Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study
Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study
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原始维苏威火山岩浆的性质和演化：实验研究
DOI：
10.1093/petrology/egu057
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发表时间：
2014
影响因子：
3.9
通讯作者：
R. Cioni
中科院分区：
文献类型：
--
作者：
M. Pichavant;B. Scaillet;A. Pommier;G. Iacono;R. Cioni
两种来自维苏威火山的镁铁质喷发产物，一种软玉和一种粗面玄武岩，已在实验室结晶，以限制原始维苏威火山岩浆的性质及其地壳演化。实验在高温(1000-12008℃)、0A1 Mpa和高压(50-200 Mpa)下进行，在无H2O流体和有H2O和CO2流体存在的条件下进行。实验从玻璃开始，除了少数几个开始于玻璃加上圣卡洛斯橄榄石晶体，以迫使橄榄石饱和。熔体中H2O浓度最高可达6A0wt%，实验fO2范围从nNO^0A1到nNO？3A4(其中nNO为镍^镍氧化物缓冲液)。单斜辉石(Mg#~93)是所研究的两个样品的液相线，其次是白榴石(5 3wt%)和金云母(Mg#~81)，分别占熔体中H2O的5 3wt%和4 3wt%。橄榄石(Fo 85)只在一次实验中自发结晶。未发现斜长石。单斜辉石进行结晶后，玻璃K_2O和Al_2O_3含量显著增加，而MgO、CaO和CaO/Al_2O_3降低，残余熔体遵循维苏威全岩从粗面玄武岩到软玉、声辉长岩和斜方辉石的演化过程。近液相线200 Mpa玻璃中H2O和CO2的浓度与文献中原始熔体包裹体的浓度重叠。通过添加橄榄石实验，重建了富Fo橄榄石结晶对应的最早演化阶段。结果表明，原始维苏威火山熔体为粗面玄武岩(K2O$4A5^5A5 wt%，MgO？8？9wt%，Mg#？75？80，CaO/Al_2O？3？0A9？0A95)，在1150~12008C和300~5200 Mpa范围内，以富Fo橄榄石(90^91)为液相结晶。原始维苏威火山熔体富挥发分(原始熔体包裹体中1A5^4A5 wt%H2O和600^4500ppm CO2)，并被氧化(从NNO塔0A4到NNO塔1A2)。上升的原始岩浆对碳酸盐围岩的同化作用可以解释橄榄石从结晶序列中消失的原因，并解释了缺乏代表橄榄石结晶岩浆的岩石。提出了碳酸盐同化作用与补给系统类型的关系：对于小体积的原始岩浆批次，碳酸盐同化作用被促进。相反，对于寿命更长、体积大、不太频繁补充的、因此更演化、更冷的储集层来说，岩浆和碳酸盐的相互作用是有限的。来自维苏威火山和其他坎帕尼亚火山的原始岩浆具有类似的氧化还原状态。然而，维苏威尖晶石的铬#是独特的，因此维苏威地幔来源中的橄榄岩成分不同于其他寒武纪岩浆。
Two mafic eruptive products from Vesuvius, a tephrite and a trachy-basalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to !12008C) and both at 0A1 MPa and at high pressures (from 50 to 200 MPa) under H 2 O-bearing fluid-absent and H 2 O-and CO 2 -bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crys-tals to force olivine saturation. Melt H 2 O concentrations reached a maximum of 6A0 wt % and experimental fO 2 ranged from NNO ^ 0A1 to NNO þ 3A4 (where NNO is nickel^nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H 2 O in melt 53 wt %, and by phlogopite (Mg# up to 81) for H 2 O in melt 43 wt %. Olivine (Fo 85) crystallized spontaneously in only one ex-perimental charge. Plagioclase was not found. Upon progressive crys-tallization of clinopyroxene, glass K 2 O and Al 2 O 3 contents strongly increase whereas MgO, CaO and CaO/Al 2 O 3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H 2 O and CO 2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was re-constructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K 2 O $ 4A5^5A5 wt %, MgO ¼ 8^9 wt %, Mg# ¼ 75^80, CaO/Al 2 O 3 ¼ 0A9^0A95) that crystallize Fo-rich olivine (90^91) as the liquidus phase between 1150 and 12008C and from 300 to 5200 MPa. Primitive Vesuvius melts are volatile-rich (1A5^4A5 wt % H 2 O and 600^ 4500 ppm CO 2 in primitive melt inclusions) and oxidized (from NNO þ 0A4 to NNO þ1A2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is pro-posed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma^carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.