Jilly-Rehak Christine E, Huss Gary R, Nagashima Kazu, Schrader Devin L
Department of Geology & Geophysics, University of Hawai'i at Mānoa, 1680 East-West Rd. POST 517A, Honolulu HI 96822, USA.
Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Mānoa, 1680 East-West Rd. POST 602, Honolulu HI 96822, USA.
Geochim Cosmochim Acta. 2018 Feb 1;222:230-252. doi: 10.1016/j.gca.2017.10.007. Epub 2017 Oct 19.
The presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early Solar System; however, the process of aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) chondrites are particularly well-suited for the study of aqueous alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots of the alteration process through time. We studied oxygen isotopes in secondary-minerals from six CR chondrites of varying hydration states to determine how aqueous fluid conditions (including composition and temperature) evolved on the parent body. Secondary minerals analyzed included calcite, dolomite, and magnetite. The O-isotope composition of calcites ranged from δO ≈ 9 to 35 ‰, dolomites from δO ≈ 23 to 27 ‰, and magnetites from δO ≈ -18 to 5 ‰. Calcite in less-altered samples showed more evidence of fluid evolution compared to heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted on a single trend, with the exception of grains from the extensively hydrated chondrite MIL 090292. The MIL 090292 magnetite diverges from this trend, possibly indicating an anomalous origin for the meteorite. If magnetite and calcite formed in equilibrium, then the relative O fractionation between them can be used to extract the temperature of co-precipitation. Isotopic fractionation in Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (~60°C). Assuming that the CR parent body experienced closed-system alteration, a similar exercise for parallel calcite and magnetite O-isotope arrays yields "global" alteration temperatures of ~55 to 88 °C. These secondary mineral arrays indicate that the O-isotopic composition of the altering fluid evolved upon progressive alteration, beginning near the Al Rais water composition of ΔO ~ 1 ‰ and δO ~ 10 ‰, and becoming increasingly O-enriched toward a final fluid composition of ΔO ~ -1.2 ‰ and δO ~ -15 ‰.
球粒陨石中含水矿物的存在表明，水在早期太阳系的地质演化中发挥了重要作用；然而，水岩蚀变过程仍未得到充分理解。类雷纳佐碳质（CR）球粒陨石特别适合用于研究水岩蚀变。样品范围从几乎无水到完全蚀变，基本上代表了随时间变化的蚀变过程的各个阶段。我们研究了六种不同水化状态的CR球粒陨石中次生矿物的氧同位素，以确定母体上的水流体条件（包括成分和温度）是如何演化的。分析的次生矿物包括方解石、白云石和磁铁矿。方解石的O同位素组成范围为δO≈9至35‰，白云石为δO≈23至27‰，磁铁矿为δO≈ -18至5‰。与高度蚀变的样品相比，蚀变程度较低的样品中方解石显示出更多流体演化的证据，这可能反映了较低的水/岩比。除了来自高度水化的球粒陨石MIL 090292的颗粒外，大多数磁铁矿都落在单一趋势线上。MIL 090292磁铁矿偏离了这一趋势，可能表明该陨石有异常来源。如果磁铁矿和方解石是在平衡状态下形成的，那么它们之间的相对O分馏可用于推断共沉淀温度。Al Rais碳酸盐 - 磁铁矿组合中的同位素分馏显示出较低的沉淀温度（约60°C）。假设CR母体经历了封闭系统蚀变，对平行的方解石和磁铁矿O同位素阵列进行类似分析可得出“全局”蚀变温度约为55至88°C。这些次生矿物阵列表明，蚀变流体的O同位素组成随着渐进蚀变而演化，开始时接近Al Rais水的组成，即ΔO约1‰和δO约10‰，并朝着最终流体组成ΔO约 -1.2‰和δO约 -15‰逐渐富集O。
