地壳污染在整个公元 1329-2005 年意大利埃特纳火山喷发记录中的作用

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Marie K Takach, Wendy A Bohrson, Frank J Spera, Marco Viccaro
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引用次数: 0

摘要

埃特纳火山近乎连续的火山喷发记录可追溯到约 700 年前,这为研究一座高度活跃火山的地球化学演变提供了绝佳的机会。尤其令人感兴趣的是,要阐明在过去各种活动中观察到的碱元素(K 和 Rb)和 87Sr/86Sr 选择性富集的原因。最近,这种碱富集趋势从 17 世纪开始显现,1971 年后加速,并伴随着喷发量、频率和爆炸性的增加。对这一特征的解释包括富碱岩浆的补给和/或来自火山下基底的地壳污染。本研究根据全岩主要氧化物、微量元素、87Sr/86Sr比率和矿物成分数据,通过数百个开放系统相平衡和微量元素计算,定量研究了地壳污染在1971年后埃特尼岩浆成分中的作用。通过对埃特纳约 4-ka 坠落分层矿床中记录的高全岩氧化镁(12-17 wt.%)、镍(135-285 ppm)和铬(920-1330 ppm)母岩浆成分进行分馏结晶,令人满意地再现了现有的 1971 年前石化数据。通过对代表埃特纳地下最上层 10-15 千米沉积岩的岩性--矽卡岩和萤石混合物--的模型同化,再现了所观测到的 1971 年后的全岩和玻璃趋势以及相平衡。值得注意的是,模型显示 K2O(重量百分比)和 Rb(ppm)在矽卡岩/萤石的部分熔化过程中表现不一致。此外,在1971年后的样本中观察到的87Sr/86Sr的升高与壁岩部分熔融产生的放射性Sr的增加是一致的。在最佳拟合模型中,可观测到1971年后K2O、Rb和87Sr/86Sr的变化趋势,其中约17%的安山岩熔体被同化,可能还有≤2%的次要止水壁岩成分(百分比相对于原始岩浆的起始质量)。之前的研究表明,偏硅尖晶石和石榴石辉石可按不同比例熔化,以重现埃特纳地球化学产物中观察到的长期和短期变化。我们认为,1971 年之后观察到的碱富集特征完全可以通过地幔异质性和地壳污染的结合来解释。特别是,高达约 20% 的地壳输入加上原始熔体的地幔异质性可以很好地解释地球化学信号。地壳污染对1971年后熔岩的影响在一定程度上是由于岩浆的频繁补给使中上层地壳热启动并加强了部分熔融。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
The Role of Crustal Contamination Throughout the 1329–2005 CE Eruptive Record of Mt. Etna Volcano, Italy
The nearly continuous volcanic eruption record at Mt. Etna dating back ~700 years provides an excellent opportunity to investigate the geochemical evolution of a highly active volcano. Of particular interest is elucidating the cause of selective enrichment in alkali elements (K and Rb) and 87Sr/86Sr observed in various episodes of past activity. More recently, this alkali enrichment trend started to manifest in the 17th century and accelerated after 1971, and was accompanied by an increase in the volume, frequency, and explosivity of eruptions. Explanations for this signature include recharge of alkali-enriched magmas and/or crustal contamination from the subvolcanic basement. This study quantitatively examines the role of crustal contamination in post-1971 Etnean magma compositions via hundreds of open-system phase equilibria and trace element calculations based upon whole-rock major oxides, trace elements, 87Sr/86Sr ratios, and mineral compositional data. Available pre-1971 petrochemical data are satisfactorily reproduced by fractional crystallization of a high whole-rock MgO (12–17 wt.%), Ni (135–285 ppm), and Cr (920–1330 ppm) parental magma composition that is documented in Etna’s ~4-ka fall-stratified deposit. Observed post-1971 whole-rock and glass trends and phase equilibria are reproduced via modeled assimilation of a skarn and flysch mixture, lithologies that represent the uppermost 10–15 km of sedimentary rocks beneath Etna. Notably, models show that K2O (wt.%) and Rb (ppm) behave incompatibly during partial melting of skarn/flysch. Additionally, the observed elevation of 87Sr/86Sr in post-1971 samples is consistent with the addition of radiogenic Sr from wallrock partial melts. In best-fit models, which yield observed post-1971 K2O, Rb, and 87Sr/86Sr trends, ~17% anatectic melt is assimilated and there may be a subordinate stoped wallrock component of ≤2% (percentage is relative to the starting mass of pristine magma). Previous work has shown that metasomatized spinel lherzolite and garnet pyroxenite can be melted in different proportions to reproduce long- and short-term changes observed in Etna’s geochemical products. We propose that the alkali enrichment signature observed after 1971 can be fully explained through the combination of mantle heterogeneity and crustal contamination. In particular, up to ~20% crustal input coupled with mantle heterogeneity of primitive melts explains the geochemical signals quite well. The influence of crustal contamination on post-1971 lavas is, in part, the result of frequent recharge of magmas that thermally primed the middle to upper crust and enhanced its partial melting.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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