Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge
Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge
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方济各榴辉岩块石榴石中的氧同位素分带：地幔楔中岩石缓冲流体相互作用的证据
DOI：
10.1007/s00410-013-0915-0
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发表时间：
2012
影响因子：
3.5
通讯作者：
J. Valley
中科院分区：
文献类型：
--
作者：
J. C. Errico;J. Barnes;A. Strickland;J. Valley
对来自混杂岩系高级地块的榴辉岩和角闪岩石榴石以及包裹这些地块的阳起石的氧同位素组成进行了测定，以限制它们的流体历史。来自3个地区(环山、汉密尔顿山和詹纳海滩)的5个榴辉岩块的SIMS氧同位素分析表明，在所分析的12个石榴石中，有9个石榴石的δ18O值从核到边缘突然下降了~1-3‰，距离边缘约120±9.50μm。相反，一个地块(环山)的角闪岩石榴石的δ18O值从核到边缘逐渐增加，这意味着与榴辉岩块的历史不同。榴辉岩石榴石芯中δ18O的值在5.7~11.6‰之间，保留了榴辉岩原岩的成分。石榴石边缘的δ18O值突然下降到3.2%-11.2%‰，表明在榴辉岩相变质作用(450C-600C)的生长末期，与较低的δ18O流体发生了相互作用。我们推测这种流体来自蛇纹岩化的地幔楔形体。包裹地块的阳起岩皮中高含量的镁、镍和铬也支持了与超镁铁质岩石的相互作用。利用绿泥石和多硅白云母与洋脊的阳起岩进行氧同位素测温表明，环山和汉密尔顿山的温度为185-240℃。Rind形成温度和较低的δ18O石榴石边缘表明，这些块体从石榴石生长末期开始通过低温退变与超镁铁质岩石接触。我们提出了洋壳在俯冲开始时被俯冲并嵌入上覆地幔楔体的构造模式。随着俯冲作用的继续，高品位块体与周围超镁铁质岩石之间的交代交换记录在低δ18O石榴石边缘，后来随着温度的下降而形成地壳。
The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ18O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ18O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ18O garnet rims, and later as temperatures decrease, with rind formation.
影响因子：
3.9
作者：
R. Halama;T. John;P. Herms;F. Hauff;V. Schenk
通讯作者：
R. Halama;T. John;P. Herms;F. Hauff;V. Schenk