Peridotite-melt interaction under transitional conditions between the spinel and plagioclase facies beneath the Mid-Atlantic Ridge: Insight from peridotites at 13°N
Peridotite-melt interaction under transitional conditions between the spinel and plagioclase facies beneath the Mid-Atlantic Ridge: Insight from peridotites at 13°N
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大西洋中脊下方尖晶石相和斜长石相过渡条件下橄榄岩-熔体相互作用：来自 13°N 橄榄岩的见解
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
S. G. Simakin
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作者：
A. Pertsev;Nikolay S. Bortnikov;L. Aranovich;E. A. Vlasov;V. Beltenev;V. N. Ivanov;S. G. Simakin
摘要大西洋中脊13°N处发现的橄榄岩(含透辉石的方辉橄榄岩)分为两个成分群。橄榄岩P1为不含斜长石的岩石，矿物组成均一，辉石强烈亏损高度不相容元素。橄榄岩P2具有与基性熔体相互作用的证据：镁铁质细脉；矿物组成差异很大；矿物在高度不相容元素(Na、Zr和LREE)中富集；中等不相容元素(Ti、Y和HREE)矿物在与镁铁质集合体接触处从P1级到丰度高4-10倍；以及橄榄岩中的铬尖晶石+金红石和镁铁质细脉中的铬尖晶石+钛铁矿以及镁铁质细脉中的镍黄铁矿+金红石。橄榄岩P2中矿物的不相容元素异常富集发生在尖晶石-斜长石相界面，对应的压力约为0.8~0.9 Gpa。温度和氧逸度由尖晶石-斜方辉石-橄榄石平衡估算。橄榄岩P1具有均匀的矿物组成，记录了最后一次完全重结晶的温度为940-1050°C，FMQ缓冲氧逸度在计算误差范围内。在橄榄岩P2中，局部组合具有不同的共存矿物组成，这反映了在加热到岩浆温度(高于1200C)和随后的再平衡过程中重复的部分重结晶，温度降至910C，氧逸度显著高于FMQ缓冲液(ΔLOG$$)。
F_{O_2}
$$=1.3-1.9)。镁铁脉被认为是富含镁、镍的基性熔体与橄榄岩相互作用的结晶产物。与钙辉石平衡重构的熔体的地球化学类型定义为T-≈：(La/Sm)N≈1.6和(Ce/Yb)N MoRb 2.3，这与大西洋中脊这段现代玄武岩熔岩的成分变化很好地一致，包括有关淬火玄武岩玻璃的新数据。
AbstractPeridotites (diopside-bearing harzburgites) found at 13°N of the Mid-Atlantic Ridge fall into two compositional groups. Peridotites P1 are plagioclase-free rocks with minerals of uniform composition and Capyroxene strongly depleted in highly incompatible elements. Peridotites P2 bear evidence of interaction with basic melt: mafic veinlets; wide variations in mineral composition; enrichment of minerals in highly incompatible elements (Na, Zr, and LREE); enrichment of minerals in moderately incompatible elements (Ti, Y, and HREE) from P1 level to abundances 4–10 times higher toward the contacts with mafic aggregates; and exotic mineral assemblages Cr-spinel + rutile and Cr-spinel + ilmenite in peridotite and pentlandite + rutile in mafic veinlets. Anomalous incompatible-element enrichment of minerals from peridotites P2 occurred at the spinel-plagioclase facies boundary, which corresponds to a pressure of about 0.8–0.9 GPa. The temperature and oxygen fugacity were estimated from spinel-orthopyroxene-olivine equilibria. Peridotites P1 with an uniform mineral composition record the temperature of the last complete recrystallization at 940–1050°C and FMQ buffer oxygen fugacity within the calculation error. In peridotites P2, local assemblages have different compositions of coexisting minerals, which reflects repeated partial recrystallization during heating to magmatic temperatures (above 1200°C) and subsequent reequilibration at temperatures decreasing to 910°C and an oxygen fugacity significantly higher than FMQ buffer (Δlog $$
f_{O_2 }
$$ = 1.3–1.9). Mafic veins are considered to be a crystallization product from basic melt enriched in Mg and Ni via interaction with peridotite. The geochemical type of the melt reconstructed by the equilibrium with Ca-pyroxene is defined as T-MORB: (La/Sm)N ≈ 1.6 and (Ce/Yb)N ≈ 2.3, which is well consistent with compositional variations of modern basaltic lavas in this segment of Mid-Atlantic Ridge, including new data on quenched basaltic glasses.