High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain

Abstract

The prograde, high pressure, transition from antigorite serpentinite to enstatite-olivine rock occurs along a tectonically undisturbed pro®le at Cerro del Almirez, SE Spain. The reactant assemblage is antigorite + olivine with tremolite rimming precursor diopside. The product assemblage of tremolite + chlorite + enstatite + olivine has a spinifex-like texture with arborescent or radiating olivine elongated parallel to [001] and with radially grown enstatite. Product enstatite is very poor in Al 2 O 3 . Due to numerous oriented submicroscopic inclusions of chromian magnetite, product olivine has a brownish pleochroism and a bulk chromium content similar to precursor antigorite. Titanian clinohumite with a ¯uorine content of 0.45±0.50 wt% persisted beyond the breakdown of antigorite. The partitioning of iron and magnesium amongst the silicate phases is almost identical to that at lower pressures. Average K d values Mn/Mg and Ni/Mg are 0.17 and 0.70 for antigorite-olivine pairs and 1.83 and 0.22 for orthopyroxene-olivine pairs, respectively. These data are useful in discriminating generations of olivine grown on each other. From the ®eld data a phase diagram topology for a portion of the system CaO-MgO-SiO 2 -H 2 O is derived. This topology forms the basis for extrapolations into inaccessible P-T regions.

The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt: petrological characteristics and tectonomagmatic evolution

“…c CaO vs. Al 2 O 3 (wt.%) of orthopyroxene (opx) from the Abu Dahr serpentinized harzburgite. The fields of orthopyroxenes from the Alpine-type peridotites (Arai 1980), Avacha highly depleted harzburgite mantle xenoliths (Ishimaru et al 2007), regionally metamorphosed ultramafics (Evans and Trommsdorff 1974;Trommsdorff et al 1998) and contact metamorphosed ultramafics (Arai 1975;Frost 1975) are all given for comparison during subduction; thus they reflect their concentration in the mantle wedge (Pearce 2008).…”

Geochemistry of metasomatized peridotites above subducting slab: a case study of hydrous metaperidotites from Happo-O'ne complex, central Japan

“…Orthopyroxenes show high Mg#s-0.90 -0.91. Chromian spinels with high TiO 2 contents (3.1 wt% on an average, up to 5.7 wt%), which are enclosed mainly by orthopyroxenes, have high Cr#s of 0.90 -0.97 and low Mg#s of 0.1 -0.24 (Table 2) (Khedr and Arai, 2009 Nozaka, 2005) relative to the prograde tremolite (Trommsdorff et al, 1998).…”

“…2 and 3) produced by high -pressure breakdown of antigorite serpentinite from SE Spain (Garrido et al, 2005); owing to the prograde origin and mineral assemblages of chlorite harzburgites, they are free of Na 2 O, very low in CaO, and enriched with HFSEs, REEs, Th, and U (Trommsdorff et al, 1998). Their antigorite -serpentinite precursor is enriched with fluorine and chlorine (Garrido et al, 2005), which results in a high mobility of Ti in these harzburgites (Audétat and Keppler, 2005) to form Ti -clinohumite; this is not the case with the Happo -O'ne peridotites.…”

On the crystal structure and compressional behavior of talc: a mineral of interest in petrology and material science

“…The petrological investigation of peridotites suggested talc as an important phase in the oceanic crust (e.g. Cannat 1993;Trommsdorff et al 1998;Mével 2003). Talc is expected to play a significant role as a transport vehicle for water into the mantle via subduction zones (e.g.…”