5:15 PM - 6:45 PM
[MGI31-P04] Drilling experiments of mafic and ultramafic rocks: Implication for in-situ identification of the lithological boundary using drilling parameters
Keywords:Drilling parameter, mafic rock, ultramafic rock, Rate of penetration
Mantle convection plays an important role in the plumbing of deep material and plate tectonics. International Ocean Discovery Program (IODP) have been conducted to drill the oceanic plate to penetrate the Moho transition and reach the upper mantle. The Moho boundary was derived from the seismic velocity discontinuity, which is inferred the boundary between mafic rocks in the crust and ultramafic rocks in the upper mantle. However, another petrological model has been proposed that the seismic Moho boundary inferred the alteration front of fresh and serpentinized ultramafic rocks (e.g., Clague and Straley 1977) because serpentinization has the effect of reducing seismic velocity. Seismic velocity is affected by mineral alteration, so it is difficult to distinguish gabbro and serpentinite from seismic velocity data alone. Therefore, we tried to apply drilling parameters such as, weight on bit (WOB), rotation per minute (RPM), torque, and rate of penetration (ROP). Drilling parameters were obtained continuously, so we may be able to estimate the lithological transition from the difference in drilling parameters. Therefore, we conducted drilling experiments using mafic and ultramafic rocks to investigate the rock dependence of drilling parameters.
Experimental samples were used gabbro (2 samples) and mafic granulite (1 sample) as mafic rocks, and peridotite (2 samples) and serpentinized peridotite (2 samples) as ultramafic rocks. Samples were collected from the Oman ophiolite and the Horoman Complex. Drilling experiments were conducted under dry and wet conditions using the pressurized high-velocity (PHV) rotary shear apparatus located at Kochi Core Center. In the drilling experiments, torque and ROP were measured while WOB varied from 0.3–0.9 kN, and RPM varied from 1.5–0.1 /min.
Experimental results show that ROP for drilling in serpentinized peridotite was about 2 times higher than that for mafic rocks and peridotite under both dry and wet conditions. This result is likely caused by the reduction of fracture strength with serpentinization. Measured torque were almost the same in drilling experiments on all samples, so the rock dependence in drilling parameters was clearly shown in ROP. For all samples, ROP under wet condition was about 2 times higher than that under dry condition. This high ROP under wet condition is likely occurred due to the increase in crack growth velocity.
IODP Expedition 360 drilled gabbro at the Southwest Indian Ridge, and IODP Expedition 399 drilled serpentinized peridotite at the Mid-Atlantic Ridge with JOIDES Resolution. Drilling parameters were recorded on both expeditions, so we compared the results of ROP, which were normalized under the same RPM and torque conditions using Equivalent strength method (Hamada et al., 2018). Normalized ROP for the formation of serpentinized peridotite were much higher than that for gabbro, which is consisted with our experimental results. Therefore, ROP can be applied to distinguish the serpentinite layer. Based on our study, in addition to physical properties, drilling parameters such as ROP are also important to estimate the lithological transition around the Moho.
Experimental samples were used gabbro (2 samples) and mafic granulite (1 sample) as mafic rocks, and peridotite (2 samples) and serpentinized peridotite (2 samples) as ultramafic rocks. Samples were collected from the Oman ophiolite and the Horoman Complex. Drilling experiments were conducted under dry and wet conditions using the pressurized high-velocity (PHV) rotary shear apparatus located at Kochi Core Center. In the drilling experiments, torque and ROP were measured while WOB varied from 0.3–0.9 kN, and RPM varied from 1.5–0.1 /min.
Experimental results show that ROP for drilling in serpentinized peridotite was about 2 times higher than that for mafic rocks and peridotite under both dry and wet conditions. This result is likely caused by the reduction of fracture strength with serpentinization. Measured torque were almost the same in drilling experiments on all samples, so the rock dependence in drilling parameters was clearly shown in ROP. For all samples, ROP under wet condition was about 2 times higher than that under dry condition. This high ROP under wet condition is likely occurred due to the increase in crack growth velocity.
IODP Expedition 360 drilled gabbro at the Southwest Indian Ridge, and IODP Expedition 399 drilled serpentinized peridotite at the Mid-Atlantic Ridge with JOIDES Resolution. Drilling parameters were recorded on both expeditions, so we compared the results of ROP, which were normalized under the same RPM and torque conditions using Equivalent strength method (Hamada et al., 2018). Normalized ROP for the formation of serpentinized peridotite were much higher than that for gabbro, which is consisted with our experimental results. Therefore, ROP can be applied to distinguish the serpentinite layer. Based on our study, in addition to physical properties, drilling parameters such as ROP are also important to estimate the lithological transition around the Moho.