Petrology and geochemistry of Patuxent Range 91501, a clast-poor impact melt from the l-chondrite parent body and Lewis Cliff 88663, an L7 chondrite

Abstract

We have performed petrologic and geochemical studies of Patuxent Range (PAT) 91501 and Lewis Cliff (LEW) 88663. PAT 91501, originally classified as an L7 chondrite, is rather a unique, near total impact melt from the L-chondrite parent body. Lewis Cliff 88663 was originally classified as an "achondrite (?)", but we find that it is a very weakly shocked L7 chondrite. PAT 91501 is an unshocked, homogeneous, igneous-textured ultramafic rock composed of euhedral to subhedral olivine, low-Ca pyroxene, augite and chrome-rich spinels with interstitial albitic plagioclase and minor silica-alumina-alkali-rich glass. Only ∼ 10% relic chondritic material is present. Olivine grains are homogeneous (Fa25.2-26.8). Low-Ca pyroxene (Wo1.9-7.2 En71.9-78.2Fs19.9-20.9) and augite (Wo29.8-39.0 En49.2-55.3 Fs11.8-14.9) display a strong linear TiO2-Al2O3 correlation resulting from igneous fractionation. Plagioclase is variable in composition; Or3.0-7.7 Ab79.8-84.1 An8.2-17.2. Chrome-rich spinels are variable in composition and zoned from Cr-rich cores to Ti-Al-rich rims. Some have evolved compositions with up to 7.9 wt% TiO2. PAT 91501 bulk silicate has an L-chondrite lithophile element composition except for depletions in Zn and Br. Siderophile and chalcophile elements are highly depleted due to sequestration in centimeter-size metal-troilite nodules. The minerals in LEW 88663 are more uniform in composition than those in PAT 91501. Olivine grains have low CaO and Cr2O3 contents similar to those in L5-6 chondrites. Pyroxenes have high TiO2 contents with only a diffuse TiO2-Al2O3 correlation. Low-Ca pyroxenes are less calcic (Wo1.6-3.1 En76.5-77.0 Fs20.4-21.4), while augites (Wo39.5-45.6 En46.8-51.1 Fs7.6-9.4) and plagioclases (Or2.6-5.7 Ab74.1-83.1 An11.2-23.3) are more calcic. Spinels are homogeneous and compositionally similar to those in L6 chondrites. LEW 88663 has an L-chondrite bulk composition for lithophile elements, and only slight depletions in siderophile and chalcophile elements that are plausibly due to weathering and/or sample heterogeneity.