The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.Magmas underneath active volcanoes are stored over a large range of depths in so-called trans-crustal magmatic systems (Cashman et al., 2017). Mantle-derived mafic magmas can assimilate the overlying crust during ascent and overprint original chemical signatures. Magma interaction with crustal rocks is best followed by observing changes in isotopic compositions. In Iceland, oxygen isotope ratios (δ18O) of crustal rocks deviate significantly from mantle values due to high-temperature interaction of low-δ18O fluids with the Icelandic crust (Gautason and Muehlenbachs, 1998). As a result, oxygen isotopes have been widely used to study the role of crustal rocks in Iceland's basalt petrogenesis, either in the form of crustal material subducted into the mantle (source contamination) and/or as contaminants throughout the magmatic column (crustal contamination) (Eiler et al., 2000a; Kokfelt et al., 2006; Thirlwall et al., 2006; Bindeman et al., 2008; Hartley et al., 2013).Fresh mid-oceanic ridge basalt (MORB) glasses typically have δ18O values in the range of +5.5‰ ± 0.2‰ (Eiler et al., 2000b), whereas Icelandic basaltic glasses and melt inclusions (MIs) from Iceland's rift zones have δ18O as low as +2.5‰ (Breddam, 2002; Burnard and Harrison, 2005; Peate et al., 2010; Hartley et al., 2013; Halldórsson et al., 2016). Although the origin of this shift toward 18O-depleted values is a matter of debate, there is general consensus that assimilation of low-δ18O, hydrothermally altered crust (Eiler et al., 2000a; Hartley and Thordarson, 2013) and δ18O heterogeneities in the mantle source (e.g., Thirlwall et al., 2006) control δ18O variations of Icelandic glasses. However, our understanding of where and to what extent melts are affected by crustal contamination in trans-crustal magmatic systems across Iceland is limited, primarily because it is challenging to quantify the δ18O values of components that are truly mantle-derived.Our objectives are to pinpoint the depths in the Icelandic crust at which contamination affects δ18O values of Icelandic basalts and to quantify the extent of crustal contamination as melts migrate through the Icelandic crust. We address these objectives with δ18O and trace element (TE) analyses of a well-characterized subglacial and Holocene basalt sample suite of melt inclusions (MIs) and groundmass glasses (Caracciolo et al., 2020, 2021) from the Bárðarbunga volcanic system (Figs. S1–S3 in the Supplemental Material1). Located in the Eastern Rift Zone, the Bárðarbunga volcanic system is one of the most active systems in Iceland (Larsen et al., 2015), and it is situated above the inferred location of the Iceland mantle plume (Harðardóttir et al., 2018). The Bárðarbunga volcanic system is an ideal candidate for evaluating the effects of crustal contamination, because the crust reaches > 40 km thickness (Jenkins et al., 2018) and the plumbing architecture is likely controlled by multilevel stacked reservoirs in which melts are processed over a range