Partial Melting Of Primitive Mantle Research Articles

Experiment constraints on orthopyroxene enrichment in the Kaapvaal craton lithospheric mantle

Peridotite xenoliths from the Kaapvaal cratonic lithosphere are characterized by high modal orthopyroxene (Opx), which is inconsistent with the mineralogy of residues of partial melting of primitive mantle. The high orthopyroxene content has been explained by the reaction between Opx-poor peridotite and silica-rich melts. To assess and further constrain this model, we conducted a series of experiments on the interaction of Si-rich melt with Opx-poor harzburgite at 1.5 GPa and 1100 to 1250 °C. Orthopyroxene is enriched in all experiments. In the low temperature run at 1100 °C, melt is consumed and phlogopite is precipitated with the generation of orthopyroxene. Mg# (atomic 100*Mg/(Mg+Fe)) values in olivines also exhibit a decrease compared with the starting olivines. At temperatures of ≥1200 °C, phlogopite is absent as the temperatures are higher than its upper stability field. The residues are composed of high modal orthopyroxene and high Mg# olivine. Only the residues in the high temperature runs are consistent with Kaapvaal cratonic mantle xenoliths. Combined our experiments with the distribution, geochronological and geochemical data of the xenoliths, we propose a two-stage ‘metasomatism-depletion’ model to explain the generation of Opx-rich xenoliths in Kaapvaal craton. In the early Archean, high-degree melting of primitive mantle produced a highly depleted lithospheric mantle, which was Opx-poor. Subsequently, Si-rich melts/fluids were released from the subducting oceanic slabs to interact with the Opx-poor mantle, generating an Opx-rich, but hydrous and fertile mantle. The metasomatic mantle is similar to the result of the run at 1100 °C. In the late Archean, the metasomatic mantle was re-melted in post-collisional extensional environments, which produced voluminous lavas with residues of highly depleted, Opx-rich peridotites. The Opx-rich residual peridotites are consistent with the runs at ≥1200 °C. Thus, Opx enrichment in the Kaapvaal craton lithospheric mantle can be well explained by the two-stage ‘metasomatism-depletion’ model.

Petrology of spinel lherzolite xenoliths from Youkou volcano, Adamawa Massif, Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo89.4−90.5), enstatite (En89 − 91Fs8.7−9.8Wo0.82−1.13), clinopyroxene, spinel (Cr#Sp = 9.4–13.8), and in some cases amphibole (Mg# = 88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al2O3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al2O3 = 6.07–6.56 wt% in clinopyroxene) and the low CaO/Al2O3 ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO 2 content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO 2 (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO 2 (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.

Melting relations of the hydrous primitive mantle in the CMAS–H 2O system at high pressures and temperatures, and implications for generation of komatiites

The hydrous phase relations of primitive mantle compositions in the CaO–MgO–Al 2O 3–SiO 2–H 2O system with 1, 2, and 5 wt.% H 2O have been investigated at 4, 6.5, and 8 GPa. The stability field of orthopyroxene in the residue expands with increasing H 2O content at 4–8 GPa. Dissolution temperature of garnet decreases more rapidly than that of the other phases at 4 and 6.5 GPa. The dissolution curves of garnet and pyroxenes cross at 6 GPa and 1700°C in the 2 wt.% H 2O-bearing system. Garnet is stable until high degree of melting at 8 GPa. Compositions of aluminum depleted komatiites are consistent with the liquid formed by more than 30% melting of the primitive mantle with H 2O up to 5 wt.% at 8 GPa. Aluminum undepleted komatiite can be formed by more than 50 wt.% melting of dry and wet primitive peridotite (with water up to 2 wt.% H 2O) at around 4–6 GPa. Variation in chemistry of komatiites might be explained by partial melting of primitive mantle with various water contents.

Geochemical constraints on petrogenic processes on Venus

Abundances of three incompatible elements (K, U, Th) have been determined by robotic spacecraft in five surface materials on Venus. We present these data normalized to terrestrial normal mid‐ocean ridge basalt (NMORB) derived from the depleted mantle. Relative to NMORB, all of the Venus materials studied are enriched in all of these elements. The moderately enriched (Venera 9 and 10, Vega 1 and 2) basaltic rocks are similar to one another in the sense of their enrichment trends (UN&gt;KN) but differ from the highly enriched Venera 8 material, where the trend is KN&gt;UN. This difference implies that the enrichment pattern for the basaltic materials is not controlled by crystallization of an NMORB‐like magma or by contamination of such a magma by highly enriched Venera 8 material within the crust, and the Venera 8 material cannot have evolved from the magma of any of the basaltic rocks. Our calculations show the K‐U‐Th pattern for any of the Venus rocks analyzed was not controlled by batch partial melting of primitive mantle. The Venera 8 material could be produced as a partial melt from eclogitic tholeiite, but none of the basaltic materials could be because the sense of their UN&gt;KN trends is opposite to the enrichment trends in calculated models (KN&gt;UN). The Venus basalts differ from fresh terrestrial rocks (NMORBs and oceanic island‐arc volcanics) in having nearly constant K/U ratios, while terrestrial rocks have nearly constant Th/U ratios. This may suggest an unusual composition of mantle source(s) of the Venus basalts and/or unusual fractionation process(es) on that planet.