Protracted construction of fast-spread gabbroic crust: Constraints from SHRIMP Pb/U dates and chemical analysis of zircon from Hess Deep

Wednesday, 17 December 2014
Barbara E John and Rose Pettiette, University of Wyoming, Laramie, WY, United States
Core samples from ODP Hole 894G and IODP Hole 1415N provide the opportunity to study both the processes and timescales of crustal accretion of fast spread ocean crust initially formed at the East Pacific Rise. Here we report combined SHRIMP chemical analyses and U-Pb geochronology on zircon from a suite of ten samples from depths of 26–148 mbsf in ODP Hole 894G, and one sample from IODP Hole U1415N.

U-Pb zircon dating of gabbro, olivine gabbro, gabbronorite and olivine gabbronorite (1-10% oxide) collected from Hole 894G yield 230Th-corrected 206U/238Pb ages from 1.090 to 1.311 Ma with errors as low as 2.5%. In the uppermost 40 m of Hole 894G the one zircon-bearing sample yielded a date of 1.211±0.041Ma (MSWD =0.07). The next five sample dates from 50-100mbsf are older, with a weighted mean age of 1.281±0.019 Ma (MSWD=1.19). In the deepest section between 100-150 mbsf, two zircon-bearing samples have a weighted mean age of 1.234±0.050Ma (MSWD=3.5). These changes in zircon age (at ~40 and 100 mbsf) coincide with marked petrologic, textural and geochemical variations. Together with petrologic observations, these Pb/U dates define 3 macro-units, each tens of meters thick, showing no systematic pattern of age downhole. The range of zircon solidus dates implies this ~120m (true thickness) of fast spread crust was constructed over a protracted period of time (≥ 70ka), as thin pulses of magma emplaced randomly with depth, via a ‘piecemeal’ mode of crustal accretion.

The one sample of lower crust hosting zircon sampled from IODP Hole U1415N (olivine- and orthopyroxene-bearing gabbro w/ 0.5% oxide) yields a weighted mean solidus age of 1.321± 0.097Ma (MSWD 1.06).

Zircon chemistry is variable between samples downhole. Ti and Hf concentrations from spot analyses within individual samples range from tight clusters to linear trends of decreasing Ti with increasing Hf. Measured Ti concentrations in all zircon from 894G vary from 5 to 50 ppm (mean 23 ppm); calculated Ti-in-zircon temperatures vary by sample (Tmean= 783°; Tmax= 864°; Tmin =680°), with intrasample temperature ranges from 40-165°C (DTmean= 120°C). Zircons from U1415N have measured Ti concentrations of 19-50 ppm (calculated Tmean= 829°; Tmax= 966°; Tmin=777°; DT=189°C), consistent with the more primitive chemistry of this lower crustal sample.