The Columbia River Basalt Group (CRBG) is the youngest continental flood basalt province, proposed to be sourced from the deep-seated plume that currently resides underneath Yellowstone National Park. If so, the earliest erupted basalts from this province, such as those in the Picture Gorge Basalt (PGB), aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. Using geochemical and isotopic data, this study explores potential mantle sources and magma evolution of the PGB. Long known geochemical signatures of the PGB include overall large ion lithophile element (LILE) enrichment and relative depletion of high field strength elements (HFSE) typical of other CRBG main-phase units. Basaltic samples of the PGB have 87Sr/86Sr ratios on the low end of the range displayed by other CRBG lavas and mantle-like δ18O values. The relatively strong enrichment of LILE and depletion of HFSE coupled with depleted isotopic signatures suggest a metasomatized upper mantle as the most likely magmatic source for the PGB. Previous geochemical modeling of the PGB utilized the composition of two high-MgO primitive dikes exposed in the northern portion of the Monument Dike swarm as parental melt. However, fractionation of these dike compositions cannot generate the compositional variability illustrated by basaltic lavas and dikes of the PGB. This study identifies a second potential parental PGB composition best represented by basaltic flows in the extended spatial distribution of the PGB. This composition also better reflects the lowest stratigraphic flows identified in the previously mapped extent of the PGB. Age data reveal that PGB lavas erupted first and throughout eruptions of main-phase CRBG units (Steens, Imnaha, Grande Ronde Basalt). Combining geochemical signals with these age data indicates cyclical patterns in the amounts of contributing mantle components. Eruption of PGB material occurred in two pulses, demonstrated by a ~0.4 Ma temporal gap in reported ages, 16.62 to 16.23 Ma. Coupling ages with observed geochemical signals, including relative elemental abundances of LILE, indicates increased influence of a more primitive, potentially plume-like source with time.

中文翻译：

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哥伦比亚河玄武岩群图片峡谷玄武岩的地幔来源和地球化学演化

哥伦比亚河玄武岩群 (CRBG) 是最年轻的大陆溢流玄武岩省，提议源自目前位于黄石国家公园下方的深层羽流。如果是这样，那么该省最早喷发的玄武岩，例如图片峡谷玄武岩 (PGB) 中的玄武岩，有助于理解和模拟羽流撞击以及玄武岩火山作用的后续演化。本研究利用地球化学和同位素数据，探索了 PGB 的潜在地幔来源和岩浆演化。长期以来已知的 PGB 地球化学特征包括总体大离子亲石元素 (LILE) 富集和其他 CRBG 主相单元典型的高场强元素 (HFSE) 相对消耗。 PGB 玄武岩样本的 87Sr/86Sr 比率位于其他 CRBG 熔岩和类地幔 δ18O 值显示范围的低端。 LILE 相对较强的富集和 HFSE 的消耗，加上同位素的消耗特征表明，交代化的上地幔是 PGB 最可能的岩浆来源。先前的 PGB 地球化学模型利用了纪念碑岩脉群北部暴露的两个高 MgO 原始岩脉的成分作为母体熔体。然而，这些岩脉成分的分馏不能产生玄武岩熔岩和 PGB 岩脉所说明的成分变化。这项研究确定了第二种潜在的亲代 PGB 成分，最好由 PGB 扩展空间分布中的玄武岩流代表。这种成分也更好地反映了先前绘制的 PGB 范围内确定的最低地层流量。年龄数据显示，PGB 熔岩首先喷发，并贯穿主相 CRBG 单元（Steens、Imnaha、Grande Ronde 玄武岩）的喷发。将地球化学信号与这些年龄数据相结合表明了地幔成分数量的周期性模式。 PGB 物质的喷发发生在两个脉冲中，由报告年龄（16.62 至 16.23 Ma）约 0.4 Ma 的时间间隙证明。将年龄与观测到的地球化学信号（包括 LILE 的相对元素丰度）结合起来，表明更原始的、潜在的羽状源的影响随着时间的推移而增加。