Crystallography of hornblende amphibole in LAP04840 R chondrite and implication for its metamorphic history

Abstract

LAP04840 is an unusual R chondrite that includes abundant hornblende amphibole. LAP04840 shows a texture of equilibrated chondrite composed of 59.3% olivine, 13.5% orthopyroxene, 13.3% hornblende, 6.2% plagioclase, 6.0% Fe-Ni sulfide, and 1.7% accessory minerals. Hornblende replaces olivine and pyroxene in both chondrules and matrices, suggesting its secondary origin. All major phases in LAP04840 are homogeneous: olivine (Fa₃₇), orthopyroxene (En₇₀Wo₁), and plagioclase (An₈Or₂). Hornblende is also nearly homogeneous, but the total sum by electron microprobe analysis is 96-98 wt%, suggesting the presence of Fe³⁺ and a hydroxyl group. Synchrotron Fe-XANES analysis gives a Fe³⁺/ΣFe ratio of ∼ 0.6 and micro-FT-IR analysis confirms the presence of a hydroxyl group. Thus, the structural formula is (Na_0.40K_0.04)(Ca_1.46Mn_0.02Fe²⁺_0.06Na_0.46)(Al_0.08Fe²⁺_0.43Fe³⁺_0.75Cr_0.08Mg_3.60)(Si_7.02Al_0.98)O₂₂(OH)₂. Single crystal X-ray diffraction of LAP04840 hornblende gives the following lattice constants: a = 9.7957(9) Å, b = 18.0788(12) Å, c = 5.2949(5) Å, β = 104.747(3)°. The relatively short distances of [M(1)-O = 2.069 Å], [M(2)-O = 2.081 Å], and [M(3)-O = 2.058 Å] suggest the feasible preference of small Fe³⁺ at these sites. The mineralogy and petrology of LAP04840 are consistent with its classification as an R6 chondrite. However, the presence of hornblende and biotite is quite unique among not only R chondrites but also asteroidal meteorites in general. The presence of these hydrous minerals suggests metamorphism under high pressure and an aqueous environment probably at depth in the parent body. A thermometer using hornblende and plagioclase equilibria gives T = 670-690 °C. Further, a barometer using Al content in hornblende gives P = ∼ 0.1 GPa. Although these estimates bear some uncertainties, it is likely that the size of the R chondrite parent body was large enough to induce such metamorphism.