Extended chronologies of aqueous alteration in the CM2 carbonaceous chondrites: Evidence from carbonates in Queen Alexandra Range 93005
Extended chronologies of aqueous alteration in the CM2 carbonaceous chondrites: Evidence from carbonates in Queen Alexandra Range 93005
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CM2 碳质球粒陨石中水蚀变的扩展年表：来自亚历山德拉皇后山脉 93005 碳酸盐的证据
DOI：
10.1016/j.gca.2012.06.005
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发表时间：
2012
影响因子：
5
通讯作者：
Lee M
中科院分区：
文献类型：
--
作者：
Lee M
南极CM2碳质球粒陨石QUE 93005含有四种不同成分的碳酸盐，即breununite、方解石、白云岩和贫钙白云岩。这些碳酸盐可以形成单矿物颗粒，也可以共生形成由白云石+霰石、白云石+方解石组成的双矿物颗粒，或者含有霰石、贫钙白云石和方解石共生的多矿物颗粒。所有晶粒类型的碳酸盐都含有铁-镍硫化物和/或镁-铁层状硅酸盐。在双矿物颗粒中，白云石首先结晶，被风辉石覆盖或部分被方解石取代。多矿物颗粒呈中心层状，霰石首先在孔隙边缘结晶，随后被蚀刻，然后生长过度，部分被贫钙白云石取代，贫钙白云石在被方解石覆盖之前部分溶解。方解石和白云石也有胶结裂缝，这些裂缝与水蚀球粒的细粒边缘交叉，并且是由球粒在水化过程中膨胀形成的。总体而言，QUE 93005的矿化顺序为：(1)白云岩，(2)风辉岩，(3)贫钙白云岩，然后(4)方解石。碳酸盐组成和矿物学的长期变化反映了溶液组成的变化，也可能反映了物源的变化。镁铁层状硅酸盐在方解石结晶之前取代白云岩、霰石和贫钙白云岩，大部分或全部硫化物在层状硅酸盐和方解石结晶之后形成。在硫化物结晶之后，碳酸盐颗粒的边缘被磨损，要么是由于撞击“园艺”，要么是由于气体或蒸汽快速损失时基质的流化。通过Mn-Cr体系测定白云石的结晶年龄表明，QUE 93005的水蚀变开始于太阳系最古老固体形成后的3.93±0.23Ma或之前。总体而言，QUE 93005蚀变过程中的水岩比和fo2与CM1s和CR1s相似，但QUE 93005整体蚀变程度较低，说明蚀变时间尺度较短，可能是由于流化过程中粒间液态水的损失。
The Antarctic CM2 carbonaceous chondrite QUE 93005 contains four compositionally distinct carbonates, namely breunnerite, calcite, dolomite and a Ca-poor dolomite. These carbonates can form monomineralic grains, or may be intergrown as bimineralic grains consisting of dolomite plus breunnerite and dolomite plus calcite, or polymineralic grains containing an intergrowth of breunnerite, Ca-poor dolomite and calcite. Carbonates in all grain types have inclusions of Fe–Ni sulphides and/or Mg–Fe phyllosilicates. In the bimineralic grains, dolomite crystallised first to be overgrown by breunnerite or partially replaced by calcite. Polymineralic grains are concentrically layered, with breunnerite crystallising first on pore margins to be later etched, then overgrown and partially replaced by Ca-poor dolomite that was itself partly dissolved prior to being overgrown by calcite. Calcite and dolomite have also cemented fractures that cross-cut the fine-grained rims to aqueously altered chondrules and were formed by expansion of the chondrules during their hydration. Overall, the sequence of mineralisation in QUE 93005 was: (1) dolomite, (2) breunnerite, (3) Ca-poor dolomite then (4) calcite. This secular change in carbonate composition and mineralogy reflects changing solution composition and probably also provenance. Mg–Fe phyllosilicates replaced dolomite, breunnerite and Ca-poor dolomite prior to calcite crystallisation, and most or all of the sulphides formed after both the phyllosilicates and calcite. Following sulphide crystallisation, the edges of carbonate grains were abraded, either by impact ‘gardening’ or as a consequence of fluidisation of the matrix during rapid loss of gas or vapour. Determination of the crystallisation age of dolomite via the Mn–Cr system indicates that aqueous alteration of QUE 93005 began on or before 3.93±0.23Ma after the formation of the oldest solar system solids. Overall, the water/rock ratio and fO2during alteration of QUE 93005 was similar to that of the CM1s and CR1s, but the lower degree of alteration of QUE 93005 overall suggests that alteration timescales were shorter, possibly due to loss of intergranular liquid water during fluidisation.
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