Three-dimensional Evolution of Melting, Heat and Melt Transfer in Ascending Mantle beneath a Fast-spreading Ridge Segment Constrained by Trace Elements in Clinopyroxene from Concordant Dunites and Host Harzburgites of the Oman Ophiolite
Three-dimensional Evolution of Melting, Heat and Melt Transfer in Ascending Mantle beneath a Fast-spreading Ridge Segment Constrained by Trace Elements in Clinopyroxene from Concordant Dunites and Host Harzburgites of the Oman Ophiolite
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受阿曼蛇绿岩一致纯辉岩和宿主方辉石中单斜辉石微量元素约束的快速扩张脊段下升地幔熔融、传热和熔体传递的三维演化
DOI：
10.1093/petrology/egw020
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发表时间：
2016
影响因子：
3.9
通讯作者：
Arai Shoji
中科院分区：
文献类型：
--
作者：
Akizawa Norikatsu;Ozawa Kazuhito;Tamura Akihiro;Michibayashi Katsuyoshi;Arai Shoji
蛇绿岩地幔橄榄岩中的纯橄榄岩带和岩脉被解释为海底地幔内的化石熔融通道。整合的纯橄榄岩带(即通过水平地幔流动从脊轴向外移位的化石熔体通道)尤为重要，因为它们可能代表了大洋中脊玄武岩(MORB)的母质熔体被输送到古脊轴以下较浅深度的熔体通道。我们在阿曼蛇绿岩北部到中部的一个推断的古脊段的地幔剖面中，进行了广泛深度的野外观测和采样，采集了整合的纯橄榄岩带(CDB)及其寄主方辉橄榄岩。CDB向上增厚，频数减少。在相同的地层水平上，它们在区段中心比在区段末端附近更厚和更频繁。CDB主要由橄榄石组成，含有少量尖晶石和非常少量的辉石。与橄榄石相比，单斜辉石具有较小的颗粒尺寸和间隙位置。用主元素电子探针和激光烧蚀电感耦合等离子体质谱分析了CDB及其寄主方辉橄榄石中的组成矿物。橄榄石(>；90·5)具有较高的Fo[10 0 × mg/(mg + Fe)]，铬尖晶石(>；0·50)具有较高的Cr#[Cr/(Cr + Al)]，单斜辉石具有较低的Al_2O_3(<；3·5 wt%)。寄主方辉橄榄岩中单斜辉石的球粒陨石标准化微量元素配分模式一致地表现为从重稀土(HREE)到中稀土(MREE)的平缓下降和从MREE到轻稀土(LREE)的急剧下降(= 高度亏损)，但CDB中的单斜辉石表现出较弱的轻稀土亏损，这更多地取决于古山脊段的地层水平和位置。相反，CDB中单斜辉石中的HREE浓度高于或接近于宿主方辉橄榄石的浓度。用一维稳态开放体系减压熔融反应模型评价了CDB及其寄主方辉橄榄岩中单斜辉石的微量元素组成。模拟结果表明，在高压下产生的富LREE熔体通过熔体通道向上输送到浅地幔(直至莫霍面过渡带)，在那里它与从部分熔融的橄榄岩中积累的高度亏损的熔体混合，产生正常的(N)-MORB类熔体。地幔在岩段中心的石榴石稳定场开始上升(= 熔融)，但在岩段端部附近的石榴石稳定场或尖晶石稳定场。这表明地温梯度沿古脊段的变化：在段中心较高，而在段末端较低。这一推断得到了以下结论的支持：在岩段中心存在较厚(达250 cm)的CDB，且CDB在岩段中心比在岩段末端附近更频繁地出现，并得到了层析类N-MORB熔体仅在靠近岩段末端的最上层渗入寄主橄榄岩的地球化学证据支持。
Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations and sampling of concordant dunite bands (CDB) and their host harzburgite at selected outcrops covering a wide depth range in the mantle section along an inferred paleo-ridge segment in the northern to central part of the Oman ophiolite. The CDB increase in thickness and decrease in frequency upward. They are thicker and more frequent in the centre of the segment than near the segment ends when compared at the same stratigraphic level. The CDB consist mostly of olivine with minor spinel and very rare amounts of pyroxene. Clinopyroxene has a small grain size and an interstitial position relative to olivine. The constituent minerals in the CDB and their host harzburgite were analyzed by electron microprobe for major elements and by laser ablation inductively coupled plasma mass spectrometry for trace elements. Most of the CDB have refractory major element mineral compositions, such as high Fo [100 × Mg/(Mg + Fe)] in olivine (>90·5), high Cr# [Cr/(Cr + Al)] in chromian spinel (>0·50), and low Al2O3(<3·5 wt %) in clinopyroxene. Chondrite-normalized trace element patterns of clinopyroxene in the host harzburgites consistently show a gentle decrease from heavy REE (HREE) to middle REE (MREE) and a sharp decrease from MREE to light REE (LREE) (= highly depleted), but those in the CDB show weaker LREE depletion, which is more variable depending on the stratigraphic level and position along the paleo-ridge segment. In contrast, the HREE concentrations in clinopyroxene in the CDB are higher than or similar to those of the host harzburgites. Trace element compositions of clinopyroxene in the CDB and their host harzburgites are evaluated with a one-dimensional, steady-state, open-system decompressional melting–reaction model. The modeling results suggest that an LREE-enriched melt generated at high pressure was transported upwards through melt channels to the shallow mantle (up to the Moho transition zone), where it mingled with highly depleted melts accumulated from fractionally melted peridotites to generate normal (N)-MORB-like melts. The mantle started upwelling (= melting) in the garnet stability field in the segment centre, but either in the garnet or in the spinel stability field near the segment ends. This suggests a variation of geothermal gradient along the paleo-ridge segment: higher in the segment centre and lower near the segment ends. This inference is supported by the presence of thicker (up to 250 cm) CDB as well as more frequent occurrence of CDB in the segment centre than near the segment end and by the geochemical evidence for chromatographic N-MORB-like melt percolation into the host peridotite only in the uppermost horizons near the segment ends.