Incompatible trace element partitioning and residence in anhydrous spinel peridotites and websterites from the Ronda orogenic peridotite
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Peridotite
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Anhydrous
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Leucogranite
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Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multianvil apparatus at 6 GPa and 1400°C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivine + clinopyroxene + garnet ± orthopyroxene). The multiply saturated hydrous melts have higher (Mg + Fe)/Si and Al/Ca compared with hydrous melts produced at lower pressures. The melt compositions are similar to those determined near the dry peridotite solidus at ∼1700°C, when compared on an H2O-free basis. Melt H2O contents were determined to be ∼11 wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (>70 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H2O-rich melt then at the base of this zone, at ∼220 km, these results imply that the melt must contain 15–16 wt % H2O. The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na2O but enriched K2O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K2O and H2O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K2O and H2O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of ∼0·2 wt % melting of a garnet peridotite source rock enriched with 1·7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.
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Knowledge of the trace element distributions and concentrations in main mineral phases of mantle-derived rocks is particularly useful for understanding the partial melting, depletion and enrichment processes, and metasomatism in the earth’s mantle. Trace element data on mantle peridotites were usually obtained with large uncertainty by neutron activation analysis in China previously. We have determined trace element concentration of 173 grains of clinopyroxene (cpx) in 50 peridotite xenoliths from different areas in China using the pro-
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Neutron Activation Analysis
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Carbonatite
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Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed.
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Ferrobasalts from DSDP sites 214 and 216 on the Ninetyeast Ridge are characterized by high absolute iron (FeO>12.9 wt %), FeO/MgO>1.9, and TiO 2 >2.0 wt %. Their trace element abundances indicate a tholeiitic affinity; however, they are distinct from midocean ridge incompatible element‐depleted tholeiites owing to higher contents of Ba, Zr, and Sr and flat to slightly light‐REE‐enriched, chondritenormalized REE patterns. Calculations using major and trace element abundances and phase compositions are generally consistent with a model relating most major elements and phase compositions in site 214 and 216 ferrobasalts by fractionation of clinopyroxene and plagioclase. However, some incompatible element abundances for site 216 basalts are not consistent with the fractional crystallization models. Basalts from site 214 can be related to andesitic rocks from the same site by fractionating clinopyroxene, plagioclase and titanomagnetite. Site 254 basalts, at the southern end of the Ninetyeast Ridge, and island tholeiites in the southern Indian Ocean (Amsterdam‐St. Paul or Kerguelen‐Heard volcanic provinces) possibly represent the most recent activity associated with a hot spot forming the Ninetyeast Ridge. These incompatible‐element‐enriched tholeiites have major element compositions consistent with those expected for a parental liquid for the site 214 and 216 ferrobasalts. However, differences in the trace element contents of the basalts from the Ninetyeast Ridge sites are not consistent with simple fractional crystallization derivation but require either a complex melting model or a heterogeneous mantle source.
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