Abstract Carbonatite melts derived from the mantle are enriched in CO 2 - and H 2 O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO 2 . Experimental studies have also shown that a CO 2 –H 2 O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO 2 –H 2 O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution–precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO 2 –H 2 O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO 2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Abstract Sulfides play a crucial role in the distribution of chalcophile elements in the Earth's mantle. In this work, combined petrography and mineral chemistry of sulfide and diopside in a pyroxenite from an ultramafic–alkaline–carbonatite complex of NE India, related to the Kerguelen plume, was carried out and a considerably high sulfur concentration in the parental melt of the pyroxenite was obtained. Two types of sulfide, with similar compositions, were detected in pyroxenite: Type A are multifaceted polygons, elliptical and spherical in shape, occurring as poikilitic inclusions in diopside; and Type B are intergranular sulfides of irregular shapes in silicate grains. These sulfides are often partially replaced by magnetite. Mineral chemistry suggests that both types of sulfide are products of re-equilibration of high-temperature monosulfide solid solution and represent a low-temperature ( c. 400°C) mineral phase of the Cu–Fe–S system. Petrographical features suggest that the sulfides were separated as immiscible melt droplets at the time of sulfur saturation and fractionation of diopside in the coexisting silicate magma. Our study implicates that both high- and low-temperature sulfides can form in the plume-associated ultramafic rocks.
Abstract The Sung Valley ultramafic–alkaline–carbonatite complex (UACC) of Meghalaya, NE, India, is a result of magmatic activity related to the Kerguelen mantle plume spanning from 101 to 115 Ma. In the present study, an integrated crystal size distribution (CSD), mineral chemistry, and melt inclusion analysis are carried out on the ijolites present within this UACC. The CSD analysis shows that these ijolites were formed in multiple stages through changes in the crystallization environment, such as cooling and nucleation rates. Raman spectroscopy of mineral inclusions of rutile, aphthitalite, apatite, carbonate–silicate melt inclusions, and disordered graphite within clinopyroxene and titanite, respectively, indicates a heterogeneous composition of the parental magma. These mineral and melt inclusion phases further suggest localized changes in oxygen fugacity ( f O 2 ) due to redox reactions in the lower crust. SEM–EDX analysis of the exposed melt inclusions reveals the presence of alkali-bearing diopside, phlogopite, and andradite, along with an unidentified carbonated silicate daughter phase. The studied melt inclusions are dominated by carbonate, whereas silicates are subordinate. The presence of this fully crystallized carbonate–silicate melt as calcite, diopside, phlogopite, magnetite, apatite, and andradite suggests the presence of “nano-calciocarbonatites” in these ijolites. Our study provides insights into different mechanisms of the loss of alkalies from initially entrapped alkaline carbonate melt in clinopyroxenes. The predominant occurrence of calcite as the only carbonate phase in the studied melt inclusions is a result of silicate–carbonate melt immiscibility, calcite-normative system in these inclusions, dealkalization of the alkaline carbonates in the presence of external fluid, and/or redistribution of the alkalies to the daughter alkali-bearing silicates.
The C-O-H-S fluids play a significant role in various geological processes. The participation of these fluids in redox reactions remarkably influences carbonation process, melting behaviour, and physicochemical properties of the mantle rocks. In this study, we report rare, reduced fluid inclusions in shallow upper mantle-derived pyroxenites associated with the Kerguelen mantle plume from Northeast India. Raman spectroscopy reveals primary inclusions of hydrocarbon (CH) fluid and calcite along with the nearly perpendicular exsolution lamellae of magnetite in clinopyroxene (referred to herein as Cpx 1) of these pyroxenites. A rare occurrence of pseudo-secondary polyphase fluid inclusions of hydrogen sulfide (H2S), carbon monoxide (CO), rutile, and calcite was also recorded in other clinopyroxenes (referred to herein as Cpx 2) of these pyroxenites. Mineral chemical data suggests that the studied Cpx are purely diopside and Cpx 1 hosted magnetite exsolutions are coeval with these clinopyroxenes. This is evident from the notable enrichment in MgO in these magnetite exsolutions relative to accessory magnetite, showing that they certainly do not represent a sub-solidus phase. Single Cpx geothermobarometric calculations suggest that these pyroxenites were formed at 1.2- 1.8 GPa pressure and 752- 941 °C temperature. We use a theoretical thermodynamic model to validate our natural observations of fluid inclusions. Overall, our results show that mantle plume-derived oxidized C-O-H-S fluids interacted with crystallizing diopside from pyroxenites in the shallow mantle. During this interaction, these fluids participated in redox reactions leaving the reduced fluid products such as CH, CO and H2S and minerals such as calcite and rutile trapped as co-genetic inclusions along with magnetite exsolutions in host diopsides at oxygen fugacity conditions equivalent to FMQ-2. Therefore, considering the widely accepted fact that the shallow lithospheric mantle is predominantly oxidized, these natural observations of fluid inclusions, for the first time provide direct evidence of redox heterogeneity of the shallow mantle.  
