Partial Melting of Lower Oceanic Crust Gabbro: Constraints From Poikilitic Clinopyroxene Primocrysts
Partial Melting of Lower Oceanic Crust Gabbro: Constraints From Poikilitic Clinopyroxene Primocrysts
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DOI：
10.3389/feart.2018.00015
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发表时间：
2018-03
影响因子：
2.9
通讯作者：
J. Leuthold;C. Lissenberg;B. O’Driscoll;O. Karakas;T. Falloon;Dina N. Klimentyeva;P. Ulmer
中科院分区：
文献类型：
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作者：
J. Leuthold;C. Lissenberg;B. O’Driscoll;O. Karakas;T. Falloon;Dina N. Klimentyeva;P. Ulmer
一批又一批的岩浆沿着沿着扩张的洋脊在板块下、上升、停滞和喷发，形成洋壳。因此，必须了解岩浆在洋中脊分化的过程和条件。虽然分离结晶被认为是岩浆分异的主要机制，但开放系统的火成岩杂岩也经历熔融-同化-同化-杂交（MASH，Hildreth和Moorbath，1988）过程。在这里，我们研究晶体尺度的记录，部分熔融的下地壳辉长岩累积从缓慢蔓延的大西洋洋脊（凯恩Megamullion;收集与杰森·皮尔森）和快速蔓延的东太平洋隆起（赫斯深; IODP探险345）。单斜辉石oikocrysts在这些辉长岩保存显着的晶内地球化学变化，指出结晶溶解发作的辉长岩共晶组合。Kane Megamullion和Hess Deep单斜辉石核心1原生晶及其斜长石包裹体表明，结晶来自高温玄武岩（分别> 1，160和> 1，200 °C），接近单斜辉石饱和温度（<50%和<25%结晶）。阶梯状的相容Cr（和共变Al）和不相容Ti、Zr、Y和稀土元素（REE）从异形核1到过生长核2逐渐减少，而Mg#和Sr/Sr* 比值逐渐增加。我们发现，部分再吸收的纹理和地球化学分区的结果，从部分熔融的稀土贫下洋壳辉长岩堆（原岩）侵入热原始幔源熔体，随后过度生长结晶（再施肥）从混合熔体。此外，向晶体外缘，Ti、Zr、Y和REE含量显著增加，Al、Cr、Mg#、Eu/Eu* 和Sr/Sr* 降低，表明结晶来自于结晶后期的相对分异熔体或原位捕获熔体。原始热反应熔体的侵入和间隙分异熔体的渗流是大洋下地壳中两种不同的MASH过程。它们可能是产生大洋中脊玄武岩中观察到的广泛成分变化的基本机制。我们还提出，这种过程在缓慢和快速扩展的洋脊上都起作用。热数值模拟表明，在缓慢扩张的海脊处，下地壳部分熔融的程度可以局部增加到50%，但整体地壳熔融量很低（小于约100%）。5%的总地幔和地壳熔体;约。20%在快速扩展的山脊）
Successive magma batches underplate, ascend, stall and erupt along spreading ridges, building the oceanic crust. It is therefore important to understand the processes and conditions under which magma differentiates at mid ocean ridges. Although fractional crystallization is considered to be the dominant mechanism for magma differentiation, open-system igneous complexes also experience Melting-Assimilation-Storage-Hybridization (MASH, Hildreth and Moorbath, 1988) processes. Here, we examine crystal-scale records of partial melting in lower crustal gabbroic cumulates from the slow-spreading Atlantic oceanic ridge (Kane Megamullion; collected with Jason ROV) and the fast-spreading East Pacific Rise (Hess Deep; IODP expedition 345). Clinopyroxene oikocrysts in these gabbros preserve marked intra-crystal geochemical variations that point to crystallization-dissolution episodes in the gabbro eutectic assemblage. Kane Megamullion and Hess Deep clinopyroxene core1 primocrysts and their plagioclase inclusions indicate crystallization from high temperature basalt (>1,160 and >1,200°C, respectively), close to clinopyroxene saturation temperature (<50% and <25% crystallization). Step-like compatible Cr (and co-varying Al) and incompatible Ti, Zr, Y and rare earth elements (REE) decrease from anhedral core1 to overgrown core2, while Mg# and Sr/Sr* ratios increase. We show that partial resorption textures and geochemical zoning result from partial melting of REE-poor lower oceanic crust gabbroic cumulate (protolith) following intrusion by hot primitive mantle-derived melt, and subsequent overgrowth crystallization (refertilization) from a hybrid melt. In addition, toward the outer rims of crystals, Ti, Zr, Y and the REE strongly increase and Al, Cr, Mg#, Eu/Eu*, and Sr/Sr* decrease, suggesting crystallization either from late-stage percolating relatively differentiated melt or from in situ trapped melt. Intrusion of primitive hot reactive melt and percolation of interstitial differentiated melt are two distinct MASH processes in the lower oceanic crust. They are potentially fundamental mechanisms for generating the wide compositional variation observed in mid-ocean ridge basalts. We furthermore propose that such processes operate at both slow- and fast-spreading ocean ridges. Thermal numerical modeling shows that the degree of lower crustal partial melting at slow-spreading ridges can locally increase up to 50%, but the overall crustal melt volume is low (less than ca. 5% of total mantle-derived and crustal melts; ca. 20% in fast-spreading ridges)