of depths (Hansen and Grönvold, 2000; Maclennan, 2019; Caracciolo et al., 2020, 2021). Our new results suggest that the Bárðarbunga volcanic system is a progressively assimilating, multi-level magmatic system, and that this process is likely to occur in other parts of the active rifts in Iceland.Oxygen isotope and TE analyses were performed by secondary ion mass spectrometry (SIMS) on MIs (n = 133) and groundmass glasses (n = 29) at the NordSIMS facility at the Swedish Museum of Natural History (Stockholm, Sweden). Also, oxygen isotope analyses (n = 16) were performed via laser fluorination (LF) at the University of Texas at Austin, USA, on groundmass glasses from the same localities (see the Supplemental Material for analytical methods). SiO2-corrected SIMS δ18O values of MIs vary between +3.2‰ and +6.4‰, whereas groundmass glasses have δ18O values between +2.6‰ and +5.5‰, which on average are lower than those of MIs (Fig. 1; Fig. S7). Most MIs (78%) record δ18O > +4‰, while 66% of groundmass glasses have δ18O values >+4‰. In contrast, LF data of groundmass glasses, which are generally in good agreement with SIMS data (Fig. S5), record a narrower range of δ18O values, between +3.7‰ and +4.2‰ (Fig. 1A). Oxygen isotope ratios of MIs and groundmass glasses correlate with melt MgO content (Fig. 1A). Primitive MI compositions (MgO > 8 wt%) record the largest spread in δ18O values (+3.4‰ to +6.4‰) (Fig. 1A) and in TE ratios (Fig. S6), and the variability of TE ratios becomes narrower as MgO decreases (Fig. S6). The most primitive MIs preserve the most incompatible TE-enriched (Zr/Nb < 8, La/Sm > 2.2) and depleted (Zr/Nb > 15, La/Sm < 1.3) signatures, whereas the most evolved MIs and glasses record lower δ18O and intermediate TE ratios.Evidence suggests that the Icelandic mantle is isotopically, chemically, and lithologically heterogeneous, and many studies have demonstrated that the mantle underneath Iceland contains a geochemically enriched, 18O-depleted component (Skovgaard et al., 2001; Macpherson et al., 2005; Kokfelt et al., 2006). The relationship between Sr-Nd-Pb isotope signatures and low δ18O values (down to +4.3‰) found in lavas from Reykjanes Peninsula indicates the presence of a geochemically enriched, low-δ18O mantle domain beneath the Reykjanes Peninsula (Thirlwall et al., 2006). A comparable low-δ18O component has also been documented in samples from north, south, and central Iceland (Breddam, 2002; Maclennan et al., 2003; Burnard and Harrison, 2005; Macpherson et al., 2005). The geochemical features of this enriched component likely reflect a mantle source that contains recycled subducted oceanic lithosphere (Breddam, 2002; Gurenko and Chaussidon, 2002; Macpherson et al., 2005; Thirlwall et al., 2006; Peate et al., 2010).Collectively, MIs and groundmass glasses from the Bárðarbunga volcanic system exhibit a large variation of δ18O values (Fig. 1). Notably, melts with δ18O similar to MORB are only found in some depleted to moderately enriched primitive MIs (Zr/Nb > 10), whereas primitive enriched