Nitrogen in peridotite xenoliths: Lithophile behavior and magmatic isotope fractionation
Nitrogen in peridotite xenoliths: Lithophile behavior and magmatic isotope fractionation
复制标题
橄榄岩捕虏体中的氮：亲石行为和岩浆同位素分馏
DOI：
10.1016/j.gca.2009.05.054
复制
发表时间：
2009
影响因子：
5
通讯作者：
P. Burnard
中科院分区：
文献类型：
--
作者：
R. Yokochi;B. Marty;G. Chazot;P. Burnard
为了记录地幔岩石和矿物中氮的起源和形态，对也门阿塔克、德国埃菲尔和法国中央高原的含水和无水橄榄岩捕虏体的 N 和 Ar 含量和同位素组成进行了研究。通过具有精细温度分辨率的逐步燃烧提取氮气和氩气，然后在铂坩埚中熔化。在单个橄榄岩捕虏体中观察到高达 25.4‰ 的大同位素不平衡，金云母中的 δ15N 值低至 -17.3‰，而单斜辉石和橄榄石则显示出正 δ15N 值。该韦尔岩样品中金云母、单斜辉石和整个岩石的相同 Sr 同位素比与共同储层的结晶一致，表明金云母的轻 N 特征可能是从宿主岩浆吸收 N 过程中同位素分馏的结果。金云母中的氮浓度总体较高（7.6–25.7ppm），而块状橄榄岩捕虏体的氮浓度则在 0.1 至 0.8ppm 之间。金云母的高氮含量至少部分归因于宿主岩浆-矿物相互作用，并且也可能表明氮以 NH4+ 的形式出现，在镁铁质岩浆中的矿物生长过程中取代了 K+。这种形态与以下事实一致：对于来自受俯冲相关交代作用和岩浆作用影响的地幔区域的一组样品，N 和 Rb 含量具有良好的相关性。这些样品的 N/Rb 比率与俯冲带岩浆的估计值相当，但比表征俯冲板片的 N/Rb 比率低一个数量级。这种差异表明在弧前区域氮相对于碱优先释放。分析的地幔捕虏体中矿物的 N/40Ar* 比率远高于 MORB 和 OIB 中囊泡的比率，这要求地幔中氮形态的出现比 Ar 更相容，夹带过程中流体相的显着损失，或者挥发性元素在地幔流体源中的停留时间较长，以大幅增加后者的 40Ar* 预算。
In order to document the origin and speciation of nitrogen in mantle-derived rocks and minerals, the N and Ar contents and isotopic compositions were investigated for hydrous and anhydrous peridotite xenoliths from Ataq, Yemen, from Eifel, Germany, and from Massif Central, France. Nitrogen and Ar were extracted by stepwise combustion with a fine temperature resolution, followed by fusion in a platinum crucible. A large isotopic disequilibrium of up to 25.4‰ is observed within single peridotite xenoliths, with δ15N values as low as −17.3‰ in phlogopite whereas clinopyroxene and olivine show positive δ15N values. Identical Sr isotopic ratios of phlogopite, clinopyroxene and whole rock in this wehrlite sample are consistent with crystallization from a common reservoir, suggesting that the light N signature of phlogopite might be the result of isotopic fractionation during N uptake from the host magma. The nitrogen concentration is systematically high in phlogopite, (7.6–25.7ppm), whereas that of bulk peridotite xenoliths is between 0.1 and 0.8ppm. The high N content of phlogopite is at least partly due to host magma–mineral interaction, and may also suggest the occurrence of N as NH4+ that substituted for K+during mineral growth in mafic magmas. Such speciation is consistent with the fact that N and Rb contents correlate well for a set of samples from mantle regions that were affected by subduction-related metasomatism and magmatism. The N/Rb ratios of these samples are comparable with values estimated for subduction zone magmas, but are one order of magnitude lower than the N/Rb ratios characterizing subducting slabs. This difference suggests preferential release of N relative to alkalis in the forearc region. N/40Ar∗ratios of minerals from analyzed mantle xenoliths are much higher than those of vesicles in MORBs and OIBs, requiring either the occurrence of nitrogen speciation in the mantle more compatible than Ar, significant loss of fluid phase during entrainment, or long residence time of volatile elements in the mantle source(s) of fluids to increase drastically the40Ar∗budget of the latter.