Abstract:
Peridotite is the major rock type of which Earth’s upper mantle
consists and exposed at the Earth surface due to volcanic and orogenic
processes. Such exposed peridotites provide access to study geodynamic
processes of the mantle. Horoman from Northern Japan is an orogenic
peridotite formed by partial melting of the depleted mantle at a mid-ocean
ridge at ~1 Ga. Samples from this orogenic peridotite contains Pargasite
(Prg), a Ca-amphibole, which have been formed as a result of fluid/meltperidotite interaction or mantle metasomatism subsequent to the formation
of peridotite. Insitu trace element, and trace element and Rb-Sr and Sm-Nd
isotopic composition of bulk-mineral fractions of constituent minerals (Prg,
Cpx and Opx) separated from two samples were measured using SIMS,
ICP-MS and ID-TIMS.
Chondrite normalized REE patterns of Prg show similar trends regardless of
its two modes of occurrence (patchy and interstitial) in a single sample.
Nevertheless, it shows two main REE patterns as (1) nearly flat heavy REE
and middle to light REE depleted pattern and (2) enriched middle REE
relative to light and heavy REE among samples. The REE patterns of Prg
are almost parallel to those of coexisting Cpx. The primitive mantle
normalized trace element patterns of all the Prg show similar trends
characterized by Sr and Li negative anomaly and Nb positive anomaly.
87Sr/86Sr and 87Rb/86Sr, and 143Nd/144Nd and 147Sm/144Nd ratios for Prg,
Cpx and Opx in each sample measured are plotted individually on isochron
diagrams. Rb-Sr data from one sample (HR±0) form a well fitted isochron.
3
rd Annual Whereas Rb-Sr data from the other sample (HR-70) are randomly
distributed and do not fit for an isochron. All Sm-Nd data from individual
samples are randomly distributed and do not fit for an isochron. The
isochron obtained for sample HR±0 gives an age of 58.8 ± 5.5 Ma with an
initial 87Sr/86Sr ratio of 0.702452 ± 0.000007 (2σ).
The well fitted mineral isochron obtained for Prg, Opx and Cpx from HR±0
indicate that minerals were in isotopic equilibrium. This further indicates
that the isotopic re-homogenization of the system occurred after formation
of the Prg. Thus, the age obtained should represent the time-integrated
isotopic variation with different parent/daughter ratios after isotopic reequilibration by the metasomatic mineral phase, Prg. In addition, the drill
core sample of the HR±0 used for separation of these minerals (2.5 cm x 7
cm core) suggest that the isotopic re-homogenization of the system occurred
at least within a sphere of 2.5 cm diameter. The sample, however, with
randomly distributed 87Sr/86Sr isotope ratios with no isochron suggest that
isotopic re-equilibration among mineral phases has not achieved in HR-70.
Previously published whole-rock Sr isotope data showing no meaningful
isochron also suggest that the system has not achieved isotopic rehomogenization in m scale. The Sm-Nd isotopic disequilibrium between
minerals suggests that this system has been disturbed only locally implying
contrasting isotopic behavior to Rb-Sr.