MIs (Zr/Nb = 7–8) have lower δ18O (Fig. 1B; Fig. S7). Assuming that depleted mantle (DM) and enriched mantle (EM) components are present underneath the Bárðarbunga volcanic system as elsewhere in Iceland (Thirlwall et al., 2004, 2006; Macpherson et al., 2005) and that variations in TE ratios, such as Zr/Nb (e.g., Fitton et al., 1997), reflect source heterogeneity, we tested to see whether our data set could be reproduced by binary mixing between the DM and EM domains (Fig. 1B). Our modeling shows that by taking into account high degrees of partial melting of DM (15%) and small degrees of partial melting for EM (5%) (Stracke and Bourdon, 2009), we are only able to reproduce a small subset of the MIs, with most of the data falling to lower values than the modeled envelope (gray field in Fig. 1B).Decreasing δ18O values with decreasing MgO content in our set of MIs and groundmass glasses (Fig. 1A) are consistent with those of previous studies (Hemond et al., 1988; Nicholson et al., 1991; Hartley et al., 2013) and likely indicates that crustal assimilation processes play a fundamental role in controlling melt δ18O values by driving them toward increasingly lower value. Indeed, the relationship between TE ratios, δ18O, and MgO contents suggests that as melt evolution proceeds, melts acquire a low-δ18O character (Fig. 1) and the TE compositional variability collapses to a narrower range (Fig. S6) as a result of concurrent mixing and crystallization (Maclennan, 2008) coupled with assimilation of hydrothermally altered, 18O-depleted Icelandic crust.We tested the idea of three distinct endmember components (EM, DM, and the crust) by modeling the assimilation of low-δ18O basaltic crust into mantle-derived melts by binary mixing processes. Binary mixing was modeled between the different pairs of endmembers and for different TE ratios (Fig. 2). The modeling was carried out assuming a crust with δ18O of 0‰, in agreement with δ18O values measured in drill core samples of the altered upper Icelandic crust (Hattori and Muehlenbachs, 1982), and TE ratios Zr/Nb = 11.8 and La/Sm = 1.5 (see Table 1, and the Supplemental Material).Overall, the model shows that the distribution of magma compositions from Iceland's rift zones (Marshall et al., 2022) is consistent with the existence of DM and EM components that undergo partial melting. In particular, the δ18O values of mixtures lying along the mantle array (gray field in Fig. 1B) are then progressively shifted toward even lower δ18O values upon assimilation. Therefore, any melt composition associated with the Bárðarbunga volcanic system lies within the binary mixing lines according to a three-step process (Fig. 2):Enriched and depleted mantle domains undergo partial melting, producing enriched and depleted primary melts.Primary enriched and depleted melts mix in different proportions.The mixed melts ascend throughout the crust. Their initial mantle-like δ18O value is lowered as they progressively assimilate low-δ18O crustal material while rising toward higher levels in the crust.Enriched