Temporally variable crustal contributions to primitive mantle-derived Columbia River Basalt Group magmas
Temporally variable crustal contributions to primitive mantle-derived Columbia River Basalt Group magmas
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地壳的时间变化对原始地幔来源的哥伦比亚河玄武岩群岩浆的贡献
DOI：
10.1016/j.chemgeo.2021.120197
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发表时间：
2021
期刊：
影响因子：
3.9
通讯作者：
Peters, Bradley J.
中科院分区：
文献类型：
--
作者：
Day, James M.D.;Nutt, Kimberley L.R.;Mendenhall, Brendon;Peters, Bradley J.
Steens地层是最早和最原始的地层之一（>7 wt. MgO）喷发产物的哥伦比亚河玄武岩群（CRBG）和CRBG-黄石-蛇河大火成岩省。本文报道了Steens组下部和上部新的主量、微量和高亲铁性元素丰度和87 Sr/86 Sr、143 Nd/144 Nd和187 Os/188 Os数据，以研究可能的地幔来源和岩浆分异作用及地壳混染作用于熔岩的性质。所研究的Steens组玄武岩是相对镁铁质（7-9重量% MgO），不相容的微量元素富集，并具有较弱的Nb和Ta异常相比，其他CRBG熔岩。最原始的玄武岩在结晶时的同位素组成与来自相对亏损的地幔源一致（87 Sr/86 Sr = ~0.7033; εNdi = ~ + 6.5; γOsi = ~ + 1）。Steens组的原始岩浆成分没有为俯冲板片成分提供令人信服的证据，而是表明起源于原始地幔来源，与其他中生代大陆溢流玄武岩（CFB;例如，北大西洋火成岩省（North Atlantic Igneous Province）CRBG中硫化物饱和的开始发生在比其他CFB（~ 8wt%）更低的MgO（<7wt%）下，导致Steens地层中的高Os含量。总体而言，Steens组的Os含量、εNdi值随MgO含量的降低而降低，187 Os/188 Os随MgO含量的降低而升高。这些地球化学特征是一致的，随着时间的推移，增加地壳污染的母熔体，一个功能，也共享的CRBG作为一个整体。根据可能的地幔和地壳成分的Os和Nd同位素的计算，以不同的CRBG的形成表明，地壳污染的数量从~1到2%，从Steens组岩浆活动到超过6%，在Grande Ronde和Wanapum喷发的逐步增加。这些结果表明，在CRBG岩浆活动的后期，更大的体积阶段，约16.5 Ma后，地壳污染和增强的潜在的隐蔽CO2脱气。除非幔源熔体能够产生足够的温室气体排放，否则在中新世中期气候最佳期（17 Ma）的开始和最大CO2释放之间可能存在偏移，表明CRBG喷发是当时气候变化的一个贡献因素，但不是触发因素。
The Steens Formation is one of the earliest and most primitive (>7 wt. MgO) eruptive products of the Columbia River Basalt Group (CRBG) and the CRBG-Yellowstone-Snake River large igneous province. New major-, trace-, and highly siderophile-element abundance and87Sr/86Sr,143Nd/144Nd and187Os/188Os data are reported for the lower and upper Steens Formation to examine likely mantle sources and the nature of magmatic differentiation and crustal contamination acting on lavas. Examined Steens Formation basalts are relatively mafic (7–9 wt% MgO), incompatible trace element enriched, and have weaker Nb and Ta anomalies compared to other CRBG lavas. The most primitive basalts have isotopic compositions at the time of crystallization consistent with originating from a mantle source that was relatively depleted (87Sr/86Sr = ~0.7033; εNdi = ~ + 6.5; γOsi = ~ + 1). Primary magma compositions for the Steens Formation do not provide compelling evidence for a subducted slab component, instead suggesting derivation from primitive mantle sources more similar to those of other Mesozoic continental flood basalts (CFB; e.g., Deccan, North Atlantic Igneous Province). Onset of sulfide saturation in the CRBG occured at lower MgO (<7 wt%) than in other CFB (~ 8 wt%) leading to the high Os contents in the Steens Formation. Collectively, the Steens Formation exhibits decreasing Os contents, εNdi values and increasing187Os/188Os with decreasing MgO. These geochemical signatures are consistent with increasing crustal contamination to parent melts with time, a feature that is also shared for the CRBG as a whole. Calculations based on Os and Nd isotopes of likely mantle and crust components to different formations of the CRBG indicates a progressive increase in the quantities of crustal contamination from ~1 to 2% from Steens Formation magmatism to more than 6% during Grande Ronde and Wanapum eruptions. These results would indicate increasing crustal contamination and enhanced potential cryptic degassing of CO2in the later, more voluminous stages of CRBG magmatism, after ~16.5 Ma. Unless mantle-derived melts can produce sufficient greenhouse gas emission, there is likely an offset between the inception of the mid-Miocene Climatic Optimum at 17 Ma and maximum CO2release, indicating that CRBG eruption was a contributing factor to climate change at that time, but was not the trigger for it.
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