Partitioning of elements between silicate melt and H2ONaCl fluids at 1.5 and 2.0 GPa pressure: Implications for mantle metasomatism (original) (raw)
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Partition coefficients between a synthetic andesite melt and 1.5 and 3.0 molal (m) NaCi-H20 fluids have been measured at 1.5-2.0 GPa and 1250°C. Use of the double-capsule method allowed collection of silicate glass and of solute quenched from the fluid. Measured solubilities of silicate melt in both 1.5m and 3.Om NaCl fluid are 51 wt% at 1.5 GPa and 53 wt% at 2.0 GPa, much higher than solubilities measured for pure HZ0 at the same conditions. Wet chemical analysis of the run products yielded fluid/ melt partition coefficients (D Ruid'me't) ranging from 0.43 to 1.31 and Ti, suggesting a near-congruent dissolution of the melt in fluid. D values for many elements increase in direct proportion to melt solubility, as expected for congruent solution. Concentrations of alkalis in the fluid positively correlate with total Cl concentration, suggesting that Cl complexes with alkalis in the fluid. Results imply that NaCl-bearing aqueous fluids can dissolve large amounts of silicate material, but do not strongly fractionate elements in equilibrium with silicate melts. Thus, the geochemical signature left by metasomatic H20-NaCl fluids will not be distinctly different from that of silicate melts.
Geochemistry, Geophysics, Geosystems, 2014
Both silicate melts and aqueous fluids are thought to play critical roles in the chemical differentiation of the Earth's crust and mantle. Yet their relative effects are poorly constrained. We have addressed this issue by measuring partition coefficients for 50 trace and minor elements in experimentally produced aqueous fluids, coexisting basanite melts, and peridotite minerals. The experiments were conducted at 1.0-4.0 GPa and 950-1200 C in single capsules containing (either 40 or 50 wt %) H 2 O and trace elementenriched basanite glass. This allowed run products to be easily identified and analyzed by a combination of electron microprobe and LAM-ICP-MS. Fluid and melt compositions were reconstructed from mass balances and published solubility data for H 2 O in silicate melts. Relative to the basanite melt, the solutes from H 2 Ofluids are enriched in SiO 2 , alkalis, Ba, and Pb, but depleted in FeO, MgO, CaO, and REE. With increasing pressure, the mutual solubility of fluids and melts increases rapidly with complete miscibility between H 2 O and basanitic melts occurring between 3.0 and 4.0 GPa at 1100 C. Although LREE are favored over HREE in the fluid phase, they are less soluble than the HFSE (Nb, Ta, Zr, Hf, and Ti). Thus, the relative depletions of HFSE that are characteristic of arc magmas must be due to a residual phase that concentrates HFSE (e.g., rutile). Otherwise, H 2 O-fluids have the capacity to impart many of the geochemical characteristics that distinguish some rocks and melts from the deep mantle lithosphere (e.g., MARID and lamproites). et al., 1997]. A more recent method has been to trap fluids within the interstices of diamond aggregates and then to analyze these by laser ablation microprobe and inductively coupled plasma mass spectrometry (LAM-ICP-MS) [e.g., Stalder et al., 1998;].
Geochimica Et Cosmochimica Acta, 1998
To constrain the trace element composition of aqueous fluids in the deep crust and upper mantle, mineral-aqueous fluid partition coefficients (D min/fluid) for U, Th, Pb, Nb, Ba, and Sr have been measured for clinopyroxene, garnet, amphibole, and olivine in experiments at 2.0 GPa and 900°C. Ciinopyroxeneand garnet-fluid partition coefficients are similar for Nb (0.01-0.7) and Ba ( ~ 10-4-10-5), whereas values of D cpx/ttuid for Sr (0.5-4), Th (0.6-9), and Pb (0.04-0.09) are -10× (Th, Pb) to ~ 1000× (Sr) higher than O garnet/fluid. At the same fO2 (FMQ + 1), garnet-fluid partition coefficients for U are -10× higher than those for clinopyroxene. Amphibole-fluid partition coefficients are uniformly high ( -1 ) for all elements studied, and, with the exception of Ba, interelement fractionations are similar to clinopyroxene. The olivine-fluid partition coefficient for Nb is similar to values measured for the other silicates, whereas D °~Vm~/~u~J for U, Th, Pb, Sr, and Ba are significantly lower.
Low-degree partial melting trends recorded in upper mantle minerals
Earth and Planetary Science Letters, 1998
The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline-olivine normative, 64 wt% SiO 2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO 2 and 1-2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO 2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T ³ 1230ºC and P ³ 1:0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr 2 O 3 and poorer in SiO 2 , K 2 O, Na 2 O, Al 2 O 3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO 2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO 2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO 2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.
