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Melting experiments on SiO_2-rich lamproites to 6.4 GPa and their bearing o the sources of lamproite magmas

机译:富含SiO_2的斑岩熔融实验到6.4 GPa及其与斑岩岩浆的来源。

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摘要

Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 deg C and 2 to 6.4 GPa have been determined experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All samples have extended melting intervals (500-600 deg C). Bulk composition has a significant control on the nature of the initial liquidus phases, with orthopyroxene occurring at low pressures (< 4 GPa) in the relatively calcium-poor Oscar and Cancarix lamproites. At higher pressure (> 6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase at all pressures in the Middle Table Mountain lamproite. Near-solidus phases assemblages at high pressure (> 5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet (Oscar); clinopyroxene + garnet + coesite + K-Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K-Ti-silicate (Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different relative contributions from the veins and wall-rocks to the partial melts.
机译:已通过实验确定了三种富含二氧化硅的三氧化锰:透明质白云石金云母三氧化钛(Oscar,West Kimberley);在800至1700℃和2至6.4 GPa的温度和压力下,超固相关系。富山红铁矿堇青石(坎卡里克斯,穆尔西亚-阿尔梅里亚);和金云母过渡辉绿岩红岩(怀俄明州中桌山)。所有样品的熔化间隔均延长(500-600摄氏度)。本体组成对初始液相线相的性质有明显的控制,邻苯二茂铁发生在相对钙含量低的奥斯卡和坎卡里克斯红闪石中在低压(<4 GPa)下发生。在更高的压力(> 6 GPa)下,邻位邻苯二甲醚被石榴石和斜靠的ene烯取代,成为奥斯卡红闪石中的近液相相,在Cancarix样品中被邻苯二酚和斜靠的replaced烯取代。在中等压力下,Tableno Mountain山红铁矿中的亚次氯环己烯是接近液相的相。高压下(> 5 GPa)的近固相相组合为:斜辉石+金云母+堇青石+金红石+石榴石(奥斯卡);单斜辉石+石榴石+堇青石+ K-钛硅酸盐(Cancarix);单斜辉石+金云母+磷灰石+ K-Ti-硅酸盐(中桌山)。在所有组合物中,在这里研究的任何温度或压力下都从未发现橄榄石为液相。解释了相关系,表明富含二氧化硅的三价锰铁矿不能通过锂沸石源的部分熔融而得到。据认为,它们的成因涉及高度变质的古交代岩脉在哈格斯堡-伊黎索石系岩石圈地幔中。这些静脉在矿物学上被认为类似于实验观察到的高压近固相相组合。由于源脉的矿物学模式不同以及脉脉和围岩对部分熔体的不同相对贡献,富含二氧化硅的原生红铁矿岩浆的组成在克拉通之间也有所不同。

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