Zircon geochemistry at Atlantis Bank, Southwest Indian Ridge; implications for magmatic accretion at slow-spreading mid-ocean ridges

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Author(s): Doorn, Christopher James; Cheadle, Michael J.; John, Barbara E.; Coble, Matthew A.; Rioux, Matthew E.; Wooden, Joseph L.
Author Affiliation(s): Primary:
University of Wyoming, Laramie, WY, United States
Stanford University, United States
University of California Santa Barbara, United States
Oregon State University, United States
Volume Title: AGU 2018 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2018; American Geophysical Union 2018 fall meeting, Washington, DC, Dec. 10-14, 2018. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Atlantis Bank oceanic core complex (SWIR) provides extensive exposure of gabbroic lower ocean crust hosting zircon-bearing Fe-Ti oxide gabbro and felsic veins. Three ODP and IODP holes (735B, 1508 m; 1105A, 158 m; and U1473A, 809 m) have sampled this crust, spanning a spreading-parallel distance of 1.8 km. We present zircon trace element data measured by secondary ion mass spectrometry (SIMS) for zircon recovered from an array of mid-ocean ridge (MOR) borehole samples. These include 1440 SIMS trace element spot analyses of 991 grains (70 samples) collected using the Stanford-USGS SHRIMP-RG facility between 2009 and 2017. Zircons from Atlantis Bank display a broad range of trace element composition, reflecting the final stages of magmatic accretion at this slow-spread MOR. The majority of analyses range from high-Ti, low-Hf (∼65 ppm Ti, 9000 ppm Hf) to low-Ti, high-Hf (7 ppm Ti, ∼20000 ppm Hf), consistent with simple fractional crystallization. Most of the zircons (96%) have trace element compositions similar to TE data from other MOR zircons (e.g. Grimes et al., 2015). However, rare grains show low-Hf, low-Ti, and minimal Eu-anomalies (2%), and others exhibit porous textures and spot analyses with extreme Hf enrichment (2%), likely reflecting hydrothermal alteration. Ti-in-zircon thermometry suggests several samples have protracted histories of crystallization over ∼240°C. In some samples, the intra-sample variability is coupled with complex zircon textures (resorbed cores/rims and/or high U rims), suggesting complicated growth histories, including interaction with multiple melts and/or re-melting. In contrast, other samples have a restricted range in Ti-in-zircon temperatures that may reflect discrete sampling of melt during fractionation or rapid crystallization. Zircons from felsic veins sourced from Hole U1473A are characterized by enriched REE, Hf, U, Th, and Y relative to those from Hole 735B, whereas zircons from oxide gabbros show less pronounced differences. The marked enrichment in incompatible elements in U1473A zircon from felsic veins suggests the two boreholes may either sample different magmas or have undergone different degrees of differentiation.
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Atlantis II fracture zone; Expedition 360; IODP Site U1473; Indian Ocean; International Ocean Discovery Program; Leg 179; Magmatism; Mid-ocean ridges; ODP Site 1105; ODP Site 735; Ocean Drilling Program; Ocean floors; Plate tectonics; Sea-floor spreading; SloMo; Southwest Indian Ridge; Spreading centers
Coordinates: S324327 S324318 E0571618 E0571557
S324308 S324308 E0571639 E0571639
S324222 S324222 E0571641 E0571641
Record ID: 2019050417
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States