11:00 AM - 1:00 PM
[MZZ47-P05] Estimating the phosphatized ages of ferromanganese crusts by whole-rock Sr isotopic compositions
Keywords:Marine ferromanganese crusts, Sr isotopes, Phosphorites
Some marine ferromanganese crusts are phosphatized. The question of when this phosphatization happened is quite difficult. There are various dating methods for ferromanganese crust, e.g. stratigraphy using heavy elemental isotopes such as Pb, Nd, and Os, and in the Be dating method. We can see the youngest age of phosphatization from the age of boundary between non- phosphatized layer and phosphatized one. On the other hand, the question of when it has continued cannot be answered.
Hein et al. (1993) and Hyeong et al. (2013) estimate the age of phosphatization in the western Pacific from the Sr isotope stratigraphy of phosphorites obtained from seamounts. According to them, the phosphatization in the western Pacific occurs between 36 Ma and 12 Ma, and its peak is especially near the Eocene-Oligocene boundary.
In principle, if the same method is used for phosphatized ferromanganese crust, it should be possible to estimate the age at which the manganese crust was subjected to phosphatization, but in reality, it is extremely difficult to extract Sr only from the apatite phase by chemical treatments (e.g. VonderHaar et al., 1995). In general, marine ferromanganese oxides are porous, and Sr incorporated into ferromanganese oxide is constantly replacing that of younger seawater (e.g. Ito et al., 1998).
Based on these points, in this study, the whole rock Sr isotope composition was investigated using phosphorites and phosphatized ferromanganese crusts obtained from the Ogasawara Plateau and Matsubara Seamount in the western Pacific Ocean. And we explored the possibility of dating of phosphatization.
The analysis results were neatly organized by making the following three assumptions that 1) the coeval seawater Sr isotope composition in the apatite phase is preserved, 2) the Sr in the silicate phase contained in the phosphatized crust layers is negligible, and 3) all Sr contained in the ferromanganese phases are replaced by the younger seawater Sr.
Based on these assumptions, the phosphatization occurs at least 4 times in this sea area, and the seawater Sr isotope composition (age) at that time is 0.70745 (Cretaceous), 0.70770 (49 Ma) to 0.70782 (34Ma), 0.70798 (29.8Ma), 0.70880 (14.6Ma). In D886 from Matsubara Seamount, ferromanganese oxide grew on the basement of Cretaceous phosphorite, and it is thought that the depth of 8 mm to 20 mm was collectively phosphatized at 34 Ma. After that, surficial 8 mm grew. On the other hand, the growing history of the D858 from the Ogasawara Plateau is a little complicated. At least three times of phosphatizations occurred: a depth of 73 mm to 75 mm is 49 Ma, the part with a depth of 63 mm to 70 mm is 34 Ma, and the part with a depth of 53 mm to 62 mm is 14.6 Ma. In addition, the surficial, non- phosphatized ca. 50 mm seems to have grown after 14.6 Ma.
In the future, it is necessary to examine the validity of the above assumptions. But even though the method is simple, it has the potential to determine the ages of multiple phosphatizations within one crust and is likely to be effective.
Hein et al. (1993) and Hyeong et al. (2013) estimate the age of phosphatization in the western Pacific from the Sr isotope stratigraphy of phosphorites obtained from seamounts. According to them, the phosphatization in the western Pacific occurs between 36 Ma and 12 Ma, and its peak is especially near the Eocene-Oligocene boundary.
In principle, if the same method is used for phosphatized ferromanganese crust, it should be possible to estimate the age at which the manganese crust was subjected to phosphatization, but in reality, it is extremely difficult to extract Sr only from the apatite phase by chemical treatments (e.g. VonderHaar et al., 1995). In general, marine ferromanganese oxides are porous, and Sr incorporated into ferromanganese oxide is constantly replacing that of younger seawater (e.g. Ito et al., 1998).
Based on these points, in this study, the whole rock Sr isotope composition was investigated using phosphorites and phosphatized ferromanganese crusts obtained from the Ogasawara Plateau and Matsubara Seamount in the western Pacific Ocean. And we explored the possibility of dating of phosphatization.
The analysis results were neatly organized by making the following three assumptions that 1) the coeval seawater Sr isotope composition in the apatite phase is preserved, 2) the Sr in the silicate phase contained in the phosphatized crust layers is negligible, and 3) all Sr contained in the ferromanganese phases are replaced by the younger seawater Sr.
Based on these assumptions, the phosphatization occurs at least 4 times in this sea area, and the seawater Sr isotope composition (age) at that time is 0.70745 (Cretaceous), 0.70770 (49 Ma) to 0.70782 (34Ma), 0.70798 (29.8Ma), 0.70880 (14.6Ma). In D886 from Matsubara Seamount, ferromanganese oxide grew on the basement of Cretaceous phosphorite, and it is thought that the depth of 8 mm to 20 mm was collectively phosphatized at 34 Ma. After that, surficial 8 mm grew. On the other hand, the growing history of the D858 from the Ogasawara Plateau is a little complicated. At least three times of phosphatizations occurred: a depth of 73 mm to 75 mm is 49 Ma, the part with a depth of 63 mm to 70 mm is 34 Ma, and the part with a depth of 53 mm to 62 mm is 14.6 Ma. In addition, the surficial, non- phosphatized ca. 50 mm seems to have grown after 14.6 Ma.
In the future, it is necessary to examine the validity of the above assumptions. But even though the method is simple, it has the potential to determine the ages of multiple phosphatizations within one crust and is likely to be effective.