Cenozoic and Precambrian Accessory Zircons in Gabbroids of the 3rd layer of Oceanic Crust in Axial Part of the Mid-Atlantic Ridge, 6oN: U-Pb SIMS SHRIMP Data

We studied ages of 150 grains of zircon from 8 gabbroid samples, dragged at 4 sites in axial part of the MAR, Sierra-Leone area, 6oN, during 10th cruise of R/V "Academic Ioffe" (2001-2002) and 22nd cruise of R/V "Professor Logachev" (2003) by regular procedure (Williams, 1998; Ludwig, 2000; Baldwin, Ireland, 1995) using SIMS technique on SHRIMP-II. Zircon grains from cataclased and altered leucogabbronorites and fresh non-cataclased troctolite were examined. Typical zircons were mounted in epoxy, ground to half of their thickness and polished before cathodeluminescence imaging (CL) to examine an inner structure by SEM CamScan MX2500. Two groups of zircon grains different in morphology and inner structure occur in the rocks. Transparent prismatic and short-prismatic zircon grains with corroded surface prevail in gabbronorites. They display thin occilatory zoning and sectorial structure, with rare oriented light deformation lamellas. A resorption of crystal faces are rarely accompanied by colloform zircon rims. Such zircon grains are interpreted to have a magmatic origin and related to gabbronorite crystallization event. Pinkish rounded grains were found in many samples among fine (<150 mkm) fraction only; sub-idiomorphic crystals with coarse zoning and thin disparate newly formed rims occur in troctolite. These zircon grains are considered as xenogenic. The results obtained demonstrate that zircon grains different in ages in rocks studied. Transparent colorless prismatic and short-prismatic zircon grains display low U and Th (<100 ppm) and extremely low radiogenic Pb. Calculated 238U/206Pb zircon age vary from 2.39±0.19 to 0.76±0.0.04 Ma. These figures are assumed to constrain a time and a last of gabbronorite crystallization. Zircon grains the ages of which range of Paleozoic to Mesoarchean are less abundant in these rocks. Calculated concordian 238U/206Pb ages of zircons from sample L-1097/1 are 991±41, 1121±29, 1355±26 and 1852± Ma; sample I-1028/1 are 1439±44 Ma old; sample L-1097/3: 2714± 50 Ma and 2880± 18 Ma; sample I-1069/19 (troctolite): 87±7 Ma, 499±15 Ma, 657±13 Ma and 3120±27 Ma old. The most of samples contain zircons of both groups, which evidently were contained in the same portions of the basaltic melt. Their origin could be related to partial capture of materials of different ages and origin from the "graveyard" of subducted slabs by the mantle plume which ascended from the CMB. Detailed study of rocks from exhumed slabs, which are represented by ultrahigh-pressure complexes showed that zircon can be preserved in metastable status (Ernst, 1999). During ascent, the plume material and incorporated slab fragments were in the solid state. They melted when the plume head reached its buoyancy level and began to spread over the oceanic lithosphere. Zircon, as the highest temperature mineral, was last to melt and dissolve in the basaltic magma. Therefore, its relicts can preserved in the melt. Presented data indicate that zircon can be used as an important (and, possibly, sole) source of information on the composition of deep mantle beneath modern oceans.