Collaborative Research: Understanding the influence of tectonic setting on the depth of magmatic processes in the mid-ocean ridge system

South Dakota School Of Mines And Technology, Rapid City SD

Investigators

Abstract

Collaborative Research: Understanding the influence of tectonic setting on the depth of magmatic processes in the mid-ocean ridge system The rocks that pave the seafloor cover ~70% of the Earth’s surface and are produced by volcanoes along the mid-ocean ridge (MOR) system. These volcanoes arguably represent the largest geologic system in the Earth’s crust. By analyzing the lavas erupted at the MOR, geologists understand how these magmas are produced and how they chemically change as they rise to the surface. However, less is understood about depths of formation and how that may change in different parts of the MOR system. New data on depth of formation of the crystals in MOR lavas, based on CO2 in trapped droplets of magma in crystals (melt inclusions), indicate that the crystals are produced over a wide depth range. Those crystals therefore provide a record of magma compositions encountered during transport from depth and of processes controlling ocean crust formation. This study will chemically analyze microscopic melt inclusions and crystals to understand how the depth of magma formation varies globally. This new data will provide information on the depth of crystallization in different MOR environments and the carbon budget of the mantle. A core aspect of this proposal involves outreach to students at the tribal colleges in South Dakota which will consist of three parts including current programs at South Dakota Mines, mentoring students from Oglala Lakota College (OLC), and providing advice on the development of the analytical facilities at OLC. Current understanding of the evolution of the ocean crust relies heavily on submarine volcanic glasses and, more recently, melt inclusion (MI) compositions. Such data is most informative about the complex array of mixing and differentiation processes active in the upper mantle and crust - in essence “what” is happening. What has been difficult to constrain is “where” specific processes are active in terms of depth, making it challenging to resolve the influences of processes occurring in the melting regime versus those that occur during transport to the surface. New data on depth of entrapment for melt inclusions (based on CO2 saturation/concentration) indicate that much of the crystal cargo in ocean floor basalts, particularly those from plagioclase ultraphyric basalts (PUB) are the products of crystallization at a wide range of pressure. They therefore provide an opportunity to investigate how the array of magma compositions evolves during transport and how the distribution of energy and mass of the processes controlling ocean crust formation differs between tectonic settings (different spreading rates, magma supply, extent of melting, etc.). This study will conduct combined analysis by electron microprobe (for major element composition), Laser ICP-MS (trace element compositions), secondary ion mass spectrometry (glass CO2, H2O, S), and Raman spectroscopy (MI bubble density/composition). The samples selected represent a number of PUB lavas from a range of settings, on axis, pull apart basins, ultra-slow to intermediate spreading rates and from ridges characterized by different crustal thicknesses. This new information will improve understanding of a number of fundamental questions in MORB petrology including: 1) the comparative depth of crystallization in different MOR environments for olivine and plagioclase (how crystal sorting influence what is being sampled), 2) the depth dependence of the processes responsible for MOR differentiation, 3) the extent to which CO2/trace element ratios can be used to estimate the mantle C budget. A core aspect of this proposal involves outreach to students at the tribal colleges in South Dakota which will consist of three parts including current programs at South Dakota Mines, mentoring students from Oglala Lakota College (OLC), and providing advice on the development of the analytical facilities at OLC. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.