The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

Abstract

The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ~13% ferri-magnesiohornblende and ~0.4% phlogopite by volume. Other minerals include olivine (Fo(62)), orthopyroxene (En(69)Fs(30)Wo(1)), albite (An(8)Ab(90)Or(2)), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe3+ as determined by Mossbauer spectrometry and electron microprobe chemical analyses. Fe3+/Fe-tot values are olivine ≤ 5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (Na-A + Al-IV ⇌ (A)rectangle + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based oil the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 degrees C from a amphibole-plagioclase thermometer; P-H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P-CO2 unconstrained: f(O2) at QFM + 0.5 log units; log(f(HF)/f(H2O))≈-5.8; log(f(HCl)/f(H2O)) ≈-3.3; and log(f(HCl)/f(HF)) ≈-2.6. The hydrogen in LAP 04840 is very heavy, an average delta D value of +3660 +/- 75% in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.