Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins
Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins
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DOI：
10.1073/pnas.0801518105
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发表时间：
2008-06-24
影响因子：
11.1
通讯作者：
Simionovici, Alexandre
中科院分区：
文献类型：
--
作者：
Miyahara, Masaaki;El Goresy, Ahmed;Simionovici, Alexandre
和平河是少数震惊的成员的L-陨石家族，包含高压多晶型的橄榄石，林伍德石和wadsleyite，在不同的纹理和设置的碎片夹带在冲击熔体静脉。在这些设置是完整的橄榄石斑状球粒。我们遇到了几个挤压和压扁橄榄石斑状球粒夹带在冲击熔融脉的陨石与新的纹理和成分。前者的化学未分区（Fa（24-26））橄榄石斑状晶体严重扁平，并显示出与核心中非常窄的成分范围（Fa（6）-Fa（10））的富镁wadsleyite同心共生。Wadsleyite核被贫Mg且化学性质明显的环带ringwoodite（Fa（28）-Fa（38））带包围。的wadsleyite-ringwoodite接口表示高达32摩尔%的fayalite的组成差距。在这两个地区的聚焦离子束切片的透射电子显微镜研究表明，wadsleyite核心和ringwoodite带由花岗变晶状共生的多边形微晶的ringwoodite和wadsleyite，与wadsleyite微晶为主的核心和ringwoodite微晶为主的带。这两种高压多晶型物的结构和组成强烈暗示了通过窄组成（Fa 24 -26）的橄榄石熔体的分离结晶形成，从富镁的华德士来石开始，然后是来自橄榄石组成（Fa（24-26））的冲击诱导熔体的贫镁的灵伍德石。我们的发现可以消除最近从其他受冲击的L-6彗星报道的高压状态的不切实际的时间尺度的可能性。
Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa(24-26)) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa(6)-Fa(10)) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa(28)-Fa(38)) belt. The wadsleyite-ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24-26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa(24-26)). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6, chondrites.