5:15 PM - 7:15 PM
[SCG53-P07] Relationship Between Cooling Rate and the Morphology of Magnetite in the Shiosezaki Dolerite Sill, Oga Peninsula
Keywords:Magnetite, Dendrite, Secondary Arm Spacing, Dolerite
The cooling rate of magma is a critical parameter that influences the timescales of material transport within a magma reservoir, including elemental diffusion and the flotation or settling of gas bubbles and crystals. The morphology of minerals crystallized from magma is known to correlate with the cooling rate. Holness (2014) demonstrated that the average aspect ratio of plagioclase in intrusive rocks correlates with magma cooling time, proposing a cooling rate meter based on this relationship.
In the field of metallurgy, it is well known that the secondary arm spacing of dendritic crystals formed during the solidification of alloys is proportional to the cooling rate raised to the power of -0.4. Toramaru and Yamaki (1997) investigated the relationship between the secondary arm spacing of dendritic magnetite in a basaltic intrusion and the cooling rate of magma. They found that the secondary arm spacing of dendrites increases in proportion to approximately the 1/3 to 1/2 power of the distance from the boundary with the host rock. However, subsequent research on the relationship between magnetite dendritic arm spacing and cooling rate has been limited.
In this study, we examined variations in the aspect ratio of plagioclase and the secondary arm spacing of dendritic magnetite within the Shiosezaki dolerite sill, located at the southwestern tip of the Oga Peninsula, to investigate their relationship with magma cooling rate.
The Shiosezaki dolerite sill (c.a. 3.8 m thick) intrudes the Shiosezaki Conglomerate, dated to 33–34 Ma (Kano, 2007), with an intrusion age of 21.3 ± 1.1 Ma (Kano, 2007). The sill exhibits a rhythmic layering of vesicles. We collected 13 samples along the vertical section of the sill. The bulk rock chemistry of the chilled margin is classified as Hawaiite.
In the Shiosezaki dolerite sill, the average aspect ratio of plagioclase increases monotonically toward the upper and lower margins of the intrusion. The morphology of magnetite is dendritic within 15 cm from the boundaries with the host rocks but transitions to a polyhedral shape at distances greater than 1 m. The secondary arm spacing of magnetite increases in proportion to approximately the 1/2 power of the distance from the boundary.
Using the one-dimensional thermal conduction equation for the cooling of basaltic intrusions proposed by Holness (2014), we calculated the cooling rate for each sample from the Shiosezaki dolerite sill. The results show that the secondary arm spacing of dendritic magnetite is proportional to approximately the -0.3 power of the cooling rate. This trend is similar to that observed in alloys, suggesting that dendritic magnetite could serve as a useful indicator for estimating magma cooling rates.
In the field of metallurgy, it is well known that the secondary arm spacing of dendritic crystals formed during the solidification of alloys is proportional to the cooling rate raised to the power of -0.4. Toramaru and Yamaki (1997) investigated the relationship between the secondary arm spacing of dendritic magnetite in a basaltic intrusion and the cooling rate of magma. They found that the secondary arm spacing of dendrites increases in proportion to approximately the 1/3 to 1/2 power of the distance from the boundary with the host rock. However, subsequent research on the relationship between magnetite dendritic arm spacing and cooling rate has been limited.
In this study, we examined variations in the aspect ratio of plagioclase and the secondary arm spacing of dendritic magnetite within the Shiosezaki dolerite sill, located at the southwestern tip of the Oga Peninsula, to investigate their relationship with magma cooling rate.
The Shiosezaki dolerite sill (c.a. 3.8 m thick) intrudes the Shiosezaki Conglomerate, dated to 33–34 Ma (Kano, 2007), with an intrusion age of 21.3 ± 1.1 Ma (Kano, 2007). The sill exhibits a rhythmic layering of vesicles. We collected 13 samples along the vertical section of the sill. The bulk rock chemistry of the chilled margin is classified as Hawaiite.
In the Shiosezaki dolerite sill, the average aspect ratio of plagioclase increases monotonically toward the upper and lower margins of the intrusion. The morphology of magnetite is dendritic within 15 cm from the boundaries with the host rocks but transitions to a polyhedral shape at distances greater than 1 m. The secondary arm spacing of magnetite increases in proportion to approximately the 1/2 power of the distance from the boundary.
Using the one-dimensional thermal conduction equation for the cooling of basaltic intrusions proposed by Holness (2014), we calculated the cooling rate for each sample from the Shiosezaki dolerite sill. The results show that the secondary arm spacing of dendritic magnetite is proportional to approximately the -0.3 power of the cooling rate. This trend is similar to that observed in alloys, suggesting that dendritic magnetite could serve as a useful indicator for estimating magma cooling rates.