Textures and geochemistry of zircons in ODP holes 735B and 1105A, Atlantis Bank, SWIR

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Author(s): John, B. E.; Cheadle, M. J.; Rioux, M. E.; Wooden, J. L.; Baines, G.
Author Affiliation(s): Primary:
University of Wyoming, Department of Geology and Geophysics, Laramie, WY, United States
Massachusetts Institute of Technology, United States
Stanford University, United States
University of Adelaide, Australia
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Zircon is a common accessory mineral in ocean crust, and an important chronometer for studying the timing and duration of crustal accretion. Here, we present a comprehensive textural/geochemical study of zircon in 25 samples from the length of ODP Hole 735B (1508 m) and adjacent Hole 1105A (158 m) at Atlantis Bank, South West Indian Ridge (SWIR). Two zircon-bearing rock suites include i) a dioritic suite comprising amphibole granodiorite, quartz diorite and diorite dikes/veins, and ii) a suite of oxide gabbro segregations/veins. Combined TIMS U/Pb dating (Rioux et al, this meeting) and SIMS REE and other trace element (TE) chemical analyses of these zircons provide constraints on the growth and thermal history of ocean crust, and melt evolution. Zircons from both drill holes vary in morphology, but are typically pristine, colorless euhedral to anhedral grains from ∼50-1000 µm in the long dimension. Over 90% show weak sector zoning, and ∼% show oscillatory zoning in CL. Additional textures include: 1. resorbed rims in two dioritic veins; 2. high U rims in two additional dioritic veins; 3. internal resorption/recrystallization boundaries in one diorite dike and one oxide gabbro; 4. a sub-population of high U grains hosting mottled/spongy interiors, possibly indicative of disequilibrium/reaction, in one diorite dike; and 5. mineral/melt inclusions in zircons in most of the dioritic veins, and in ∼% of oxide gabbros. SIMS analyses of 390 zircons (>750 spot analyses) confirm that the zircons have TE concentrations (including U/Yb vs Hf) typical of those from ocean crust. U ranges from <10 to >800 ppm in zircons from the dioritic veins (mean 123 ppm), and 5 to >500 ppm in zircons from the oxide gabbros (mean 59 ppm). All analyzed zircons have steep positive REE slopes with distinct positive Ce and negative Eu anomalies (Ce/Ce* and Eu/Eu*), similar to other oceanic zircons. Zircons from dioritic veins are REE-enriched (ΣREE = 216-15670; mean 3000 ppm) and have significant, but variable, Ce/Ce* = 52-984 (mean 380) and Eu/Eu* = 0.10-0.36 (mean 0.22). Zircons from oxide gabbros have lower REE (ΣREE = 283-10757; mean 1600 ppm), with highly variable Ce/Ce* = 23-637 (mean 242), and Eu/Eu* = 0.22-0.44 (mean 0.34). In dioritic suite zircon, the Eu anomaly becomes deeper with increasing Hf, implying strong feldspar fractionation during zircon crystallization. Mean Ti-in-zircon temperatures vary from 870-745°C (corrected for aTiO2 =0.7, aSiO2=1.0) in individual dioritic veins, and 900 -745°C (aTiO2 =0.7, aSiO2=0.7) in individual oxide gabbros. Intra-sample zircon temperatures range from ∼60-230°C, with single grains showing 0-160°C temperature difference. Variations in Hf, Ti, Y and other elements in individual samples range from tight clusters to linear trends on element-element plots. The degree of intra-sample variability in zircon chemistry reflects complicated and varied magmatic histories. Samples with little chemical variability may reflect discrete sampling of melt during fractionation, or rapid cooling and crystallization of magma. Samples with larger scatter in zircon chemistry and complex zircon textures (resorbed cores/rims or high U rims) have more complicated growth histories that could include interaction with multiple melts and/or re-melting.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Atlantis II fracture zone; Chemical composition; Crust; Geochemistry; Indian Ocean; Leg 118; Leg 179; Lithosphere; Nesosilicates; ODP Site 1105; ODP Site 735; Ocean Drilling Program; Oceanic crust; Oceanic lithosphere; Orthosilicates; Silicates; Southwest Indian Ridge; Textures; Thermal history; Zircon; Zircon group
Coordinates: S324327 S324318 E0571618 E0571557
Record ID: 2015003351
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