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Earth and Space Science. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Manasseite, ferrohogbomite and amesite in mantle plume associated carbonatite: Implications for unplumbed ultra-hydrous nature of parental carbonatite magmaAuthorskoushikseniDShubhamChoudharyiDSee all authors koushik seniDCorresponding Author• Submitting AuthorWadia Institute of Himalayan GeologyiDhttps://orcid.org/0000-0002-5679-9603view email addressThe email was not providedcopy email addressShubham ChoudharyiDWadia Institute of Himalayan GeologyiDhttps://orcid.org/0000-0002-5555-3599view email addressThe email was not providedcopy email address
Abstract Carbon (C) and sulfur (S) bearing volatiles in the mantle are important as they control processes like metasomatism and melting that ultimately influence Earth’s geochemical cycle. Here we report rare shallow mantle fluid inclusions in a pyroxenite, associated with the Kerguelen Plume, from Northeast India. Optical microscopy and Raman spectroscopy reveal primary inclusions of hydrocarbon (CH) fluid and calcite in diopsidic clinopyroxene (Cpx) of this pyroxenite. Another Cpx from the same rock shows a rare occurrence of pseudo-secondary polyphase fluid inclusions of hydrogen sulfide (H 2 S), carbon monoxide (CO), rutile and calcite. We infer that metasomatic interaction between silicates and C-O-H volatiles generated these fluids, which are trapped as inclusions. We further suggest two factors controlling carbon speciation in these fluids 1) exsolution of magnetite and diffusion of Ti from the Cpx to form rutile and 2) oxidation buffering due to presence of H 2 S that prevented the formation of CH and formed CO instead. Our study explains the existence of significant reduction zones at localized scale in the oxidized shallow upper mantle.
The Palaeozoic granites forming the Dalhousie pluton occur as an elongated body intruded into the core of an antiform in Salkhala metasedimentary rocks and are referred to herein as Dalhousie granites (DG). The studied mesoscopic structures, for example, well‐developed and randomly distributed megacrysts of K‐feldspar at the core part of massif, show undeformed porphyritic nature, whereas augen along with the mylonitic foliation and parallelly aligned feldspar megacrysts are dipping towards north‐north‐east (NNE) in the marginal part. The presence of aplite veins is evidence of final stage crystallization, where these rocks rapidly crystallized from siliceous residual fluids/solutions that escaped along fractures in the granites. Moreover, the granites show intrusive as well as thrusted contact with the Chamba metamorphics. The microscopic study from the marginal parts shows the dynamic recrystallization of quartz along with the fractures filled with secondary material oriented opposite to the top‐to‐the‐SW. Whereas euhedral quartz grains with undeformed grain boundaries, compositional zoning in plagioclase, and magmatic (perthite) texture of K‐feldspar are present in the core of Dalhousie pluton. In addition, a new whole‐rock geochemical dataset of DG is presented and investigated to elucidate the petrogenesis and tectonic environment. The geochemical data shows that DG (dominantly monzogranites) are formed from a pelitic source‐derived, (molar Al 2 O 3 /CaO + Na 2 O + K 2 O > 1.1) strongly peraluminous (S‐type) calc‐alkaline magma. This magma was generated by muscovite vapour absent dehydration melting. Harker bivariate plots indicate that fractional crystallization was a dominant process during the evolution of these granites. The tectonic discrimination diagrams suggest that DG were generated in syn‐collisional setting. Exceptionally high Pb in DG occurred due to primary melt generation at low‐temperature during partial melting (low‐degree melting) of the pelitic source rock. Trace and rare earth elements characteristics, such as positive anomaly of Zr show a notable amount of zircon as one of the accumulating phases. A pronounced negative Eu anomaly (Eu N /Eu* = 0.04–0.66), along with high silica content and highly varied trace element ratios in these rocks, show that these are fairly evolved granites.
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23001489' d='M0 0v6h9V0z'/%3e%3cpath fill='%23e03c31' d='M0 0v3h9V0z'/%3e%3cg stroke='%23fff' stroke-width='2'%3e%3cpath id='d' d='m0 0 4.5 3L0 6m4.5-3H9'/%3e%3cuse xlink:href='%23b' stroke='%23ffb81c' clip-path='url(%23c)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='none' stroke='%23007749' stroke-width='1.2'/%3e%3c/g%3e%3c/svg%3e)