Lithos, 2013
The partitioning of a number of trace elements (LILE, HFSE, REE, Cu, Pb, Co, Ni) between mantle minerals (olivine, pyroxenes and garnet) and silico-carbonate melts was experimentally studied at 6-12 GPa and 1300-1700°C. The starting compositions were model kimberlitic with~30 wt.% SiO 2 , which differentiated to carbonatite-like melts with b10 wt.% SiO 2 depending on the degree of crystallization. The melts were rich in CO 2 (up tõ 30 wt.%) and contained 0 to 30 wt.% H 2 O. Trace elements were added to the starting mixtures to levels of 100 ppm. They were analyzed by LA ICP MS in the products of 18 experiments. The partition coefficients of Ba, La, Ta, and Nb are very low for all phases (b0.01). These elements are especially susceptible to contamination in the experimental products. We suspect that some relatively high values obtained for these elements in previous studies are overestimated. Generally, the partition coefficients (D s/l ) of the moderately incompatible and compatible elements increase in the sequence olivine-low-Ca pyroxene-high-Ca pyroxene. Garnet shows maximum fractionation of the trace elements, such that D values for the highly incompatible elements are lower than those for pyroxenes and highest for the HREE, Sc, and V. The partition coefficients of a number of incompatible elements are rather insensitive to pressure, temperature, and melt composition. There is no correlation between the partition coefficients and the content of H 2 O, CO 2 and the overall content of trace elements. They are predominantly controlled by the composition of the crystalline phases. In particular, garnet-liquid partitioning is very sensitive to Ca in garnet, and pyroxene-liquid partitioning to Al in pyroxene. The comparison of the obtained partition coefficients with the compositions of likely kimberlite magmas shows that depleted and refertilized harzburgite from the continental lithospheric mantle is a viable candidate as the source material of kimberlites. The estimation of trace element characteristics of near-solidus mantle melts indicates that the most reliable indicator of carbonate melt metasomatism is depletion of Zr and Hf relative to Sm and Nd. Criteria based on the ratios of more incompatible elements (e.g. Nb/La) are very sensitive to the melting conditions and can be misleading.
THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY, 1998
The influence of melt composition and temperature on the partitioning of siderophile elements between magnesiowustite (MW) and silicate melt (Sil) has been studied systematically at 9 GPa. Additional experiments at pressures above 20 GPa were performed to determine the effect of pressure. The determined partition coefficients (KD(M/Mg) MW/Sil) of M = Fe, Ni, Co, Mn, Cr and V show systematic variation as a function of temperature, pressure and melt composition. Comparison to literature data indicates that variables other than those investigated, for example magnesiowustite composition may be responsible for the large differences reported for Mw-silicate liquid partitioning. According to the present results magnesiowustite fractionation can be excluded to contribute significantly to the depletion of the observed elements in the mantle.
Mantle-derived magmas-roles of variable source peridotite and variable C-H-O fluid compositions
The system forsterite-nepheline-quartz is a useful simple system analogue of melting relations in upper mantle peridotite. The liquidus phase fields at 28 kbar differ from those at low pressure by expansion of the enstatite field at the expense of forsterite. The system illustrates a large field of liquid compositions, from model basanites to model quartz tholeiites, which can be derived from one peridotite source. More refractory source compositions permit a greater compositional range of derivative liquid compositions than more fertile compositions and in particular are required as source or parent compositions for enstatite-rich liquids.
Contributions To Mineralogy and Petrology, 2000
Experiments in the systems diopside-albite (Di-Ab) and diopside-albite-dolomite (Di-Ab-Dmt), doped with a wide range of trace elements, have been used to characterise the difference between clinopyroxene-silicate melt and clinopyroxene-carbonate melt partitioning. Experiments in Di-Ab-Dmt yielded clinopyroxene and olivine in equilibrium with CO2-saturated dolomitic carbonate melt at 3 GPa, 1375 °C. The experiments in Di-Ab were designed to bracket those conditions (3 GPa, 1640 °C and 0.8 GPa, 1375 °C), and so minimise the contribution of differential temperature and pressure to partitioning. Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt systems. Notably, in the carbonate system clinopyroxene-melt partition coefficients for Si, Al, Ga, heavy REE, Ti and Zr are higher by factors of 5 to 200 than in the silicate system. Conversely, partition coefficients for Nb, light REE, alkali metals and alkaline earths show much less fractionation (<3). The observed differences compare quantitatively with experimental data on partitioning between immiscible carbonate and silicate melts, indicating that changes in melt chemistry provide the dominant control on variation in partition coefficients in this case. The importance of melt chemistry in controlling several aspects of element partitioning is discussed in light of the energetics of the partitioning process. The compositions of clinopyroxene and carbonate melt in our experiments closely match those of near-solidus melts and crystals in CMAS-CO2 at 3 GPa, suggesting that our partition coefficients have direct relevance to melting of carbonated mantle lherzolite. Melts so produced will be characterised by elevated incompatible trace element concentrations, due to the low degrees of melting involved, but marked depletions of Ti and Zr, and fractionated REE patterns. These are common features of natural carbonatites. The different behaviour of trace elements in carbonate and silicate systems will lead to contrasted styles of trace element metasomatism in the mantle.