On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere

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doi: 10.1007/s00410-009-0409-2
Author(s): Grimes, Craig B.; John, Barbara E.; Cheadle, Michael J.; Mazdab, Frank K.; Wooden, Joseph L.; Swapp, Susan; Schwartz, Joshua J.
Author Affiliation(s): Primary:
University of Wyoming, Department of Geology and Geophysics, Laramie, WY, United States
Other:
U. S. Geological Survey, United States
University of Alabama, United States
University of Wisconsin, United States
Volume Title: Contributions to Mineralogy and Petrology
Source: Contributions to Mineralogy and Petrology, 158(6), p.757-783. Publisher: Springer International, Heidelberg - New York, International. ISSN: 0010-7999 CODEN: CMPEAP
Note: In English. Supplemental information/data is available in the online version of this article. 90 refs.; illus., incl. 2 tables, sketch map
Summary: We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ∼ 0.7, a SiO2 ∼ 1.0, pressure ∼ 2 kbar); intra-sample variation is typically ∼ 60-150°C. Decreasing Ti correlates with enrichment in Hf to ∼ 2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ∼ 680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ∼ 25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ∼ 80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance. Copyright 2009 Springer-Verlag
Year of Publication: 2009
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Absolute age; Atlantic Ocean; Atlantis II fracture zone; Crust; Crystal chemistry; Crystallization; Dates; Diorites; Expeditions 304/305; Gabbros; Geochemistry; Igneous rocks; In situ; Inclusions; Indian Ocean; Integrated Ocean Drilling Program; Ion probe data; Leg 176; Lithogeochemistry; Magmatism; Major elements; Mass spectra; Metaigneous rocks; Metals; Metamorphic rocks; Metasomatic rocks; Mid-Atlantic Ridge; Mid-ocean ridges; Nesosilicates; North Atlantic; ODP Site 735; Ocean Drilling Program; Ocean floors; Oceanic crust; Orthosilicates; P-T conditions; Petrology; Plagiogranite; Plutonic rocks; Rare earths; SHRIMP data; Serpentinite; Silicates; Southwest Indian Ridge; Spectra; Trace elements; U/Pb; Ultramafics; Zircon; Zircon group
Coordinates: S324327 S324318 E0571618 E0571557
N301000 N301200 W0420300 W0420800
Record ID: 2011018406
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by Springer Verlag, Berlin, Federal Republic of Germany