and depleted mantle domains undergo partial melting, producing enriched and depleted primary melts.Primary enriched and depleted melts mix in different proportions.The mixed melts ascend throughout the crust. Their initial mantle-like δ18O value is lowered as they progressively assimilate low-δ18O crustal material while rising toward higher levels in the crust.This three-step process can explain the full range of δ18O values, TE data in the Bárðarbunga volcanic system, and most melts erupted across Iceland's neovolcanic rift zones (Fig. 2; Fig. S7).Following the mixing equation outlined in Sohn (2013) (see the Supplemental Material), we quantitatively derived the extent of oxygen isotope exchange required to explain the observed δ18O values and TE contents (Fig. 2) in each of the Bárðarbunga volcanic system MIs and groundmass glasses. The extent of oxygen isotope exchange is adopted here as a proxy for the amount of crustal material assimilated. The majority of glasses and MIs in the Bárðarbunga volcanic system require between 10% and 30% oxygen isotope exchange to explain their low-δ18O value (Fig. 3A; Fig. S9A), which is in agreement with thermodynamic limits calculated for primitive basaltic magmas (Heinonen et al., 2022). Up to 55% oxygen isotope exchange is required to explain the lowest δ18O values recorded by groundmass glasses. However, the calculated extent of exchange is strongly dependent on the chosen δ18O of the assimilant, which is difficult to constrain and likely to be heterogeneous across the crust. For example, if melts were to assimilate crust with δ18O = −2‰, we can reproduce the lowest δ18O values (around +2.6‰) with 35–40% oxygen isotope exchange (Fig. S9C).Having constrained the extent of oxygen isotope exchange, we next seek to establish where this process occurs within the Bárðarbunga volcanic system. The equilibration pressure of glasses and MIs can be estimated by applying the Olivine–Plagioclase–Augite–Melt (OPAM) barometer (Yang et al., 1996; Hartley et al., 2018). OPAM equilibration pressures for the Bárðarbunga samples are 1.0–6.3 kbar (3.5–22.5 km), and ~60% of the samples are in the 2–4 kbar range (7.1–14.3 km) (Caracciolo et al., 2020). The distribution of equilibration pressures (Fig. 3A) is consistent with a multi-tiered, trans-crustal magmatic system in which melts fractionate and mix within an interconnected stacked sill network (Maclennan, 2019; Caracciolo et al., 2020, 2021). Equilibration pressures correlate with the extent of oxygen isotope exchange (Fig. 3A). The most contaminated melts, which record the lowest δ18O values, show the lowest equilibration pressures in the 1–2 kbar range (3.5–7.1 km). For example, melts stored and equilibrated at ~4 kbar (14.3 ± 4.7 km) have experienced 15% crustal contamination on average, but melts equilibrated at ~1.5 kbar (5.4 ± 4.7 km) have experienced 35% contamination on average (Fig. 3A). Although this correlation has a low R2 (0.19), it is highly statistically significant (p < 0.0001, n = 86). The low R2 value is largely