14:45 〜 15:00
[SCG42-05] M2Mマントル掘削計画と951-フルプロポーザル: ハワイ沖ノースアーチの成熟した地殻掘削
★招待講演
キーワード：モホール、ハワイ沖上部地殻掘削951フルプロポーザル、海洋地殻構造、熱水変質過程、地下生命圏、深海掘削
The goal of the MoHole to Mantle (M2M) project is to drill into the upper mantle, through intact oceanic crust formed at a fast spreading rate. The bathymetry and seismic structure of fast-spread crust are relatively uniform, and the great majority of crust subducted into the mantle during the past 200 Ma was produced at fast spreading ridges. Many first-order hypotheses about crustal accretion and interaction between the lithosphere, hydrosphere, and biosphere can and will be tested by drilling to the Mantle.
Full penetration and sampling of the upper mantle, the Mohorovičić discontinuity (Moho), and intact crust in oceanic lithosphere will provide hitherto unattainable information on the composition, microstructure, and melting of the upper mantle, the construction and cooling of oceanic crust, the biological and chemical exchanges between the crust and oceans, and the linkages between these processes.
Despite more than a half century’s effort, we have not yet managed to reach kilometers below the seafloor to the Moho and the underlying mantle. A major technical obstacle to establishing a drill hole several kilometers deep is collapse of an uncased borehole, due to substantial lithostatic load and increasing temperatures. To prevent borehole collapse, riser-drilling technology is required to stabilize the borehole and remove cuttings. In 2005, the riser-drilling vessel D/V Chikyu was commissioned with the expressed ultimate goal to explore the upper mantle. After 15 years of riser operations by Chikyu, mantle drilling now comes into the realm of technical feasibility. Consequently, it is imperative to initiate the foundational steps for upper-mantle drilling in the near future, to prepare for successfully drilling a complete section of the oceanic crust and upper mantle during the 25 to 30 years of Chikyu operational lifetime.
In October 2021, we submitted IODP proposal 951-Full to use Chikyu on the North Arch off Hawaii, to drill a complete section down to cumulate gabbros of mature 80-Ma upper oceanic crust at a half spreading rate of 3.5 cm/a. This region is one of the three candidate sites of the M2M project. Our overarching goal is to drill a pilot hole to inform the design of a future MoHole and gain experience in deep hard-rock drilling with Chikyu.
The average age of Earth’s oceanic crust is ~63 Ma and the average age of oceanic crust being subducted is ~79 Ma. Although, there are some drillholes in old (>110 Ma) and young (<20 Ma) oceanic crust, there is no deep hole (>100 m) into mature intact average-age oceanic crust that records the full history of seawater-basalt exchange.
With the proposed drilling, we will investigate the physical, chemical, and biological architecture and evolution of the oceanic crust to test three major hypotheses, that: 1) the North Arch crust spread at an intermediate rate will comprise an intact 740–820 m-thick extrusive sequence, overlying a thick (880-1770 m) sheeted dike complex; 2) hydrothermal exchanges between the oceanic crust and seawater are episodic and the extent of fluid-rock exchange does not just reflect the age of the crust but also external factors that enhance fluid flow and reaction; and 3) in oceanic crust cooler than the thermal limit to life, water-rock interactions sustain microbial life as deeply as seawater-derived fluid penetrates. Drilling will provide additional insights into North Arch volcanism and hazardous giant landslides from the Hawaiian volcanoes.
The drilling operation will be conducted in three stages. Stage 1 will entail coring sediment to below the sediment-basalt interface (Hole A), drilling and coring basement to 1130 mbsf and setting casing (Hole B). Stage 2 (Hole B) will be coring and setting casing to 1730 mbsf. Stage 3 will entail coring to 2500 mbsf and penetrating through the Layer 2-3 transition and into the uppermost cumulate gabbros without casing.
Full penetration and sampling of the upper mantle, the Mohorovičić discontinuity (Moho), and intact crust in oceanic lithosphere will provide hitherto unattainable information on the composition, microstructure, and melting of the upper mantle, the construction and cooling of oceanic crust, the biological and chemical exchanges between the crust and oceans, and the linkages between these processes.
Despite more than a half century’s effort, we have not yet managed to reach kilometers below the seafloor to the Moho and the underlying mantle. A major technical obstacle to establishing a drill hole several kilometers deep is collapse of an uncased borehole, due to substantial lithostatic load and increasing temperatures. To prevent borehole collapse, riser-drilling technology is required to stabilize the borehole and remove cuttings. In 2005, the riser-drilling vessel D/V Chikyu was commissioned with the expressed ultimate goal to explore the upper mantle. After 15 years of riser operations by Chikyu, mantle drilling now comes into the realm of technical feasibility. Consequently, it is imperative to initiate the foundational steps for upper-mantle drilling in the near future, to prepare for successfully drilling a complete section of the oceanic crust and upper mantle during the 25 to 30 years of Chikyu operational lifetime.
In October 2021, we submitted IODP proposal 951-Full to use Chikyu on the North Arch off Hawaii, to drill a complete section down to cumulate gabbros of mature 80-Ma upper oceanic crust at a half spreading rate of 3.5 cm/a. This region is one of the three candidate sites of the M2M project. Our overarching goal is to drill a pilot hole to inform the design of a future MoHole and gain experience in deep hard-rock drilling with Chikyu.
The average age of Earth’s oceanic crust is ~63 Ma and the average age of oceanic crust being subducted is ~79 Ma. Although, there are some drillholes in old (>110 Ma) and young (<20 Ma) oceanic crust, there is no deep hole (>100 m) into mature intact average-age oceanic crust that records the full history of seawater-basalt exchange.
With the proposed drilling, we will investigate the physical, chemical, and biological architecture and evolution of the oceanic crust to test three major hypotheses, that: 1) the North Arch crust spread at an intermediate rate will comprise an intact 740–820 m-thick extrusive sequence, overlying a thick (880-1770 m) sheeted dike complex; 2) hydrothermal exchanges between the oceanic crust and seawater are episodic and the extent of fluid-rock exchange does not just reflect the age of the crust but also external factors that enhance fluid flow and reaction; and 3) in oceanic crust cooler than the thermal limit to life, water-rock interactions sustain microbial life as deeply as seawater-derived fluid penetrates. Drilling will provide additional insights into North Arch volcanism and hazardous giant landslides from the Hawaiian volcanoes.
The drilling operation will be conducted in three stages. Stage 1 will entail coring sediment to below the sediment-basalt interface (Hole A), drilling and coring basement to 1130 mbsf and setting casing (Hole B). Stage 2 (Hole B) will be coring and setting casing to 1730 mbsf. Stage 3 will entail coring to 2500 mbsf and penetrating through the Layer 2-3 transition and into the uppermost cumulate gabbros without casing.