Experimental Study of the Formation of Chromium-Bearing Priderite and Yimengite as Products of Modal Mantle Metasomatism

Doklady Earth Sciences - The results of experiments on the synthesis of exotic titanates (priderite and yimengite) simulating metasomatic conditions of alteration of the mantle minerals (chromite...     

南乌拉尔玛克苏托夫榴辉岩蓝片岩杂岩体的岩石学

玛克苏托夫 (Maksyutov)榴辉岩是乌拉尔—昆仑—祁连—秦岭—大别山元古代褶皱带之一部分。榴辉岩呈团块或布丁夹于片麻岩内 ,由于显生宙与花岗岩侵入有关的退变质叠加影响 ,时代变化自元古代至下古生代。榴辉岩矿物组分恒定但不同程度受绿帘蓝片岩相退变质叠加 ,榴辉岩中石榴石以富铁为主 (f=50 %～ 75% ) ;单斜辉石为绿辉石 Jd40 .3～ 52 .2。据 Krogh石榴子石—单斜辉石温度计 ,榴辉岩形成温度为 1 0 60～ 686℃ ,压力为 2 2 .3× 1 0 5～ 1 4.1 6× 1 0 5k Pa;PT轨迹具岩浆 (榴辉岩浆 )自上地幔向下地壳上侵的特点。     

Experimental study of eclogite melting with participation of the H<Subscript>2</Subscript>O-CO<Subscript>2</Subscript>-KCl fluid at 5 GPa

In order to model the processes of formation of the highly alkaline (potassic) melts during the partial melting of the eclogite nodules in kimberlites, experiments on the melting of the model and natural eclogites in presence of the H₂O-CO₂ and H₂O-CO₂-KCl fluids at 5 GPa and 1200 and 1300°C are performed. A comparative analysis of the phase relations in the systems with H₂O-CO₂ and H₂O-CO₂-KCl demonstrate that KCl in the fluid equilibrated with eclogites intensifies their melting. It is related to both high Cl concentration in the forming silicate melt (2.0–5.5 wt %) and its enrichment in K₂O owing to the K-Na exchange reactions with the immiscible chloride melt. Because of these reactions, the K₂O/Cl ratio in the melts increases with the KCl content in the system and reaches 2.5–3.5 in the silicate melts coexisting with the immiscible chloride liquid. However, the ratio KCl/(H₂O + CO₂ + KCl) in the fluid does not influence on the ratio K₂O/Cl in the melts. Thus, the solubility KCl in the melts, apparently, does not depend on presence of the H₂O-CO₂ fluid, at least, within the concentration range used in the experiments (up to 20 wt %). The experiments show that the deliberated chloride liquid is necessary to form the potassium-rich chlorine-bearing silicate melts during the eclogite melting. It corresponds to the KCl content in the system above 5 wt %.     

Interaction of model peridotite with H2O-KCl fluid: Experiment at 1.9 GPa and its implications for upper mantle metasomatism

Mineralogical and geochemical data suggest that chloride components play an important role in the transformation and partial melting of upper mantle peridotites. The effect of KCl on the transformation of hydrous peridotite rich in Al₂O₃, CaO, and Na₂O was examined in experiments aimed at studying interaction between model NCMAS peridotite with H₂O-KCl fluid under a pressure of 1.9 GPa, temperatures of 900–1200°C, and various initial H₂O/KCl ratios. The experimental results indicate that KCl depresses the solidus temperature of the hydrous peridotite: this temperature is <900°C at 1.9 GPa, which is more than 100°C lower than the solidus temperature (1000–1025°C) of hydrous peridotite in equilibrium with KCl-free fluid. The reason for the decrease in the melting temperature is that the interaction of KCl with silicates prevails over the effect of chloride on the water activity in the fluid. Experimental data highlight the key role of Al₂O₃ as a component controlling the whole interaction process between peridotite and H₂O-KCl fluid. Garnet, spinel, and pargasite-edenite amphibole in association with aluminous orthopyroxene are unstable in the presence of H₂O-KCl fluid at a chloride concentration in the fluid as low as approximately 2 wt % and are replaced by Cl-bearing phlogopite (0.4–1.1 wt % Cl). Interaction with H₂O-KCl fluid does not, however, affect clinopyroxene and forsterite, which are the Al poorest phases of the system. Chlorine stabilizes phlogopite at relatively high temperatures in equilibrium with melt at temperatures much higher than the solidus (>1200°C). The compositional evolution of melt generated during the melting of model peridotite in the presence of H₂O-KCl fluid is controlled, on the one hand, by the solubility of the H₂O-KCl fluid in the melt and, on the other hand, by phlogopite stability above the solidus. At temperatures below 1050°C, at which phlogopite does not actively participate in melting reactions, fluid dissolution results in SiO₂-undersaturated (35–40 wt %) and MgO-enriched (up to 45 wt %) melts containing up to 4–5 wt % K₂O and 2–3 wt % Cl. At higher temperatures, active phlogopite dissolution and, perhaps, also the separation of immiscible aqueous chloride liquid give rise to melts containing >10 wt % K₂O and 0.3–0.5 wt % Cl. Our experimental results corroborate literature data on the transformation of upper mantle peridotites into phlogopite-bearing associations and the formation of ultrapotassic and highly magnesian melts.     

Experimental Study of the Formation of Ba–Cr Titanates in the Fluid-Bearing Chromite–Rutile/Ilmenite System at Т = 1000–1200°С and Р = 1.8–5.0 GPa

Doklady Earth Sciences - The results of experimental study of crystallization of chromium-bearing Ba-titanates (redlegeite, lindsleyite, and hawthorneite) in the chromite–rutile/ilmenite...     

Phlogopite Formation in the Orthopyroxene–Garnet System in the Presence of H2O–KCl Fluid in Application to the Processes of Mantle Metasomatism

Doklady Earth Sciences - The results of experimental studies are presented for reactions in the orthopyroxene–garnet–phlogopite system in the presence of H2O–KCl fluid at...