after ref. 7 and references therein.…”
Section: Mantle Wedge Peridotites In the Bohemiam Massifmentioning
The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.
“…A negative Ti anomaly in orthopyroxene is observed only in clinopyroxene-free harzburgite 19 . This finding is consistent with the absence of clinopyroxene porphyroclast, which constrains Stage 1 temperatures to above the clinopyroxene-out curve in the hydrous peridotite system (>1120 °C 20 ). Figure 3 Panels (a,b) show primitive-mantle (PM) normalized 18 trace element patterns of orthopyroxene (Opx) in Type A and Type B peridotites, with stages are denoted by subscript numbers in the figure.…”
Section: Mantle Wedge Peridotites In the Bohemiam Massifsupporting
confidence: 85%
“… 30 clinopyroxene (Cpx)-out curve is from ref. 20 other phase boundaries are after ref. 7 and references therein.…”
Section: Mantle Wedge Peridotites In the Bohemiam Massifmentioning
The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.
“…8a,b). Based on this observation, higher Sr/Ba ratio in arc Kessel et al, (2005aKessel et al, ( , 2015 decreasing from ~ 12 to 3.5 with increasing T from 700 to 800 °C while the system approached the wet solidus. However, in the case of REE, different mineral phases controlled the partitioning behavior in the study of Kessel et al (2005a,b) and in our experiments, resulting in distinctive REE patterns.…”
The equilibrium between aqueous fluids and allanite-bearing eclogite has been investigated to constrain the effect of temperature (T) and fluid composition on the stability of allanite and on the mobility of major and trace elements during the dehydration of eclogites. The experiments were performed at 590-800 °C and 2.4-2.6 GPa, and fluids were sampled as synthetic fluid inclusions in quartz using an improved entrapment technique. The concentrations and bulk partition coefficients were determined for a range of major (Mg, Ca, Na, Fe, Al, Ti) and 16 trace elements as a function of T and fluid composition. The results reveal a significant effect of T on element partitioning between the fluids and the solid mineral assemblage. The partition coefficients increase by more than an order of magnitude for most of the major and trace elements, and several orders of magnitude for light rare-earth elements (LREE) from 590 to 800 °C. The addition of various ligand species into the fluid at 700 °C results in distinctive trends on element partitioning. The concentrations and corresponding partition coefficients of most of the elements are enhanced upon addition of NaF to the fluid. In contrast, NaCl displays a nearly opposite effect by suppressing the solubilities of major elements and consequently affecting the mobility of trace elements that form stable complexes with alkali(alumino)-silicate clusters in the fluid, e.g. high field strength elements (HFSE). The results further suggest that fluids in equilibrium with orthopyroxene and/or diopsidic clinopyroxene are peralkaline (ASI ~ 0.1-0.7), whereas fluids in equilibrium with omphacitic pyroxene are more peraluminous (ASI ~ 1.15).
“…As, Sb, B, Li, Cs, Pb, U, Ba, Sr) into the mantle wedge beneath arc systems (Tatsumi et al, 1986;Kessel et al, 2005;Kessel et al, 2015;Cannat et al, 1995;Carlson, 2001;Mével, 2003;Scambelluri et al, 2004;Savov et al, 2007, Cannat et al, 2010Deschamps et al, 2011). The limited available experimental data shows that during dehydration melting of peridotite (Tatsumi et al, 1986;Kessel et al, 2005;Spandler et al, 2014;Kessel et al, 2015), the LREE will preferentially partition to the fluid/melt over the HREE. The excess LREE in serpentinized peridotites shown here coupled with their preferential mobility could then result in a high LREE/HREE and possibly high REE/HFSE ratio flux from the serpentinized peridotite to the mantle wedge during dehydration melting, along with the fluid mobile elements.…”
Section: Ree Sink and Impact On Mantle Reservoirsmentioning
Serpentinized abyssal peridotites are evidence for active communication between the Earth's hydrosphere and the upper mantle, where exchange and retention of both major and trace elements occur. Bulk rock Nd isotopes in serpentinized abyssal peridotites imply interaction of seawater with the peridotite. In contrast, the Nd isotopes of clinopyroxenes from serpentinized abyssal peridotites retain their primary magmatic signature. It is currently unclear if, how and where seawater-derived Nd and other REE are being added or exchanged with the mantle peridotite minerals during serpentinization. To remedy this knowledge gap, we present in situ trace and major element concentrations, bulk rock and sequential leaching experiment trace element concentrations as well as Nd, Sr isotope data on refertilized and depleted serpentinized abyssal peridotites from the Southwest Indian Ridge. The secondary serpentine matrix and magnetite veins in these peridotites have elevated LREE concentrations, with variable negative Ce anomalies and large Rb, Sr, Pb and U enrichments that resemble seawater trace element patterns. The LREE concentrations in the serpentine phase are higher than those expected for the primary mantle mineralogy (olivine, orthopyroxene) based on data from relic clinopyroxenes and equilibrium partition coefficients. These data are consistent with seawater-derived REE addition to the peridotite during serpentinization. The bulk rocks have more radiogenic Sr and more unradiogenic Nd isotopes than their clinopyroxene (up to 8 Nd units lower than clinopyroxene). Sequential leaching experiments designed to mobilize secondary carbonates and Feoxides show even more unradiogenic Nd isotope ratios in the leachates than the bulk rock and clinopyroxene, approaching seawater compositions (up to 15 Nd units lower than clinopyroxene). Mass balance calculations using trace elements or Nd isotopes suggest that up to 30% of the bulk peridotite Nd budget is of seawater origin and was added to the peridotite, as opposed to being exchanged. These data allows for more accurate constraints of the water/rock mass exchange ratios during serpentinization in these rocks (~1000 to 17000) that are at least an order of magnitude lower than using typical mantle and seawater endmembers. These data provide strong evidence that serpentinization may also be a sink for the light REE, with implications for the refertilization of the peridotite during serpentinization, and the cycling of these elements through the subduction system and into the upper mantle.
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