Multiple Episodes of Fluid Infiltration Along a Single Metasomatic Channel in Metacarbonates (Mogok Metamorphic Belt, Myanmar) and Implications for CO₂ Release in Orogenic Belts

Abstract

Fluid infiltration into metacarbonates is a key mechanism to induce orogenic decarbonation, which influences the global carbon cycle and long‐term climate evolution. Little is known regarding the fluid pathways during episodic infiltration events and how flow patterns control time‐integrated CO2 outflux. We investigate the “vein‐like” polycrystalline mineral reaction zones (PMRZs) in dolomite marbles (Mogok metamorphic belt, Myanmar), which are formed by metasomatism via the infiltration of Si–Al–K–Ti–Zr‐bearing fluids. The petrographic textures and mineral U–Pb chronology reveal three episodes of fluid influx in a single PMRZ: (1) the initial episode (Stage‐I) transformed most dolomite into Mg‐rich silicates/oxides and calcite at ∼35–36 Ma indicated by baddeleyite cores; (2) baddeleyite rims gave ages of ∼23–24 Ma, representing a subsequent infiltration episode (Stage‐II) that modified Stage‐I minerals via a dissolution–precipitation mechanism; (3) the final episode (Stage‐III) is recorded by zircon replacing baddeleyite, which yielded ages of ∼17 Ma. Stage‐III fluid has a higher SiO2 activity and XCO2 [CO2/(CO2 + H2O)] than Stage‐I/Stage‐II fluids. Thermodynamic and mass‐balance analyses indicate that Stage‐I infiltration causes >62–67% loss of CO2 by both dolomite‐consuming reactions and calcite dissolution, whereas the latter two infiltration episodes induce <12–18% loss of CO2 via calcite dissolution. Our results provide compelling evidence that repeated episodes of infiltration (each separated in time by 7–13 Ma) occurred along a single channel in marbles. The initial infiltration episode may create high‐permeability regions, offering favorable channels for later‐stage fluids that transfer obviously less CO2 than the initial metasomatism. This considerably complicates a quantitative assessment of CO2 liberation from metacarbonates during orogenesis.

Massive carbon storage in convergent margins initiated by subduction of limestone

“…Although there are effects from fluid derived from underlying dehydrating lithologies, fluid infiltration in subducted marbles mostly occurs at low pressures of <2.0 GPa 9 , 33 because two-thirds of the water in the fully hydrated oceanic crust would be lost at forearc depths 12 . Furthermore, marbles are notably impermeable to fluid flow 34 , 35 compared to silicate rocks, thus fluid infiltration usually occurs along a single channel in the subducted marbles 9 , 36 . The water content of the limestone used in our experiments is 0.9 wt.% and was probably higher during experiments due to hydrogen diffusion, thus could represent the water content of natural marbles at subarc depths.…”

Petrology and geochemistry of ultramafic rocks in the Mogok belt, Myanmar: Cumulates from high‐pressure crystallization of hydrous arc melts

“…During this period, episodic fluid infiltrations (i.e., $36-35 Ma, $24-23, Ma, and $17 Ma) into metacarbonates (marbles) of this region may have also contributed to regional metasomatism and subsequent ruby mineralization (Guo et al, 2021). A recent study reported titanite SIMS U-Pb ages of $25 Ma for ruby-bearing marbles, suggesting that the ruby mineralization was probably related to the high-temperature metamorphism during the India-Asia collision (Zhang et al, 2021).…”

“…As discussed above, our data for the Mogok ultramafic rocks show that they are probably Alaskan-type cumulates generated from subduction-related arc magmas. Based on regional geological evolu- belonging to cumulates, were generated in the Jurassic-Lower Cretaceous, they were unrelated with the Cenozoic metamorphism and magmatism in this region, which provided heat for gemstone mineralization (Guo et al, 2021;Searle et al, 2020). Second, if the Mogok ultramafic rocks formed during the Upper Cretaceous-Cenozoic, their emplacement could provide additional heat for the regional metamorphism and magmatism.…”

“…The Myanmar Tethys on the one hand is geologically important because of the occurrence of variable lithological units, including ophiolites (Liu et al, 2016, b;Searle et al, 2017), ultramafic, felsic to alkaline intrusions (Gardiner et al, 2018;Gardiner, Searle, Robb, & Morley, 2015;Mitchell et al, 2004;Searle et al, 2017Searle et al, , 2020, and high-temperature metamorphic rocks (Barley, Doyle, Khin, Pickard, & Rak, 2003;Mitchell et al, 2004;Searle et al, 2017Searle et al, , 2020Searle & Haq, 1964). On the other hand, it is also economically significant due to the appearance of world-class coloured gemstone (i.e., ruby and sapphire) and jade in the Mogok metamorphic belt (MMB) and regional areas in central Myanmar (Chen et al, 2018;Guo et al, 2021;Searle et al, 2020;Themelis, 2008). The MMB comprises a series of high-grade (up to granulite-facies) metasedimentary and metaigneous rocks, whose formation has been suggested to be associated with the Tethyan subduction, India-Asia collision, and post-collisional extension (Mitchell et al, 2004;Searle et al, 2017).…”

~25 Ma Ruby Mineralization in the Mogok Stone Tract, Myanmar: New Evidence from SIMS U–Pb Dating of Coexisting Titanite

“…Metasomatic skarns occur in contact zones between the marbles and intrusions as a function of the infiltration of fluids/melts. Metasomatism may form various mineralogical reaction zones in which rare gems are produced [5,8,30,35].…”