This research addresses the origin of the unusual felsic and intermediate intrusive rocks collected from the slope of the southern Mariana Trench. These unusual rocks appear to represent the middle part of the ocean crust under the Izu-Bonin/Mariana (IBM) volcanic arc. Unexpectedly, these early rocks have an island arc geochemical signature with Pb isotope ages indicating they are Cretaceous in age. If verified, this would indicate that older arc or crustal fragments comprise some of the IBM arc basement. which has significant ramifications for the evolution of the seafloor in the IBM arc area and possibly for volcanic arcs in general. Research goals are to determine at what time during arc evolution these rocks formed and/or whether they are parts of older or arcs and if these intrusive units enhanced intracrustal magmatic differentiation and middle crust development. Geochemical and isotopic data will be collected to examine possibly inhereted features and the development of the intrusive middle crustal section. Understanding the formation of how intra-oceanic arc crust forms is critical for development of continental crust and its modeling. Broader impacts of the work include support of an investigator and graduate student from a gender under-represented in the sciences, international collaboration, and support of faculty at minority-serving institution. The work enhances work being carried out by the NSF-funded MARGINS program.
Subduction zones are places where oceanic lithosphere sinks beneath an adjacent section of lithosphere, forming a deep trench on the ocean floor and a parallel chain of volcanoes fed by magma rising from below. In this project, we studied an unusual suite of magmatic rocks that were dredged from the northwestern wall of the Mariana Trench, in order to determine their origin. The Mariana Trench is the Earth's deepest trench, reaching 10,916 meters below sea-level, and it is bounded on the north and west by an escarpment exposing rocks formed by subduction zone magmatism, and on the east and south by the subducting Pacific lithospheric plate. The rocks we studied were collected at a depth of 7,000 meters in 1982 by a research vessel operated by the University of Hawaii. The unusual characteristic of these rocks was that they resembled rocks from the Earth's continents, in conflict with their oceanic setting. The results of our study show that the rocks formed from the same type of magma as a volcanic rock called boninite, which typically erupts when subduction is initiated along the boundary between two lithospheric plates. About 48 million years ago, when subduction of the Pacific plate just started, boninite magma formed, and the volcanic rocks that solidified from the boninite magma are now found on the island of Guam and adjacent seafloor. Using the mineralogy and chemical composition of the dredged rocks, we established that they formed from boninite magma after an extended period of differentiation and magma processing during cooling and solidification within the lithosphere. The result of the magmatic processing is that the rocks develop features such as high silica content and abundant quartz and feldspar minerals (see figure), similar to the continental crust. The significance of the work and its intellectual merit is threefold: 1) it shows that boninite magma can differentiate to form rocks with a composition similar to continental crust; 2) it shows that rocks having continental characteristics can form even at the earliest stages of subduction within subduction zone lithosphere; and 2) it shows that rare occurrences of rocks similar to those studied, which are found in deformed sections of oceanic lithosphere on the continents called ophiolites, are linked to subduction initiation. The petrologic and geochemical research carried out in this project formed the doctoral dissertation of one graduate student at Florida International University, and complementary parts of the work were carried out by four undergraduate students, all of whom are members of groups under-represented in the Geosciences and STEM fields. The undergraduates were supported by a Research Experiences for Undergraduates (REU) supplement.
