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Petrology of silicate inclusions in the Sombrerete ungrouped iron meteorite: Implications for the origins of IIE‐type silicate‐bearing irons
Author(s) -
Ruzicka Alex,
Hutson Melinda,
Floss Christine
Publication year - 2006
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2006.tb00452.x
Subject(s) - silicate , ilmenite , geology , electron microprobe , crystallization , fractional crystallization (geology) , apatite , empa , mineral redox buffer , chromite , pyroxene , meteorite , mineralogy , plagioclase , partial melting , mineral , pigeonite , liquidus , olivine , parent body , chondrite , geochemistry , basalt , chemistry , quartz , augite , materials science , phase (matter) , mantle (geology) , metallurgy , paleontology , physics , organic chemistry , astronomy
— The petrography and mineral and bulk chemistries of silicate inclusions in Sombrerete, an ungrouped iron that is one of the most phosphate‐rich meteorites known, was studied using optical, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and secondary ion mass spectrometry (SIMS) techniques. Inclusions contain variable proportions of alkalic siliceous glass (˜69 vol% of inclusions on average), aluminous orthopyroxene (˜9%, Wo 1–4 Fs 25–35 , up to ˜3 wt% Al), plagioclase (˜8%, mainly An 70–92 ), Cl‐apatite (˜7%), chromite (˜4%), yagiite (˜1%), phosphate‐rich segregations (˜1%), ilmenite, and merrillite. Ytterbium and Sm anomalies are sometimes present in various phases (positive anomalies for phosphates, negative for glass and orthopyroxene), which possibly reflect phosphate‐melt‐gas partitioning under transient, reducing conditions at high temperatures. Phosphate‐rich segregations and different alkalic glasses (K‐rich and Na‐rich) formed by two types of liquid immiscibility. Yagiite, a K‐Mg silicate previously found in the Colomera (IIE) iron, appears to have formed as a late‐stage crystallization product, possibly aided by Na‐K liquid unmixing. Trace‐element phase compositions reflect fractional crystallization of a single liquid composition that originated by low‐degree (˜4–8%) equilibrium partial melting of a chondritic precursor. Compositional differences between inclusions appear to have originated as a result of a “filter‐press differentiation” process, in which liquidus crystals of Cl‐apatite and orthopyroxene were less able than silicate melt to flow through the metallic host between inclusions. This process enabled a phosphoran basaltic andesite precursor liquid to differentiate within the metallic host, yielding a dacite composition for some inclusions. Solidification was relatively rapid, but not so fast as to prevent flow and immiscibility phenomena. Sombrerete originated near a cooling surface in the parent body during rapid, probably impact‐induced, mixing of metallic and silicate liquids. We suggest that Sombrerete formed when a planetesimal undergoing endogenic differentiation was collisionally disrupted, possibly in a breakup and reassembly event. Simultaneous endogenic heating and impact processes may have widely affected silicate‐bearing irons and other solar system matter.