Cenozoic and ancient accessory zircons in gabbroids of the 3rd layer in axial part of the Mid-Atlantic Ridge, 6oN: U-Pb SIMS SHRIMP data

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Bortnikov, Nikolai۱; Zinger, Tatyana۲; Sharkov, Evgenii۱; Lepekhina, Elena۳; Antonov, Alexander۳; Sergeev, Sergei۳
Holding Date 17 September 2008

Ages of 150 grains of zircon from 8 gabbroid samples dragged at 4 sites in axial zone of the MAR, 5j30’6"-5o32’4", during 10th cruise of R/V "Academic Ioffe" (2001-2002) and 22nd cruise of R/V "Professor Logachev" (2003) have been studied by regular procedure (Williams, 1998; Ludwig, 2000; Baldwin, Ireland, 1995) using SIMS technique on SHRIMP-II. Zircon grains from cataclased and altered leucogabbronorites with brown hornblende and non-cataclased fresh troctolite were examined. They were separated using magnetic separation and bromoform. Crystallographic morphology and inner structure of grains sized from 50 to 400 mkm 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 group of zircon grains different in color, morphology and inner structure occur in the rocks. Transparent colorless 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. Pink-semitransparent rounded grains were found among fine (<150 mkm) fraction only. Coarse-zoned cores and rims, which cross-cut this zoning were revealed. 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. Zircon grains of different age are found even within single sample.
Calculated concordian 238U/206Pb ages of zircons from sample L-1097/1 are 991±41, 1121±29, 1355±26, 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 (1), 499±15 (3), 657±13 Ma (7) and 3120±27 Ma old. An occurrence of zircon grains of different ages is evidence for an existence of the heterochronous lithospheric mantle beyond MAR. The different-aged slabs in the mantle is randomly distributed. The reasons for the presence of ancient zircon in modern gabbroids from spreading centers remain an open question. A possible explanation is that these zircons were captured from these "cemetery of slabs" by mantle plume, whcich head became melting at shallow levels in heterochronous oceanic lithosphere and involves it in magmatic processes