Primitive layered gabbros from fast-spreading lower oceanic crust.

Area of Science:

• Geology
• Oceanography
• Petrology

Background:

• Oceanic crust formation at fast-spreading ridges is dominated by plutonic rocks, crucial for understanding melt transport and crystallization.
• Direct sampling of these lower crustal rocks is difficult due to overlying volcanic layers, limiting direct study.
• Current models rely on indirect geophysical data and ophiolite analogues, which may not represent typical mid-ocean ridge processes.

Purpose of the Study:

• To investigate the composition and formation of lower oceanic crustal plutonic rocks at a fast-spreading ridge.
• To provide a more complete estimate of the bulk composition of fast-spreading oceanic crust.
• To test existing models of melt extraction and magma differentiation in oceanic crust formation.

Main Methods:

• Coring of primitive, modally layered gabbroic rocks from the lower plutonic crust at the Hess Deep rift.
• Geochemical analysis of recovered plutonic rocks.
• Integration of new data with existing geochemical data for shallow plutonic rocks, dykes, and lavas.

Main Results:

• Confirmation that modally layered gabbroic rocks are a key component of the lower oceanic crust at fast-spreading ridges.
• The most constrained estimate to date of the bulk composition of fast-spreading oceanic crust.
• Crystallization models using the bulk crustal composition accurately predict lava and plutonic rock compositions.
• Observed early orthopyroxene crystallization in plutonic rocks, which is not predicted by current models.

Conclusions:

• Primitive, modally layered gabbroic rocks are integral to the lower oceanic crust formed at fast-spreading ridges.
• The bulk composition of fast-spreading oceanic crust can be accurately modeled by simple crystallization processes.
• The early crystallization of orthopyroxene suggests that diverse melts are extracted from the mantle and undergo pre-eruptive mixing.