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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Volatile (C, N, Ar) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge
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Volatile (C, N, Ar) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge

机译：MORB中的挥发性（C，N，Ar）变异性以及地幔源异质性和脱气的各自作用：西南印第安海岭的案例

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摘要

In order to better constrain the origin of volatile variability of mid ocean ridge basalt (MORB), we have performed crushing analyses on 33 fresh basaltic glasses. Most samples originate from the Southwest Indian ridge (i.e. 78-49°E, EDUL cruise, August 97), the Southeast Indian Ridge, the Central Indian Ridge and the Rodriguez Triple Junction. δ~(13)C and δ~(18)O of CO_2, δ~(15)N of N_2 together with C, N and Ar contents were determined. δ~(15)N values vary between -5.9‰ and +2.1‰ while δ~(13)C values range between -11.4 and -4.3‰. C/N_2 ratios vary by one order of magnitude (316-3900). Most N_2/Ar ratios fall within a narrow range of values (48-90) but four samples yield higher values up to 220. Overall, the data for δ~(13)C, δ~(18)O, δ~(15)N, C/N_2 and N_2/Ar are within the range of those previously reported for Pacific and Atlantic oceans. No volatile DUPL anomaly has been detected in the present study. The correlations between N_2/Ar, C/Ar and C/N_2 together with δ~(13)C and δ~(15)N show that major volatile signatures, including δ~(15)N, are more influenced by degassing-induced fractionation than by mantle heterogeneity and/or late atmospheric contamination. A two stage degassing model (a closed-system degassing followed by a Rayleigh distillation) can be used to explain the data set. This model gives initial Indian MORB values similar to those for the Atlantic and Pacific oceanic basalts δ~(13)C_0 ～ -4.5 ‰, δ~(15)N_0 ～-6.0‰, (C/N_2)_0 ～ 130, (C/Ar)_0 ～14 000 and (N_2/Ar)_0 ～ 110 and C_0 between 1100 and 5000 ppm C. This large range of possible initial carbon concentrations (due to the lack of constraints on the extent of degassing under closed-system conditions) results in a large range of mantle flux estimates (i.e. from 0.4 to 1.8 * 10~(13) mol/yr). The correlations induced by these degassing processes permit an estimation of the relative solubilities of C, N and Ar: S_C/S_(N_2) ～ 5 and S_(Ar)/S_(N_2) ～ 1.2 as well as an evaluation of the fractionation of nitrogen isotopes between the vesicles and the melt: Δ_n = -1.6‰. The present study proposes a coherent data set for volatile mantle fluxes with φ_C ～ 1.3 ± 0.1 * 10~2 φ_(N_2) ～ 1.4 ± 0.1 * 10~4 φ_(Ar) (molar).
机译：为了更好地限制中洋洋脊玄武岩（MORB）挥发性变化的起源，我们对33块新鲜的玄武玻璃进行了压碎分析。大多数样品来自西南印度洋山脊（即78-49°E，EDUL航行，97年8月），东南印度洋山脊，中印度洋山脊和罗德里格斯三重交界处。测定了CO_2的δ〜（13）C和δ〜（18）O，N_2的δ〜（15）N以及C，N和Ar含量。 δ〜（15）N值在-5.9‰至+ 2.1‰之间，而δ〜（13）C值在-11.4至-4.3‰之间。 C / N_2比变化一个数量级（316-3900）。大多数N_2 / Ar比值都在一个狭窄的值范围内（48-90），但四个样本产生的值最高可达220。总体而言，δ〜（13）C，δ〜（18）O，δ〜（15）的数据N，C / N_2和N_2 / Ar均在先前报道的太平洋和大西洋范围内。在本研究中未发现挥发性DUPL异常。 N_2 / Ar，C / Ar和C / N_2与δ〜（13）C和δ〜（15）N的相关性表明，主要的挥发性特征（包括δ〜（15）N）受脱气诱导的影响更大地幔异质性和/或后期大气污染所造成的影响。可以使用两阶段脱气模型（闭式系统脱气，然后进行瑞利蒸馏）来解释数据集。该模型给出的印度初始MORB值与大西洋和太平洋大洋玄武岩的δ〜（13）C_0〜-4.5‰，δ〜（15）N_0〜-6.0‰，（C / N_2）_0〜130，（C / Ar）_0〜14 000和（N_2 / Ar）_0〜110和C_0在1100至5000 ppm C之间。这种可能的初始碳浓度范围很大（由于在封闭系统条件下对脱气程度没有限制）会导致大范围的地幔通量估算（即0.4至1.8 * 10〜（13）mol / yr）。由这些脱气过程引起的相关性允许估算C，N和Ar的相对溶解度：S_C / S_（N_2）〜5和S_（Ar）/ S_（N_2）〜1.2以及分馏的评估。囊泡和熔体之间的氮同位素：Δ_n= -1.6‰。本研究为Φ_C〜1.3±0.1 * 10〜2φ_（N_2）〜1.4±0.1 * 10〜4φ_（Ar）（摩尔）的地幔通量提出了一个相干数据集。

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