Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data
Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data
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DOI：
10.1007/s00410-011-0661-0
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发表时间：
2012-01-01
影响因子：
3.5
通讯作者：
Vannucci, Riccardo
中科院分区：
文献类型：
--
作者：
Francalanci, Lorella;Avanzinelli, Riccardo;Vannucci, Riccardo
利用微钻孔技术对2002-2003年意大利斯特龙博利火山爆发危机期间采集的7个样品中的斜长石和斜辉石进行了原位sr同位素数据采集，并进行了主要元素和微量元素分析。2002年12月28日，持续的中度爆炸活动被一个持续约7个月的喷发事件所打破。2003年4月5日发生了一次更猛烈的爆炸（阵发性）。喷发出两种岩浆类型，即以矿渣或熔岩的形式涌出的低挥发物和高斑状岩浆（hp -岩浆）和以浮石的形式涌出的富挥发物和低斑晶岩浆（lp -岩浆）。lp -岩浆与hp -岩浆的不同之处在于，它们的化学成分、地面物质组成和sr -同位素比率都略低。微量sr -同位素数据显示，与1984年、1985年和1996年喷发的产物一样，存在强同位素不平衡的分带矿物，Sr-87/Sr-86值从岩心到矿物边缘普遍递减。只有一些外缘证明了与宿主地面物质的平衡。具有高sr同位素比值（0.70665-0.70618）的内部矿物带被解释为“反结晶”，在之前的活动中结晶，并在距今约2.5 ka前的最近期开始的shoshunic活动中在当今系统中再循环。这一结果对Stromboli现今管道系统在中压下（约2-3公里深）的动力学具有重要意义，并允许我们提出一个模型，其中hp -岩浆储层在底部与累积晶体储层直接相连。由于两种岩浆的流变特性比管道中更相似，在管道中，通过脱气可以增强结晶，因此在该储层中，存在的hp -岩浆和输入的lp -岩浆之间可以发生有效的混合。反水晶（可能还有熔体）从底部重新进入hp -岩浆储层，通过上升的lp -岩浆穿过晶体浆状物再循环，从顶部再循环，通过下降的脱气和密集的hp -岩浆再循环，这是表面周期性斯特龙堡式爆炸的残余。根据在地质体中测量到的同位素变化，可以计算出lp岩浆进入浅层hp岩浆储层的比例约为20%。根据这一比例，我们估计2002-2003年期间lp岩浆输入的总体体积与喷涌事件中喷发的岩浆体积非常接近，表明这是一个大致恒定体积的稳态系统。结果表明，在4月5日爆发期间，直接到达地表的低密度岩浆量与进入系统的低密度岩浆量相比是最小的。
In situ Sr-isotope data by microdrilling, coupled with major and trace element analyses, have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002-2003 eruptive crisis at Stromboli volcano (Aeolian Islands, Italy). On 28 December 2002, the persistent moderate explosive activity was broken by an effusive event lasting about 7 months. A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile-poor and highly porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice. LP-magma differs from the HP-magma also for its slightly less-evolved chemistry, the groundmass composition and the lower Sr-isotope ratios. Micro-Sr-isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previously found in products erupted in 1984, 1985 and 1996 AD, with Sr-87/Sr-86 values generally decreasing from cores to rims of minerals. Only some outer rims testify for equilibrium with the host groundmass. The internal mineral zones with high Sr-isotope ratios (0.70665-0.70618) are interpreted as 'antecrysts', crystallised during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period, which occurred at about 2.5 ka ago. This result has implications for the dynamics of the present-day plumbing system of Stromboli at intermediate pressure (about 2-3 km depth) and allows us to propose a model whereby an HP-magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP-and input LP-magmas can occur in this reservoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallisation is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating the proportion of the LP-magma entering the shallow HP-magma reservoir at similar to 20%. From this proportion, we estimate that the total volume of LP-magma input during 2002-2003 closely matches the magma volume erupted in the effusive event, suggesting a steady-state system at broadly constant volume. The comparison with estimates of the LP-magma volume ejected by the paroxysm indicates that the LP-magma amount directly reaching the surface during the 5 April paroxysm is minimal with respect to that entering the system.