due to the uncertainties associated with the OPAM barometer and the estimate of the end-member compositions. Therefore, we argue that as melts are transferred upwards throughout the Bárðarbunga volcanic system, their δ18O values lower as they become more crustally-contaminated (Fig. 3B). Our data suggest that most of the oxygen isotope exchange that affects the δ18O of melts occurs in the upper and uppermost middle crust, between 3.5 km and 10 km. Although the scarcity of data > ~4.5 kbar (~15 km) does not allow us to provide reliable estimates of the extent of oxygen isotope exchange in the deep crust, our data suggest that lower crustal material has little effect on δ18O values due to the lack of interaction of lower crustal rocks with hydrothermal and/or meteoric waters. Indeed, the lower crust lacks altered, low-δ18O rocks, and it is likely built by intrusions (Greenfield and White, 2015).This study shows that melts supplied to the Bárðarbunga volcanic system, and possibly to other volcanic systems along Iceland's rift zones, experienced different degrees of crustal assimilation that influenced their δ18O values. We demonstrate that as melts ascend through the crust and approach shallow levels at around 3.5–10 km depth, they acquire lower δ18O values because they assimilate more crustal material. We therefore envision the plumbing system beneath the Bárðarbunga volcanic system as a progressively assimilating, multi-tiered system in which magmas are processed through a large range of depths (3.5–22.5 km) within stacked-sill reservoirs. Such complex multi-tiered volcanic systems that undergo continuous assimilation are likely common in all long-lived magmatic systems in thick crust, such as arc volcanoes and continental rifts.A. Caracciolo was supported by the University of Iceland Research Fund (HI17060092) and by the Nordic Volcanological Center. S.A. Halldórsson and E.W. Marshall acknowledge support from the Icelandic Research Fund (grants 196139–051 and #195638–051). The involvement of S.A. Halldórsson was partly in relation to Horizon 2020 project EUROVOLC, which is funded by the European Commission (grant 731070). M. Kahl acknowledges funding by the German Research Foundation (grant KA 3532/2-1) The NordSIMS ion microprobe facility acknowledges support by the Swedish Research Council (grant 2017-00671), the Swedish Museum of Natural History, and the University of Iceland; this is NordSIMS publication 705. We thank K. Lindén for laboratory assistance in Stockholm, R. Sohn for help with mathematical equations, and J. Cullen for laser fluorination analyses. We also thank U. Schaltegger for editorial handing and J. Troch and an anonymous reviewer for helpful reviews.

中文翻译：

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逐步同化冰岛跨地壳岩浆管道系统的氧同位素证据

