5:15 PM - 7:15 PM
[SCG55-P08] Exploration of serpentine-hosted alkaline vent in the southern Mariana forearc region with a focus on carbonates
Keywords:carbonate, serpentine, peridotite, southern Mariana forearc
Sepentinite-hosted alkaline vent sites support deep-sea chemosynthetic ecosystems based on hydrogen and methane, which have attracted attention as analog sites for early Earth and extraterrestrial biospheres [1]. The Shinkai Seep Field (SSF), discovered in 2010 at approximately 5,700 m in the southern Maria forearc region, is the deepest spring of its type and consisted Calyptogena colonies and chimneys composed of brucite and carbonate [2].
Unlike serpentinite seamounts, which have been reported in large numbers in the Northern Mariana forearc region [3], the SSF is distributed on the landward steep slope where serpentinized peridotite is exposed. On the landward slope of the Southern Mariana Trench, a unique tectonic setting in which relatively young fore-arc rifting occurs to break up a sequence of relatively old volcanic rocks distributed on the landward slope of the trench, this rifting possibly play an important role in the origin of the SSF. In this study, we focused on carbonates in rock samples collected during dredge surveys conducted approximately 300 m east and west of the southern Mariana forearc region to examine the extent of similar serpentinite-hosted vent site in this region.
The examined samples were collected by dredging during the KH-23-09 cruise conducted in 2023. Of the total 18 dredged samples, 24 rock samples containing white minerals thought to contain carbonates were collected from four locations: D03, D12, D14, and D15. Focusing on the white parts of these samples, I identified the minerals using X-ray diffraction (XRD) and searched for traces of seep water by observing the texture of thin sections.
As a result of visual observation of the samples, the samples were classified into the following four types.
I) Vein-like white depositions collected at approximately 4,000m and 6,700m (D03, D12)
II) White sediments containing biological skeletons collected at approximately 1,800m and 6,700m (D14, D15)
III) Breccia containing white sediments collected at approximately 6,700m (D14)
IV) Conglomerate with a matrix of white depositions collected at approximately 6,700m (D14)
XRD analysis and texture observation revealed that white sediments of types II) and III) are carbonate sediments composed of planktonic foraminiferal shells and reef-building coral skeletons. Moreover, white depositions of type IV) were not carbonates but fine-grained silicate minerals. White depositions of type I) were mostly composed of silicate minerals. Only two samples were identified that had veins composed of calcite, one of which was composed of Mg-calcite composed of mosaic crystals and micrite, and the other was composed of madstone containing planktonic foraminiferal shells and was not contained in ultramafic rocks. When searching for serpentine-hosted spring areas, samples that suggest the presence of serpentine-hosted springs include peridotite and serpentine containing aragonite veins in cracks, white chimney fragments composed of brucite and carbonates, and Calyotogena clam’s shells. From the above results, it was found that at least at the collection site of the samples examined this time, carbonates suggestting the target serpentine-hosted springs are not distributed.
Reference
[1] Kelley et al. (2005) Science, 307, 1428-1434.
[2] Ohara et al. (2012) Proc. Natl. Acad. Sci. U. S. A., 109, 2831–2835.
[3] Fryer et al. (2012) Annu. Rev. Mar. Sci., 4:345-373.
Unlike serpentinite seamounts, which have been reported in large numbers in the Northern Mariana forearc region [3], the SSF is distributed on the landward steep slope where serpentinized peridotite is exposed. On the landward slope of the Southern Mariana Trench, a unique tectonic setting in which relatively young fore-arc rifting occurs to break up a sequence of relatively old volcanic rocks distributed on the landward slope of the trench, this rifting possibly play an important role in the origin of the SSF. In this study, we focused on carbonates in rock samples collected during dredge surveys conducted approximately 300 m east and west of the southern Mariana forearc region to examine the extent of similar serpentinite-hosted vent site in this region.
The examined samples were collected by dredging during the KH-23-09 cruise conducted in 2023. Of the total 18 dredged samples, 24 rock samples containing white minerals thought to contain carbonates were collected from four locations: D03, D12, D14, and D15. Focusing on the white parts of these samples, I identified the minerals using X-ray diffraction (XRD) and searched for traces of seep water by observing the texture of thin sections.
As a result of visual observation of the samples, the samples were classified into the following four types.
I) Vein-like white depositions collected at approximately 4,000m and 6,700m (D03, D12)
II) White sediments containing biological skeletons collected at approximately 1,800m and 6,700m (D14, D15)
III) Breccia containing white sediments collected at approximately 6,700m (D14)
IV) Conglomerate with a matrix of white depositions collected at approximately 6,700m (D14)
XRD analysis and texture observation revealed that white sediments of types II) and III) are carbonate sediments composed of planktonic foraminiferal shells and reef-building coral skeletons. Moreover, white depositions of type IV) were not carbonates but fine-grained silicate minerals. White depositions of type I) were mostly composed of silicate minerals. Only two samples were identified that had veins composed of calcite, one of which was composed of Mg-calcite composed of mosaic crystals and micrite, and the other was composed of madstone containing planktonic foraminiferal shells and was not contained in ultramafic rocks. When searching for serpentine-hosted spring areas, samples that suggest the presence of serpentine-hosted springs include peridotite and serpentine containing aragonite veins in cracks, white chimney fragments composed of brucite and carbonates, and Calyotogena clam’s shells. From the above results, it was found that at least at the collection site of the samples examined this time, carbonates suggestting the target serpentine-hosted springs are not distributed.
Reference
[1] Kelley et al. (2005) Science, 307, 1428-1434.
[2] Ohara et al. (2012) Proc. Natl. Acad. Sci. U. S. A., 109, 2831–2835.
[3] Fryer et al. (2012) Annu. Rev. Mar. Sci., 4:345-373.