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洋中脊玄武质岩浆中早期结晶产物:高Mg#橄榄石常捕获有早期熔体,从而形成熔体包裹体.这些熔体包裹体记录了岩浆早期演化的重要信息.本研究获得的东太平洋海隆(EPR)13°N附近5个站位玄武岩具有很小的K/Ti(0.07～0.12),Tb/Lu(1.72～1.84),Sm/Nd(0.310～0.332)变化范围和相似的稀土元素配分型式,说明亏损上地幔源区具有相似的矿物组成.结合前人的Sr和Nd同位素分析结果表明这些玄武岩未经受明显的洋壳混染作用.通过对校正橄榄石分异和“铁丢失”作用后获得的65个初始熔体包裹体组成分析,表明这些熔体包裹体具有较寄主玄武岩高的MgO含量,因此,可能是寄主岩浆早期不同演化阶段的产物,从而能够指示岩浆较早期阶段的演化过程.寄主玄武岩尚处于橄榄石+斜长石共结结晶阶段,但熔体包裹体具有较寄主玄武岩更高的CaO/Al2O3比值,且该比值与MgO正相关和Na2O负相关的事实表明寄主玄武岩经历了较高压下的单斜辉石结晶分异.同时,根据单斜辉石结晶和熔体包裹体液相线组成计算其平均结晶压力为(0.83±0.25)GPa,说明这些熔体包裹体平均捕获于洋底以下～24km左右的上地幔中.熔体包裹体的Ca8/Al8,Na8与Fe8之间良好的负相关性,及其明显高于寄主玄武岩的Ca8/Al8,Na8,Fe8和K/Ti变化范围,显示这些熔体包裹体形成于不同熔融程度和深度的岩浆混合作用.另外,根据寄主玄武岩和熔体包裹体Ca8/Al8,Na8,Fe8和K/Ti的变化范围和平均值,这些玄武质岩浆仍需要在洋壳中经历其他组成端元的岩浆混合作用.由此可见,洋中脊玄武岩斑晶内熔体包裹体组成的多样性并不都反映岩浆在洋壳内的混合和结晶作用,形成于上地幔压力下的橄榄石中熔体包裹体不能直接与其他矿物斑晶中熔体包裹体共同用于指示洋壳内的岩浆过程.本次高Mg#橄榄石中熔体包裹体研究证明东太平洋海隆下部地幔中的确存在不同来源深度和熔融程度的岩浆混合作用。
The early crystallization products of mid-ocean ridge basaltic magma: Early magmatic traps were trapped by high Mg # olivine, forming melt inclusions that recorded important information about the early evolution of the magma. The eastern Pacific Ocean The basalts at five stations near 13 ° N seamounts (EOR) have small K / Ti (0.07-0.12), Tb / Lu (1.72-1.84) and Sm / Nd (0.310-0.332) with similar REE The results show that the depleted mantle source has similar mineral composition, and the Sr and Nd isotope analysis results show that these basalts have not been affected by obvious oceanic contamination.By adjusting the olivine differentiation and iron loss The analysis of the composition of the 65 initial melt inclusions obtained after the effect of “” indicates that these melt inclusions have a higher content of MgO than the host basalt and therefore may be the product of the early evolutionary stages of the host magma, indicating that the magma was earlier The evolution of the host host basalts is still in olivine + plagioclase cocrystal stage, but the melt inclusions have a higher CaO / Al2O3 ratio than host basalts, and the ratio is positively correlated with MgO and negatively related to Na2O show The host basalt underwent higher pressure crystallization of monoclinided pyroxene.At the same time, the average crystallization pressure of (0.83 ± 0.25) GPa was calculated according to the composition of liquid crystal of clinopyroxene and melt inclusions, indicating that these melts The inclusions are trapped in the upper mantle below about 24km below the ocean floor, and have a good negative correlation with Ca8 / Al8, Na8 and Fe8 in the melt inclusions and are significantly higher than that of the host basalts Ca8 / Al8, Na8 and Fe8 And K / Ti variation range, indicating that these melt inclusions formed magma mixing with different degrees of melting and depth.In addition, according to the variation range of host basalts and melt inclusions Ca8 / Al8, Na8, Fe8 and K / Ti and Average, these basaltic magmas still need to undergo magmatic mixing of the other constituent end-units in the oceanic crust. Thus, the diversity of melt inclusions in basalts of mid-oceanic ridges does not all reflect the magmatic activity of the magma in the oceanic crust And the melt inclusions in the olivine formed under the pressure of the upper mantle can not be used to indicate the magmatic process in the oceanic crust directly with the melt inclusions of other mineral plaque. The study of the melt inclusions in the stone proved that the East Pacific Mixing different depths and the degree of melting of the lower portion Rise foreign source mantle magma does exist.