Abstract Details
(2020) Triple Oxygen Isotope Clue of Enigmatic Nodular Chert Formation in Vindhyan Carbonates and an Insight into the Late Neo-Proterozoic Seawater Composition
Ghoshmaulik S, Bhattacharya SK, Sarkar A & Roy P
https://doi.org/10.46427/gold2020.822
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14d: Plenary Hall, Tuesday 23rd June 06:21 - 06:24
Sangbaran Ghoshmaulik
Sourendra Kumar Bhattacharya
Anindya Sarkar View abstracts at 4 conferences in series
Pallab Roy
Sourendra Kumar Bhattacharya
Anindya Sarkar View abstracts at 4 conferences in series
Pallab Roy
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Submitted by Anastasia Yanchilina on Monday 22nd June 14:18
Very nice & interesting work! I have several questions: (1) How do you propose the diagenesis of carbonates took place? Would it be precipitation of a new carbonate phase first at 150 m and then at 3.5 km? Would it be two different carbonate phases (as its also different temperature and depth within the sediments). (2) What is the source of the enriched silica fluid? Would it be dissolution / leaching of the sandstone and if so, the d18O of the porewater must also have been altered. How would these considerations be explained by your results? (3) Have the triple oxygens of the chert nodules say anything about their origin? (4) Would clumped isotopes on the carbonate phases be able to give any additional information on the temperature and d18O from which they formed?
(1) How do you propose the diagenesis of carbonates took place? Would it be precipitation of a new carbonate phase first at 150 m and then at 3.5 km? Would it be two different carbonate phases (as its also different temperature and depth within the sediments). Ans: Our studied unit contains abundant chert nodules, which form by the replacement of carbonate by siliceous fluid therefore essentially a diagenetic product. Moreover, the studied unit is composed of oolites and the intergranular spaces are filled with sparry calcite cement indicating post depositional diagenetic carbonate precipitation. The cluster of carbonate ?18O around two distinct value of 18‰ and 24‰ indicates that two carbonate phase exists which are either precipitated or altered at different equilibrium conditions. Although neomorphic phase changes might have occurred however our sample is devoid of any dolomite phase as revealed from the limited petrographic study indicating the absence of calcite-dolomite phase transformation. However, to understand the exact phase change for individual diagenetic event needs further XRD and petrographic study. Moreover, the resultant carbonates of first stage diagenesis are preserved in micro-domains and hence cannot be distinguished by bulk XRD analysis. (2) What is the source of the enriched silica fluid? Would it be dissolution / leaching of the sandstone and if so, the ?18O of the pore water must also have been altered. How would these considerations be explained by your results? Ans: The source of the fluid is marine water that percolated through the interstitial space of the sediments and might have acquired silica that was released during clay diagenesis from the overlying shale unit. However, the source of silica in these chert nodules is not clear. Sandstone lying below the limestone unit is not a good candidate for silica owing to its relative stratigraphic position. Instead, previous workers proposed that dissolution of the carbonate hosted siliceous skeletal fragments (e.g., sponge) might have released silica. The interaction of pore water with the sediment at a low water: rock ratio and elevated temperature leads to isotopic exchange between them. Consequently the pore water becomes progressively enriched in 18O. For the answer of the last part of your question please refer to the answer of the question 3. (3) Have the triple oxygen of the chert nodules say anything about their origin? Ans: Since the source of the siliceous fluid during chert nodule formation varies widely and they can form in a wide range of environment varying from shallow to deep, the triple oxygen isotope composition of the chert nodules provides an insight into the possible water isotopic composition and temperature of precipitation of the silica. In our study, as we are dealing with chert nodules which are essentially diagenetic product, we have considered two possible source of silica rich liquid. As discussed these two liquids are defined by their particular ? (where, ?=??17O/ ??18O) value based on previous studies. (4) Would clumped isotopes on the carbonate phases be able to give any additional information on the temperature and d18O from which they formed? Ans: The relict of the first stage diagenetic product is very rare in the sample and hence, the clumped isotope analysis of the carbonate phase is likely to yield the temperature of the second stage diagenesis. This temperature estimation along with the ?18O of the carbonate can yield the water composition of the second stage diagenesis of carbonates. It will be an additional constrain on the pore water oxygen isotopic composition.
Very nice & interesting work! I have several questions: (1) How do you propose the diagenesis of carbonates took place? Would it be precipitation of a new carbonate phase first at 150 m and then at 3.5 km? Would it be two different carbonate phases (as its also different temperature and depth within the sediments). (2) What is the source of the enriched silica fluid? Would it be dissolution / leaching of the sandstone and if so, the d18O of the porewater must also have been altered. How would these considerations be explained by your results? (3) Have the triple oxygens of the chert nodules say anything about their origin? (4) Would clumped isotopes on the carbonate phases be able to give any additional information on the temperature and d18O from which they formed?
(1) How do you propose the diagenesis of carbonates took place? Would it be precipitation of a new carbonate phase first at 150 m and then at 3.5 km? Would it be two different carbonate phases (as its also different temperature and depth within the sediments). Ans: Our studied unit contains abundant chert nodules, which form by the replacement of carbonate by siliceous fluid therefore essentially a diagenetic product. Moreover, the studied unit is composed of oolites and the intergranular spaces are filled with sparry calcite cement indicating post depositional diagenetic carbonate precipitation. The cluster of carbonate ?18O around two distinct value of 18‰ and 24‰ indicates that two carbonate phase exists which are either precipitated or altered at different equilibrium conditions. Although neomorphic phase changes might have occurred however our sample is devoid of any dolomite phase as revealed from the limited petrographic study indicating the absence of calcite-dolomite phase transformation. However, to understand the exact phase change for individual diagenetic event needs further XRD and petrographic study. Moreover, the resultant carbonates of first stage diagenesis are preserved in micro-domains and hence cannot be distinguished by bulk XRD analysis. (2) What is the source of the enriched silica fluid? Would it be dissolution / leaching of the sandstone and if so, the ?18O of the pore water must also have been altered. How would these considerations be explained by your results? Ans: The source of the fluid is marine water that percolated through the interstitial space of the sediments and might have acquired silica that was released during clay diagenesis from the overlying shale unit. However, the source of silica in these chert nodules is not clear. Sandstone lying below the limestone unit is not a good candidate for silica owing to its relative stratigraphic position. Instead, previous workers proposed that dissolution of the carbonate hosted siliceous skeletal fragments (e.g., sponge) might have released silica. The interaction of pore water with the sediment at a low water: rock ratio and elevated temperature leads to isotopic exchange between them. Consequently the pore water becomes progressively enriched in 18O. For the answer of the last part of your question please refer to the answer of the question 3. (3) Have the triple oxygen of the chert nodules say anything about their origin? Ans: Since the source of the siliceous fluid during chert nodule formation varies widely and they can form in a wide range of environment varying from shallow to deep, the triple oxygen isotope composition of the chert nodules provides an insight into the possible water isotopic composition and temperature of precipitation of the silica. In our study, as we are dealing with chert nodules which are essentially diagenetic product, we have considered two possible source of silica rich liquid. As discussed these two liquids are defined by their particular ? (where, ?=??17O/ ??18O) value based on previous studies. (4) Would clumped isotopes on the carbonate phases be able to give any additional information on the temperature and d18O from which they formed? Ans: The relict of the first stage diagenetic product is very rare in the sample and hence, the clumped isotope analysis of the carbonate phase is likely to yield the temperature of the second stage diagenesis. This temperature estimation along with the ?18O of the carbonate can yield the water composition of the second stage diagenesis of carbonates. It will be an additional constrain on the pore water oxygen isotopic composition.
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