Abstract The origin(s) of the chassignites and nakhlites, closely related martian olivine and augite cumulates, respectively, are much debated. Northwest Africa (NWA) 8694 is the third chassignite to be discovered and the most ferroan, containing 85% olivine (Fo54). Its O-isotope compositions (δ18O ∼ 4.4‰, Δ17O ∼ 0.30‰) are typical of other martian meteorites. It has adcumulate texture and contains cumulus chromite, poikilitic pigeonite (En56Fs37Wo7) and mesostasis (trapped interstitial liquid). The latter contains pyroxene and plagioclase (An23 Ab70 Or8) plus rare K- feldspar (Or74), and has a trachyandesitic to trachytic bulk composition. Melt inclusions in olivine contain a variety of phases including biotite and rare amphibole. Olivine, chromite, and pigeonite compositions are intermediate between those of the other chassignites and those of the nakhlites. Augite, which appears to mantle pigeonite, has a composition overlapping that in nakhlite NWA 998 and some other nakhlites at (En41-40Wo38-39). The augite lamellae in pigeonite 1–2 μm in apparent width, and the survival of Ca zoning in olivine, suggest a near-surface cooling environment. The bulk-rock REE concentrations in the three chassignites do not correlate with Mg# but depend on the abundance of trapped liquid. The form of REE patterns calculated for olivine subtraction is very like those of nakhlite mesostases, but the observed concentrations of LREE in NWA 8694 trapped liquid have a very steep slope. This is explained by undersampling of baddeleyite and zirconolite that occur near olivine contacts with mesostasis. Though pyroxene is unzoned, its trace element variations indicate fractional crystallization. The range of olivine compositions in the three chassignites (Fo79-54) is too large to result from the crystallization sequential growth of olivine from a single magma undergoing fractional crystallization. The Ge/Si ratios show degassing of NWA 8694 which sets this chassignite apart from other chassignites and nakhlites, implying a unique batch of magma for its genesis. Many potential parent liquids are capable of generating the NWA 8694 olivine composition, though not its alkaline mesostasis. We calculated that Nakhla parent liquid NA01a (Stockstill et al., 2005) with 10% Nakhla core olivine added would produce both olivine crystals and alkaline daughter liquids with compositions matching those of NWA 8694. This meteorite is a chassignite cumulate containing nakhlitic mesostasis, a direct link between the chassignites and the nakhlites and the association of dunitic to trachytic compositions is reminiscent of terrestrial shield volcanoes. Chassignites and nakhlites were possibly formed when solidification fronts on chamber walls were disrupted, mainly as side eruptions of olivine-charged magmas from the deeper zones, and augite-charged fractionated magmas from nearer the summit of a volcano resembling Piton de la Fournaise on Earth.

中文翻译：

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西北非洲 8694，铁质 Chassignite：弥合 NAKHLITES 和 Chassignite 之间的差距

摘要 chassignites 和 nakhlites，分别是密切相关的火星橄榄石和辉石堆积物的起源，有很多争论。西北部非洲 (NWA) 8694 是第三个被发现的辉锰矿，也是含铁量最高的，含有 85% 的橄榄石 (Fo54)。它的O同位素组成（δ18O ∼ 4.4‰, Δ17O ∼ 0.30‰）是其他火星陨石的典型组成。它具有堆积质地，包含铬铁矿积云、poikilitic 变辉石 (En56Fs37Wo7) 和中间稳态（捕获的间隙液体）。后者包含辉石和斜长石 (An23 Ab70 Or8) 以及稀有钾长石 (Or74)，并具有粗斜长岩至粗斜长石的整体成分。橄榄石中的熔体包裹体含有多种相，包括黑云母和稀有角闪石。橄榄石、铬铁矿、变辉石的成分介于其他辉绿岩的成分和 nakhlites 的成分之间。Augite，似乎覆盖着变质岩，其成分与 Nakhlite NWA 998 和其他一些 Nakhlite 的成分重叠 (En41-40Wo38-39)。表观宽度为 1-2 μm 的辉石片层和橄榄石中 Ca 分带的存在，表明近地表冷却环境。三种菱镁矿中的大块岩石 REE 浓度与 Mg# 无关，但取决于被困液体的丰度。橄榄石减法计算的 REE 模式的形式非常类似于 nakhlite 中间相的形式，但在 NWA 8694 捕获的液体中观察到的 LREE 浓度具有非常陡峭的斜率。这是通过发生在橄榄石与中间稳态接触附近的斜锆石和锆石的采样不足来解释的。虽然辉石是未分区的，但其微量元素变化表明分步结晶。三个菱镁矿 (Fo79-54) 中橄榄石组成的范围太大，无法由经历分步结晶的单个岩浆中的橄榄石结晶顺序生长引起。Ge/Si 比率显示 NWA 8694 的脱气，这使这种菱镁矿与其他菱镁矿和 Nakhlites 区分开来，暗示其成因是一批独特的岩浆。许多潜在的母液能够产生 NWA 8694 橄榄石成分，但不能产生碱性介稳态。我们计算出，添加 10% Nakhla 核心橄榄石的 Nakhla 母液 NA01a（Stockstill 等，2005）将产生橄榄石晶体和碱性子液，其成分与 NWA 8694 的成分相匹配。这颗陨石是一种含有菱镁石中间稳态的菱镁石堆积体，菱镁石和菱镁石之间的直接联系，以及英石与粗晶组合物的关联，让人联想到陆地盾状火山。当室壁上的凝固前沿被破坏时，可能会形成菱镁矿和 nakhlites，主要是来自更深区域的橄榄石充电岩浆的侧喷，以及来自类似于地球上的 Piton de la Fournaise 火山顶峰的辉石充电分馏岩浆。