地幔衍生熔体的氧同位素组成可以对岩浆在穿过地壳时的加工方式产生重要限制。地壳物质的同化是玄武岩成因的一个重要方面，因为它影响活火山喷发岩浆的化学和流变学特征。我们报告了来自冰岛 Bárðarbunga 火山系统的一组特征明确的玄武岩熔体包裹体和基体玻璃的氧同位素 (δ18O) 和微量元素 (TE) 数据，以评估管道系统中地壳岩石与上升岩浆相互作用的方式和位置。虽然熔体包裹体和地块玻璃记录的 δ18O 值范围很大（分别为 +3.2‰ 至 +6.4‰ 和 +2.6‰ 至 +5.5‰），但地块玻璃的平均记录值较低。不相容的微量元素之间的关系（例如，Zr/Nb) 和氧同位素比率最好用三组分混合来解释，其中来自具有不同 δ18O 值的贫化和富集的地幔组分的原始熔体混合并在逐渐受到冰岛地壳蚀变的污染时获得低 δ18O 特征。大多数（60%）熔体包裹体需要与冰岛地壳进行 10-30% 的氧气交换。此外，我们首次将氧同位素交换的程度与熔体平衡深度联系起来，表明大部分污染发生在 1-2 kbar（3-7 km 深度）。我们提出逐渐同化的多层管道系统是 Bárðarbunga 火山系统的一个特征，随着熔体通过更高的地壳水平迁移，与地壳相互作用产生的化学变化系统地增加。我们表明，类似的过程也可能发生在冰岛的活动裂谷带中。活火山下的岩浆储存在所谓的跨地壳岩浆系统中的大范围深度（Cashman 等，2017）。地幔衍生的镁铁质岩浆可以在上升过程中同化上覆地壳并叠加原始化学特征。岩浆与地壳岩石的相互作用最好通过观察同位素组成的变化来跟踪。在冰岛，由于低 δ18O 流体与冰岛地壳的高温相互作用，地壳岩石的氧同位素比（δ18O）与地幔值显着偏离（Gautason 和 Muehlenbachs，1998）。因此，氧同位素已被广泛用于研究地壳岩石在冰岛玄武岩成因中的作用，或者以地壳物质的形式潜入地幔（源污染）和/或作为整个岩浆柱的污染物（地壳污染）（Eiler 等，2000a；Kokfelt 等，2006；Thirlwall 等，2006； Bindeman 等人，2008 年；Hartley 等人，2013 年）。新鲜的大洋中脊玄武岩 (MORB) 玻璃的 δ18O 值通常在 +5.5‰ ± 0.2‰ 范围内（Eiler 等人，2000b），而冰岛的冰岛裂谷带的玄武岩玻璃和熔体包裹体 (MIs) 的 δ18O 低至 +2.5‰（Breddam，2002；Burnard 和 Harrison，2005；Peate 等，2010；Hartley 等，2013；Halldórsson 等， 2016）。尽管这种向 18O 耗尽值转变的起源是一个争论的问题，但普遍的共识是低 δ18O 的同化，热液蚀变的地壳（Eiler 等，2000a；Hartley 和 Thordarson，2013）和地幔源中的 δ18O 非均质性（例如，Thirlwall 等，2006）控制了冰岛玻璃的 δ18O 变化。然而，我们对冰岛跨地壳岩浆系统中地壳污染影响熔体的位置和程度的了解是有限的，主要是因为量化真正源自地幔的成分的 δ18O 值具有挑战性。我们的目标是确定冰岛地壳中污染影响冰岛玄武岩 δ18O 值的深度，并量化熔体通过冰岛地壳迁移时地壳污染的程度。我们通过 δ18O 和微量元素 (TE) 分析来解决这些目标，该分析对特征明确的冰下和全新世玄武岩样品套件进行了熔融包裹体 (MI) 和地块玻璃（Caracciolo 等人，2020 年，2021）来自 Bárðarbunga 火山系统（补充材料 1 中的图 S1-S3）。Bárðarbunga 火山系统位于东部裂谷带，是冰岛最活跃的系统之一（Larsen 等人，2015 年），它位于冰岛地幔柱的推断位置之上（Harðardóttir 等人，2018 年） . Bárðarbunga 火山系统是评估地壳污染影响的理想候选者，因为地壳厚度超过 40 公里（Jenkins 等人，2018 年），并且管道结构可能受多层堆叠储层控制，其中熔体在深度范围（Hansen 和 Grönvold，2000；Maclennan，2019；Caracciolo 等人，2020、2021）。我们的新结果表明，Bárðarbunga 火山系统是一个逐渐同化的多层次岩浆系统，并且这个过程很可能发生在冰岛活动裂谷的其他部分。氧同位素和 TE 分析是通过二次离子质谱 (SIMS) 对 MI (n = 133) 和地面质量玻璃 (n = 29) 进行的瑞典自然历史博物馆（瑞典斯德哥尔摩）的 NordSIMS 设施。此外，氧同位素分析 (n = 16) 在美国奥斯汀的德克萨斯大学通过激光氟化 (LF) 对来自同一地点的地面玻璃进行（参见分析方法的补充材料）。MI的SiO2校正SIMS δ18O值在+3.2‰和+6.4‰之间变化，而地块玻璃的δ18O值在+2.6‰和+5.5‰之间，平均低于MI（图1；图S7） ）。大多数 MI (78%) 记录 δ18O > +4‰，而 66% 的地块玻璃的 δ18O 值 > +4‰。相比之下，地块玻璃的 LF 数据通常与 SIMS 数据非常吻合（图 S5），记录的 δ18O 值范围较窄，介于 +3.7‰ 和 +4.2‰ 之间（图 1A）。MIs 和地块玻璃的氧同位素比率与熔体 MgO 含量相关（图 1A）。原始 MI 成分（MgO > 8 wt%）记录了 δ18O 值（+3.4‰ 至 +6.4‰）（图 1A）和 TE 比率（图 S6）的最大分布，并且 TE 比率的变异性随着MgO 减少（图 S6）。最原始的 MI 保留了最不兼容的 TE 富集 (Zr/Nb < 8, La/Sm > 2.2) 和耗尽 (Zr/Nb > 15, La/Sm < 1.3) 特征，而最进化的 MI 和玻璃记录较低δ18O 和中间 TE 比率。有证据表明，冰岛地幔在同位素、化学和岩性上是非均质的，许多研究表明，冰岛下方的地幔含有地球化学富集的 18O 贫化成分（Skovgaard 等，2001；Macpherson 等，2005；Kokfelt 等，2006）。Sr-Nd-Pb 同位素特征与雷克雅内斯半岛熔岩中发现的低 δ18O 值（低至 +4.3‰）之间的关系表明，雷克雅内斯半岛下方存在地球化学富集的低 δ18O 地幔域（Thirlwall 等人， 2006）。在冰岛北部、南部和中部的样本中也记录了类似的低 δ18O 成分（Breddam，2002；Maclennan 等，2003；Burnard 和 Harrison，2005；Macpherson 等，2005）。这种富集成分的地球化学特征可能反映了包含循环俯冲海洋岩石圈的地幔源（Breddam，2002；Gurenko 和 Chaussidon，2002；麦克弗森等人，2005；瑟尔沃尔等人，2006；Peate et al., 2010)。总的来说，来自 Bárðarbunga 火山系统的 MIs 和地块玻璃表现出很大的 δ18O 值变化（图 1）。值得注意的是，与 MORB 相似的 δ18O 熔体仅在一些耗尽至中等富集的原始 MI（Zr/Nb > 10）中发现，而原始富集的 MI（Zr/Nb = 7-8）具有较低的 δ18O（图 1B；图 1）。 S7)。假设与冰岛其他地方一样，Bárðarbunga 火山系统下方存在贫化地幔 (DM) 和富集地幔 (EM) 成分（Thirlwall 等人，2004 年，2006 年；Macpherson 等人，2005 年），并且 TE 比率的变化，例如由于 Zr/Nb（例如，Fitton 等，1997）反映了源异质性，我们测试了我们的数据集是否可以通过 DM 和 EM 域之间的二元混合来重现（图 1B）。我们的模型表明，通过考虑 DM 的高度部分熔化 (15%) 和 EM 的小部分熔化 (5%) (Stracke 和 Bourdon, 2009)，我们只能再现一小部分MI，大多数数据下降到低于模型包络线的值（图 1B 中的灰色区域）。