Carbon isotope composition of basalts from Loihi Seamount: Primordial or recycled carbon in the Hawaiian mantle plume?
Carbon isotope composition of basalts from Loihi Seamount: Primordial or recycled carbon in the Hawaiian mantle plume?
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DOI：
10.1016/j.epsl.2023.118248
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发表时间：
2023-09
影响因子：
5.3
通讯作者：
D. Graham;P. Michael;T. B. Truong
中科院分区：
文献类型：
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作者：
D. Graham;P. Michael;T. B. Truong
我们分析了37个海底玄武岩的囊泡CO2的碳同位素组成，加上He同位素和He和CO2浓度的囊泡（蒸汽）和玻璃（熔融）阶段的顶部，南北裂谷，和东翼的洛伊希海山。拉斑玄武岩和过渡玄武岩的泡状δ 13 C值在− 4.6至− 0.9‰之间，碱性玄武岩的泡状δ 13 C值在− 7.2至− 2.1‰之间。假设在玄武岩系统中测量的蒸汽-熔融分馏因子Δ（= δ蒸汽-δ熔融）为+2 ‰至+4 ‰，则计算出的大多数玄武岩的总δ 13 C（囊泡+玻璃）范围为-6 ‰至-2 ‰。δ 13 C的相对窄的范围类似于从富含气体的洋中脊玄武岩和冰岛玄武岩中推导出的地幔源值，以及从基拉韦厄火山的火山气中推导出的地幔源值。然而，这种相似性提出了一个难题，因为Loihi玄武岩已经脱气超过97%的初始CO2，从CO2-Ba系统学和晶体分馏模型推断。Loihi母岩浆（MgO= 18重量%）初始CO2浓度为0.6 - 1.9wt.%。大多数拉斑玄武岩和过渡玄武岩似乎遵循准封闭系统脱气历史。对这种脱气作用的校正表明，Loihi未脱气母岩浆的δ 13 C中值为− 1.5‰，δ 13 C范围为− 4至+ 1‰。在封闭系统脱气情景中，δ 13 C范围的估计仅弱依赖于Δ和初始[CO2]的选择。因此，洛伊希地幔柱源的特征是δ 13 C值高于幔源玄武岩和许多金刚石中普遍存在的− 6至− 4‰。这可能是由于地球地幔中原始碳同位素的不均匀性，超低速带硅酸盐和金属液体外核之间的核幔边界处的碳交换，或者是由于存在一小部分（< 1%质量）的硫化物碳酸盐，这些碳酸盐在构造上再循环到夏威夷羽源区。目前，来自回收的碳酸盐的来源有最多的支持证据。
We analyzed the carbon isotope composition of vesicle CO 2, plus He isotopes and He and CO 2 concentrations in the vesicle (vapor) and glass (melt) phases of 37 submarine basalts from the summit, north and south rifts, and east flank of Loihi Seamount. Tholeiites and transitional basalts lie in a narrow range of vesicle δ 13 C=− 4.6 to− 0.9‰, while alkali basalts range from− 7.2 to− 2.1‰. Calculated total (vesicle+ glass) δ 13 C for the majority of the basalts ranges from− 6 to− 2‰ assuming the vapor-melt fractionation factor Δ (= δ vapor-δ melt) is+ 2 to+ 4‰ as measured in basaltic systems. This relatively narrow range of δ 13 C resembles mantle source values deduced from gas-rich mid-ocean ridge basalts and basalts from Iceland, and for Kilauea volcano deduced from its fumarole gas. However, this similarity presents a conundrum because Loihi basalts have degassed> 97% of their initial CO 2 as deduced from CO 2-Ba systematics and crystal fractionation modeling. Loihi parental magma (MgO= 18 wt.%) had initial CO 2 concentrations of 0.6 to 1.9 wt.%. Most tholeiitic and transitional basalts appear to have followed a quasi closed-system degassing history. Correcting for this degassing indicates the median δ 13 C for Loihi undegassed parental magmas is− 1.5‰ and the δ 13 C range is− 4 to+ 1‰. Estimates of this δ 13 C range are only weakly dependent on the choice of Δ and initial [CO 2] in closed-system degassing scenarios. The Loihi mantle plume source is therefore characterized by δ 13 C values that are higher than the range of− 6 to− 4‰ prevalent in mantle-derived basalts and many diamonds. This could be due to primordial carbon isotope heterogeneity in Earth's mantle, exchange of carbon at the core-mantle boundary between ultra-low velocity zone silicates and the metallic liquid outer core, or to the presence of a small fraction (< 1% by mass) of surficial carbonate that was tectonically recycled to the Hawaiian plume source region. Currently, an origin from recycled carbonate has the most supporting evidence.