Anomalous isotopes and trace element zoning in plagioclase peridotite xenoliths of Oahu (Hawaii): implications for the Hawaiian plume

摘要

Survival of plagioclase in the residual melting column during melting can have a significant impact on the melting process beneath a mid-oceanic ridge [Asimow et al., Phil. Trans. R. Soc. London Ser. A 355 (1997) 255-281]. Here we investigate the origin of plagioclase that occurs in some rare mantle xenoliths from Oahu, Hawaii. The xenoliths are harzburgitic with less than 2 modal% clinopyroxene and are characterized by strong foliation and porphyroclastic texture. olivine and orthopyroxene are common porphyroclasts; and only one xenolith (77PAII-9) contains a single large clinopyroxene porphyroclast with thick exsolved orthopyroxene lamellae. The strongly foliated groundmass shows well-developed triple-point junctions and is dominantly composed of olivine (ol_(85-90) opx_(7-14) cpx_(<1-2) plag_(3-5) spinel_(trace)). Spinel grains are small and dispersed through the groundmass and show extreme variation in Cr/Al ratio within individual thin sections, indicating that they are out of equilibrium with the other phases in these xenoliths. A highly anorthitic plagioclase (An_(92-96)) occurs only in the groundmass and its modal abundance (～3-5%) is too high relative to the abundance of clinopyroxene (commonly <1%) for it to be a residual phase, implying that plagioclase may have an exotic origin. The porphyoclasts show strong compositional zoning near the rims and appear to be relict phases (as are all spinel grains) that had once equilibrated with melts within the stability field of spinel peridotite (pressure ～1-3 GPa). Clinopyroxene neoblasts and the single porphyroclast in PAII-9 are all characterized by lithosphere-like strongly depleted light rare earth element (chondrite-normalized) patterns. The clinopyroxene porphyroclast in PAII-9 is zoned in Al, Eu, Cr, and Na. The porphyroclastic ortho- and clinopyroxenes give a homogenized (host+exsolution) temperature of 1300℃, which is inferred to be the temperature at which the porphyroclast cores equilibrated with a MORB melt in the spinel peridotite stability field. Last subsolidus equilibration of these xenoliths occurred at a temperature of ～1000℃ and a pressure of 0.7 GPa based on groundmass (and rim of porphyroclast) equilibration. Diffusion calculations at groundmass equilibration pressure and temperature conditions show that the Al zoning and Eu anomaly in the rim of the clinopyroxene porphyroclast probably developed within about 80 kyr of subsolidus equilibration with plagioclase. These xenoliths show decoupled behavior of Sr and Os isotopic systems with some of the highest Koolau-like ~(87)Sr/~(86)Sr ratios (whole rock) but lowest, MORB-depleted lithosphere-like ~(187)Os/~(188)Os ratios (whole rock). Nd isotope measurements on plagioclase (ε_(Nd) = +1) and clinopyroxene (ε_(Nd) = +11) separates from one xenolith (99PA-200) show that the two phase are out of equilibrium, and indicate that the plagioclase was not part of the same lithospheric residue (represented by clinopyroxene, olivine, and orthopyroxene) but was subsequently 'introduced' into the depleted lithosphere by a Koolau-like melt during Koolau magmatism.