The pressure and temperature conditions and timing of glass formation in mantle-derived xenoliths from Baarley, West Eifel, Germany: the case for amphibole breakdown, lava infiltration and mineral – melt reaction
The pressure and temperature conditions and timing of glass formation in mantle-derived xenoliths from Baarley, West Eifel, Germany: the case for amphibole breakdown, lava infiltration and mineral – melt reaction
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DOI：
10.1007/s007100200003
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发表时间：
2002-03
影响因子：
1.8
通讯作者：
C. Shaw;A. Klügel
中科院分区：
文献类型：
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作者：
C. Shaw;A. Klügel
第四纪西艾弗尔火山区Baarley幔源捕虏体在脉、边缘和池中含有六种不同的玻璃。1)富含环带单斜辉石微晶的硅质不饱和玻璃，在捕虏体周围形成夹套，在捕虏体内部形成脉。这种玻璃在成分上类似于寄主碧玄岩中的基质玻璃。2)硅-不饱和碱性玻璃，含有与橄榄岩中角闪石有关的铬透辉石、橄榄石和尖晶石微晶。这种玻璃局部含有腐蚀的原生尖晶石和金云母。3)在单斜辉石中部分至完全破碎的角闪石颗粒中，与透辉石、尖晶石±橄榄石和蔷薇辉石微晶有关的二氧化硅不饱和玻璃。4)橄榄岩中金云母颗粒周围富含微晶石的边缘中硅不饱和到硅饱和的钾质玻璃。5)钾镁矾捕虏体中的硅不饱和钾玻璃。6)橄榄岩中部分溶解的斜方辉石晶体周围的富硅玻璃。斜方辉石-单斜辉石对（P = 1.5 GPa）的地质温度测定表明，裸露捕虏体的温度为850 °C，脉状捕虏体为950-1020 °C。单斜辉石-熔体热压法表明，方辉橄榄岩中的Cr-透辉石-2型玻璃对在1.4 - 1.1 GPa和1250 °C下形成，而魏氏岩中的Cr-透辉石-2型玻璃对在0.9 - 0.7 GPa和1120-1200 °C下形成。这种双峰分布的压力和温度表明，哈氏体捕虏体可能已被夹带在更深的比wehrlite exenolith.Glass在Baarley捕虏体有三个不同的起源：渗透的早期主机熔体不同的组合物从爆发主机碧玄岩;角闪石的部分熔融;这些熔体与捕虏体矿物的反应。1型玻璃的成分表明，夹套是相对演化的寄主岩浆的堆积。2型玻璃及其微晶的质量平衡模型表明，它是由浸染状角闪石的分解和熔体与周围捕虏体矿物的反应造成的。单斜辉石岩捕虏体中的3型玻璃是角闪石在低压下裂解的产物。金云母与2型熔体或夹套熔体反应形成的4型和5型玻璃。与斜方辉石共生的6型玻璃是由于斜方辉石被上述熔体的不全等溶解所致。2型玻璃池和夹套玻璃附近捕虏体橄榄石中的成分梯度可以模拟为Fe-Mg互扩散剖面，表明熔体-橄榄石接触时间在0.5 ~ 58天之间。结合单斜辉石-熔体温压法计算，这些数据表明，玻璃（熔体）是在短时间内由于角闪石的减压熔融和演化的基质熔体的渗透而形成的。这些捕虏体中的玻璃都不可能与交代作用或地幔中原位发生的任何其他过程直接相关。
Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rhönite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites.Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to 950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C. This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater depth than wehrlite xenoliths.Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type 2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts.Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other process that occurred insitu in the mantle.