随着我们的 MI 和地块玻璃（图 1A）中 MgO 含量的降低，δ18O 值降低与之前的一致研究（Hemond 等人，1988 年；Nicholson 等人，1991 年；Hartley 等人，2013 年）并可能表明，地壳同化过程通过驱使熔融 δ18O 值越来越低，在控制熔融 δ18O 值方面发挥着重要作用。事实上，TE 比值、δ18O 和 MgO 含量之间的关系表明，随着熔体演化的进行，熔体获得了低 δ18O 特征（图 3）。1）并且由于同时混合和结晶（Maclennan，2008）加上热液改变，18O耗尽的冰岛地壳的同化，TE组成变异性崩溃到更窄的范围（图S6）。我们测试了三个不同的想法通过模拟低 δ18O 玄武质地壳通过二元混合过程同化为地幔衍生熔体的端元成分（EM、DM 和地壳）。在不同的端元对和不同的 TE 比率之间模拟了二元混合（图 2）。建模假设地壳的 δ18O 为 0‰，这与在改变的上冰岛地壳的钻芯样品中测量的 δ18O 值一致（Hattori 和 Muehlenbachs，1982 年），并且 TE 比率 Zr/Nb = 11.8 和 La/Sm = 1.5（见表 1 和补充材料）。总体而言，该模型表明，冰岛裂谷带的岩浆成分分布（Marshall 等，2022）与经历部分熔融的 DM 和 EM 成分的存在一致。特别是，沿地幔阵列的混合物的 δ18O 值（图 1B 中的灰色区域）在同化时逐渐向更低的 δ18O 值移动。因此，与 Bárðarbunga 火山系统相关的任何熔体成分都位于二元混合线内，按照三步过程（图 2）：富集和贫化的地幔域经历部分熔融，产生富集和贫化的初级熔体。初级富集和贫化熔体以不同的比例混合。混合的熔体在整个地壳中上升。它们的初始地幔 δ18O 值随着它们逐渐同化低 δ18O 地壳物质而降低，同时向地壳中的更高水平上升。富集和贫化的地幔域经历部分熔融，产生富集和贫化的初级熔体。初级富集和贫化熔体混合不同的比例。混合熔体在整个地壳中上升。随着它们逐渐同化低 δ18O 地壳物质，同时向地壳中的更高水平上升，它们的初始地幔状 δ18O 值降低。这三步过程可以解释 δ18O 值的全部范围、Bárðarbunga 火山系统中的 TE 数据，以及大多数熔体在冰岛的新火山裂谷区喷发（图 2；图 S7）。按照 Sohn（2013）中概述的混合方程（见补充材料），我们定量推导了解释 Bárðarbunga 火山系统 MI 和地块玻璃中观察到的 δ18O 值和 TE 含量（图 2）所需的氧同位素交换程度。这里采用氧同位素交换的程度作为地壳物质同化量的代表。Bárðarbunga 火山系统中的大多数玻璃和 MI 需要 10% 到 30% 的氧同位素交换来解释它们的低 δ18O 值（图 3A；图 S9A），这与为原始玄武质岩浆计算的热力学极限一致（Heinonen 等人，2022 年）。需要高达 55% 的氧同位素交换来解释地块玻璃记录的最低 δ18O 值。然而，计算的交换程度很大程度上取决于选择的同化物 δ18O，这很难约束，并且可能在地壳中是异质的。例如，如果熔体以 δ18O = -2‰ 同化地壳，我们可以在 35-40% 氧同位素交换的情况下重现最低 δ18O 值（约 +2.6‰）（图 S9C）。限制了氧同位素的范围接下来，我们将寻求确定该过程在 Bárðarbunga 火山系统中发生的位置。玻璃和 MI 的平衡压力可以通过应用 Olivine-Plagiocase-Augite-Melt (OPAM) 气压计来估算（Yang et al., 1996; Hartley et al., 2018）。Bárðarbunga 样品的 OPAM 平衡压力为 1.0-6.3 kbar（3.5-22.5 km），约 60% 的样品在 2-4 kbar 范围内（7.1-14.3 km）（Caracciolo 等人，2020）。平衡压力的分布（图 3A）与多层一致，跨地壳岩浆系统，其中熔体在相互连接的叠层基台网络中分馏和混合（Maclennan，2019；Caracciolo 等，2020、2021）。平衡压力与氧同位素交换的程度相关（图 3A）。受污染最严重的熔体记录了最低的 δ18O 值，在 1-2 kbar 范围内（3.5-7.1 km）显示出最低的平衡压力。例如，在约 4 kbar (14.3 ± 4.7 km) 下储存和平衡的熔体平均受到 15% 的地壳污染，但在约 1.5 kbar (5.4 ± 4.7 km) 下平衡的熔体平均受到 35% 的污染（图 3A ）。尽管这种相关性具有较低的 R2 (0.19)，但具有高度的统计学意义 (p < 0.0001, n = 86)。低 R2 值主要是由于与 OPAM 气压计相关的不确定性和端元成分的估计。因此，我们认为，随着熔体在整个 Bárðarbunga 火山系统中向上转移，它们的 δ18O 值会随着它们受到更多地壳污染而降低（图 3B）。我们的数据表明，影响熔体 δ18O 的大部分氧同位素交换发生在 3.5 公里到 10 公里之间的中地壳上部和最上部。尽管>~4.5 kbar（~15 km）的数据稀缺性使我们无法提供对深部地壳中氧同位素交换程度的可靠估计，但我们的数据表明，下地壳物质对 δ18O 值几乎没有影响，因为缺乏下地壳岩石与热液和/或大气水的相互作用。事实上，下地壳缺乏改变，δ18O 含量低的岩石，它很可能是由侵入物形成的（Greenfield 和 White，2015 年）。这项研究表明，供应给 Bárðarbunga 火山系统以及可能供应给冰岛裂谷带其他火山系统的熔体经历了不同程度的地壳同化，即影响了它们的 δ18O 值。我们证明，随着熔体通过地壳上升并在大约 3.5-10 公里深度接近浅层，它们获得较低的 δ18O 值，因为它们同化了更多的地壳物质。因此，我们将 Bárðarbunga 火山系统下方的管道系统设想为一个逐渐同化的多层系统，其中岩浆在叠层岩床储层内通过大范围深度（3.5-22.5 公里）进行处理。这种经历持续同化的复杂多层火山系统很可能在厚地壳中的所有长寿命岩浆系统中很常见，例如弧形火山和大陆裂谷。Caracciolo 得到了冰岛大学研究基金 (HI17060092) 和北欧火山中心的支持。SA Halldórsson 和 EW Marshall 承认冰岛研究基金的支持（赠款 196139-051 和 #195638-051）。SA Halldórsson 的参与部分与 Horizo​​n 2020 项目 EUROVOLC 有关，该项目由欧盟委员会资助（赠款 731070）。M. Kahl 承认德国研究基金会的资助（赠款 KA 3532/2-1） NordSIMS 离子微探针设施承认瑞典研究委员会（赠款 2017-00671）、瑞典自然历史博物馆和冰岛大学的支持; 这是 NordSIMS 出版物 705。我们感谢 K. Lindén 在斯德哥尔摩的实验室协助，R. Sohn 对数学方程的帮助，以及 J. Cullen 对激光氟化分析的帮助。我们还要感谢 U. Schaltegger 的编辑处理，以及 J. Troch 和一位匿名审稿人的有益评论。