IODP Exp 345; primitive layered gabbros from fast-spreading lower oceanic crust

Author(s): Gillis, K. M.; Snow, J. E.; Klaus, A.
Author Affiliation(s): Primary:
University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada
Other:
University of Houston, United States
Texas A&M University, United States
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Plutonic rocks from the lower ocean crust formed at fast-spreading ridges provide a record of the history of melt transport and crystallization between the mantle and the seafloor. Despite the significance of these rocks, sampling them in situ has proven extremely challenging. This means our models for understanding the formation of the lower crust are based largely on geophysical studies and ophiolites that did not form at typical mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant recovery of primitive modally layered gabbroic rocks from the lowermost plutonic crust from a fast-spreading ridge exposed at the Hess Deep Rift. Drilling was focused at Site U1415, located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ∼110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ∼100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. All rock types are primitive (Mg#76-89) and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolite complexes. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the first robust estimate of the bulk composition of crust formed at a fast-spreading ridge. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the composition of both the lavas and plutonics. However, the recovered plutonic rocks show unanticipated early crystallization of orthopyroxene, challenging current models of melt extraction from the mantle and mid-ocean ridge basalt differentiation. The core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike-plutonic transition, representing intervals of the lower half to one third of the EPR plutonic crust. A more precise depth cannot be assigned as the results of Expedition 345 (e.g., magnetic inclinations) and site survey data (Ferrini et al., 2013) indicate that the southern slope of the intrarift ridge formed by mass wasting. However, sampling four large blocks of relatively fresh rocks proved advantageous, as it facilitated observations of the wide variety and complexity of rock types and textures present in fast spread primitive lowermost crust.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; East Pacific; Equatorial Pacific; Expedition 345; Gabbros; Hess Deep; Igneous rocks; Integrated Ocean Drilling Program; Intrusions; Layered intrusions; North Pacific; Northeast Pacific; Pacific Ocean; Plate tectonics; Plutonic rocks; Sea-floor spreading; Spreading centers
Coordinates: N021508 N021520 W1013234 W1013252
Record ID: 2015070526
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