HR: 0830h
AN: V21D-0548    [PDF]
TI: Hydrothermal Alteration of the Lower Oceanic Crust: Sr Isotopic Constraints from the CCSP CY-4 Drill Hole, Troodos Ophiolite, Cyprus
AU: * King, L E
EM: lek100@soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, Southampton, SO14-3ZH United Kingdom
AU: Teagle, D A
EM: dat@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, Southampton, SO14-3ZH United Kingdom
AU: Cooper, M J
EM: mjco@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, Southampton, SO14-3ZH United Kingdom
AU: Alt, J C
EM: jalt@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AB: Our understanding of the structure and geometry of mid-ocean ridge hydrothermal systems is severely hindered by a dearth of continuous samples into the plutonic complexes of the ocean crust as it is this region that provides the power to drive hydrothermal circulation and preserves a record of magma chamber processes. Although the Troodos ophiolite most probably formed in a supra-subduction zone environment, its well-preserved outcrops have made an invaluable contribution to our understanding of ocean ridge processes. CY-4 drilled as part of the Cyprus Crustal Study Project provides a unique continuous sample of 2263 m of the lower Troodos crust. The hole was initiated in sheeted dikes before grading into isotropic gabbros around 675 m. An intrusive transition is evident at 1330 m between the isotropic gabbros and lower layered gabbros, pyroxenites and crustal peridotites. Petrographic observations and strontium isotopic analyses presented here investigate the incursion of seawater derived hydrothermal fluids into the lower oceanic crust. Previous studies of the Troodos ophiolite have determined the primary igneous composition of the magmas ($^{87}Sr$/$^{86}Sr$ =0.7032-0.7040) and the range of hydrothermal fluids ($^{87}Sr$/$^{86}Sr$ =0.7047-0.7059). Samples from the sheeted dike complex are variably altered with strontium isotopic compositions elevated above the primary igneous signatures of the Troodos magmas but generally lower than the restricted hydrothermal range observed for dikes throughout the rest of the ophiolite. Samples proximal to the sheeted dike - gabbro boundary do yield strontium isotope ratios within the hydrothermal fluid range. Within a few hundred metres below the dike-gabbro transition most samples yield near primary strontium isotopic ratios suggesting only limited pervasive penetration of seawater-derived Sr. Most samples down to the bottom of the hole show little increase in Sr ratios. Rare hydrothermally altered samples with associated veins and secondary halos have $^{87}Sr$/$^{86}Sr$ elevated towards hydrothermal values indicating significant channelling of hydrothermal fluids at deeper levels in the Troodos crust.
DE: 0330 Geochemical cycles
DE: 1040 Isotopic composition/chemistry
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 8135 Hydrothermal systems (8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting