Lithological diversity among howardites and its implications for the HED parent body
Final Report Abstract
I studied the petrological attributes of 16 howardites focusing on the composition of HED silicates (pyroxene, plagioclase, and olivine). The ranges of endmember compositions typically detected for the main components, pyroxenes and plagioclases, are relatively narrow and match those reported in the literature (enstatite approx. 60‐80 %, anorthite approx. 90 ±5 %). However, full spectra are much broader than previously recognized. Pyroxenes may be as magnesian as En88 and as ferroan as Fs84. Anorthite contents in plagioclases vary from 46 to 98.5 %. Low Ca‐pyroxenes in basaltic and cumulate eucrites have been known to be moderately MgO‐rich (En30‐65). Feldspars have been found to be relatively CaO‐rich (An75‐96). This study now reveals that compositions of low Ca‐pyroxenes and feldspars in gabbroic eucrites are particularly variable, covering ranges of endmember concentrations broader than those of basaltic and cumulate eucrites combined. Gabbroic eucrites can be further distinguished into a relatively primitive, unequilibrated subspecies and evolved, i.e. differentiated samples. The former contain chemically heterogeneous pyroxenes and plagioclases including magnesian and calcic types (up to En69 and An96, respectively). Evolved gabbroic eucrites, on the other hand, are characterized by FeO‐rich, often metastable pyroxene phases (up to Fs84 versus a maximum of Fs61 in high‐Ca pyroxenes of cumulate eucrites). Pyroxene debris of En70+ probably originates from diogenites. Feldspars in differentiated gabbroic eucrites tend to be less calcic (An66‐88). Regarding olivine, forsterite concentrations range from 6 to 92 %. Two major sources can be considered: diogenites and gabbroic eucrites. Evolved gabbroic eucrites sometimes contain fine‐or very fine‐grained three‐phase symplectites with highly ferroan olivines (up to Fa94). Other gabbroic eucrites have been locally intruded by olivine veins and/or interstitial fillings (up to Fa86). As opposed to those occurrences, diogenitic olivine is more magnesian (Fo~43‐92 vs. Fo6‐37 in eucritic olivine) and exhibits an average Fe/Mn that is higher than that of eucritic olivine (53 ±10 vs. 45 ±3). Based on data from BSE images (at minimum magnification), the average fractions of pyroxenes and plagioclases in howardites are approximately 60 and 30 %, respectively. Olivines and silica contribute ~4 % each, and opaque phases sum up to ~2 %. Using average mineral data and assuming extensive gardening and mixing of upper rock layers on the HED parent body (as suggested by DAWN data), a bulk composition for Vesta’s upper crust can be derived. Approximate weight fractions calculated are 50 % SiO2, 18 % FeO, 13 % MgO, 9 % Al2O3, and 7 % CaO. This bulk composition can be translated into a mixing ratio of diogenite and eucrite components of approximately 7:3. I anticipate that the quality of these approximations may soon be tested by DAWN data.
Publications
- (2011) Rare Earth Elements in Bullk Chondrites and Chondrite Components. Lunar and Planetary Science Conference 42, 1711 (CD‐ ROM)
V. Bendel, A. Patzer, A. Pack, D. C. Hezel, and C. Münker
- (2011) Symplectic assemblages in howardites: First results. Workshop on the Formation of the first solids in the Solar System, Kauai, Hawaii, November 7‐9 2011
A. Patzer and H. Y. McSween
- (2011) The Leoville CV3 Chondrite Revisited: Prime Material for the Study of Refractory Trace Elements. Lunar and Planetary Science Conference 42, 1404 (CD‐ROM)
A. Patzer, D. C. Hezel, V. Bendel, and A. Pack
- (2012) Chondritic components I. Usual suspects and some oddballs in the Leoville CV3 meteorite. Meteoritics & Planetary Science 47, 142‐157
A. Patzer, D. C. Hezel, V. Bendel, and A. Pack
(See online at https://doi.org/10.1111/j.1945-5100.2011.01318.x) - (2012) Gabbroic vs. cumulate eucrites: Extending the diversity of eucritic lithologies. Lunar and Planetary Science Conference 43, 1227 (CD‐ROM)
A. Patzer und H. Y. McSween
- (2012) Ordinary (mesostasis) and not‐so‐ordinary (symplectites) late‐stage assemblages in howardites. Meteoritics & Planetary Science 47, 1475‐1490
A. Patzer and H. Y. McSween
(See online at https://doi.org/10.1111/j.1945-5100.2012.01408.x) - (2013) New data on the chemical composition of silicates in HED meteorites: Variety is the spice. Lunar and Planetary Science Conference 44, 2468 (CD‐ROM)
A. Patzer
