HR: 14:55h
AN: V22F-06    [PDF]
TI: Trace Elements in Clinopyroxenes of Peridotites From Mariana Forearc Seamounts (ODP Leg 195, Site 1200A, and ODP Leg 125, Sites 779A and 784A): Evidence for Polybaric, High-Degree, Fractional Mantle Melting
AU: * Spadea, P
EM: spadea@uniud.it
AF: Dipartimento di Georisorse e Territorio, Universita' di Udine, Via Cotonificio 114, Udine, I-33100 Italy
AU: Zanetti, A
EM: zanetti@crystal.unipv.it
AF: CNR-Istituto di Geoscienze e Georisorse, Via Ferrata 1, Pavia, I-27100 Italy
AU: D'Antonio, M
EM: masdanto@unina.it
AF: Dipartimento di Scienze della Terra, Universita' Federico II, Largo S. Marcellino 10, Napoli, I-80138 Italy
AU: Vannucci, R
EM: vannucci@crystal.unipv.it
AF: Dipartimento di Scienze della Terra, Universita' di Pavia, Via Ferrata 1, Pavia, I-27100 Italy
AB: The clinopyroxenes from peridotite clasts drilled during Ocean Drilling Program Leg 195 and Leg 125 in Mariana forearc serpentinite mud volcanoes are depleted to extremely depleted in FeO, Al$_(2)$O$_(3)$, Na$_(2)$O, and enriched in MgO and CaO with respect to those from abyssal peridotites far from intra-oceanic subduction areas. Consistently, they contain very low amounts of incompatible trace elements. Although strongly LREE depleted, trace element signatures of Leg 195 clinopyroxenes show a significant variability. Cpx from Site 1200A 6R-2, 66 cm and 17G1, 106 cm harzburgites, have REE patterns (Nd$_(N)$/Yb$_(N)$0.03; Sm$_(N)$/Ho$_(N)$=0.26-0.30;Yb$_(N)$=2.0-2.7) consistent with 15% of melt removal from the Depleted MORB Mantle source via fractional melting under spinel-facies conditions. Cpx from Site 1200A 6R-1, 93-96 and 10R-1, 90 cm harzburgites have REE patterns characterised by a stronger MREE/HREE fractionation (Sm$_(N)$/Ho$_(N)$=0.08-0.14) which can be explained by a polybaric fractional melting process, started in the garnet peridotite stability field (minimum 3-4%) and proceeded ($>$10%) during the ascent through the spinel peridotite stability field. Cpx from Site 1200A 16R-2, 12 cm and 1W-1, 56 cm harzburgites occur as euhedral to anhedral small grains enclosed in coarse orthopyroxene and are have extremely low REE content (maximum at Lu$_(N)$=0.6-1.2 C1; Ho$_(N)$/Yb$_(N)$=0.16-0.18), and strong HREE fractionation. According to petrographic evidence, they formed by sub-solidus exsolution from a pristine Ca-rich orthopyroxene stable at high temperature during decreasing temperature. Clinopyroxenes from Leg 125 harzburgites are also strongly depleted in incompatible trace elements. Cpx from Site 779A 26R-2, 98 cm and 779A 26R-2, 20 cm cores show extremely fractionated patterns in the MREE-HREE region, and can be modelled by 6-8% garnet-facies peridotite fractional melting followed by $\sim$12% spinel peridotite fractional melting. As a whole, the MREE-HREE distributions are consistent with those reported by Parkinson and Pearce (1998) for clinopyroxenes from Site 779A 14R-2, 40 cm harzburgite core. Differently, clinopyroxenes from Site 784A 45R-CC, 10 cm and 784A 45R-1, 108 cm harzburgites show less fractionated patterns (Nd$_(N)$/Yb$_(N)$ up to 0.25), very low HREE content (Yb$_(N)$=0.9-1.2) and sometimes relatively large LREE content (Ce$_(N)$ up to 0.13 in 784A 45R-CC, 10 cm cpx). The overall REE distributions are not consistent with large degrees of fractional melting under spinel-facies conditions (up to 20% as deduced by HREE) and suggest some melt entrapment after the melting event. In summary, the REE composition of Leg 195 and Leg 125 clinopyroxenes indicates that: 1) the melting process for most peridotites started under garnet-facies conditions and continued during the upwelling of the asthenosphere at shallow, spinel-facies, levels; 2) the effects of melt migration or entrapping are apparent only in a couple of samples; 3) the late stages of the lithospheric evolution comprise sub-solidus re-equilibration, with clinopyroxene exsolution from primary high-Ca orthopyroxene. Reference: Parkinson, I.J.and Pearce, J.A., J. Petrol., 39, 1577-1618 (1998).
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting