Low-temperature, anhydrous (Fe-Ca)-rich garnets do not grow cubic: new evidence from blueschists of the Franciscan Complex (Cazadero, USA)

Abstract

Garnet is a key mineral of several metamorphic and igneous rock types in the lithosphere. Despite being traditionally considered the archetypal cubic mineral, crystallizing in Ia $$\:\overline{3}$$ d space group, there is increasing evidence that almandine and spessartine garnets from low-temperature metapelites and metabasites are optically anisotropic, and therefore possibly not cubic. Here, we study the best-known occurrence of optically anisotropic garnet - the metabasites of the Franciscan Complex (Cazadero, USA) - integrating polychromatic polarization microscopy (PPM), electron microprobe analysis (EMPA) with and without flank method for Fe 3+ /ΣFe estimation, field emission scanning electron microscopy coupled with focus ion beam (FIB−FESEM), electron energy-loss spectroscopy (EELS), single-crystal X-ray diffraction (SCXRD) and electron backscatter diffraction (EBSD). The result of this multi-analytical approach is a comprehensive optical, chemical and crystallographic characterization of these birefringent garnets. The Cazadero garnet has optical sector zoning according to twelve pyramidal sectors forming a rhombic dodecahedron. Within sectors, a concentric oscillatory zoning is also observed. From a chemical point of view, no sector zoning is detected, whereas the concentric oscillatory zoning is maintained. Chemical maps and profiles show a typical growth distribution characterized by a bell-shaped distribution of Mn. Considering the Fe 3+ contents measured by both the EMPA flank method and EELS, a distinctive chemical zoning ranges from Alm 53 Sps 20 Grs 16 Pyr 01 Adr 10 in the core to Alm 66 Sps 04 Grs 20 Pyr 03 Adr 07 in the rim. Therefore, the analyzed garnets are almandine with a non-negligible andradite component, that reaches up to 10 mol%. Accurate X-ray diffraction refinements reveal that garnets are orthorhombic with I 2/ a 12/ d ( Fddd ) space group, with pseudo-tetragonal unit cell ( c – a from − 0.009 Å to 0.014 Å). Symmetry lowering is due to Al-Fe 3+ ordering within the octahedral sites, which is also supported by Y site distortion (difference in average < Y – O> distance from 0.0021 to 0.0041 Å). Our study provides new constraints on the origin of anhydrous almandine-rich garnets from low-grade metamorphic rocks occurring worldwide: we propose that their non-cubic structure (orthorhombic at Cazadero) is a growth feature, and is given by their Fe 3+ content, coupled with low Mg and high Ca.

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