Petrogenesis of garnet and spinel peridotites : a study with particular reference to the role of chromium in geothermometry and geobarometry
Petrogenesis of garnet and spinel peridotites : a study with particular reference to the role of chromium in geothermometry and geobarometry
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1983
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Kg Nickel
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作者：
Kg Nickel
尖晶石和石榴石橄榄岩是上地幔的主要岩石类型。本文研究了澳大利亚维多利亚玄武岩包裹体中的尖晶石橄榄岩的岩石学、矿物学、体积岩石和矿物化学，为其岩石成因以及与寄主岩石和寄主岩石中出现的其他xenolithic岩石类型的关系提供了证据。通过超镁铁质体系的实验研究，推断石榴石橄榄岩形成的P、T条件，特别是文献报道的金伯利岩中xenolithic产状条件，探讨石榴石与尖晶石橄榄岩的关系，为寄主岩、石榴石橄榄岩与同时发生的捕虏体、巨晶体和矿物之间的关系提供线索。来自澳大利亚维多利亚州两个邻近地区（Bullenmerri湖和Leura山）的超镁铁质包裹体包括有或没有水相的尖晶石橄榄岩、辉石岩、辉石岩和角闪石。矿物化学提供了在45千米深的温度范围内的单个结核的平衡结晶的证据。温度估算显示无水组合>含硅云母组合>含角闪石组合之间存在系统差异。辉橄榄岩和直辉石中“MgO”、“CaO”、“Al_2”、“O_3”和相容元素含量的整体岩石变化被模拟为早期部分熔融事件，造成不同程度的衰竭。提取液为苦味酸成分。尖晶石辉橄榄岩中的角闪石发育独立于早期部分熔融事件和后期部分熔融事件，它们的结晶是水的加入引起的近等化学变质反应的结果。水化事件发生于玄武岩形成之前，但不是玄武岩形成的先决条件。水合交代作用发生在LVZ以上的上地幔，但这很可能不是碱性岩浆的形成区域。碱性岩浆侵位和通过岩石圈/上地幔可能是局部交代和水化作用的原因。辉长岩、辉长岩、某些辉长岩和角闪石是岩浆在地幔深处分馏和/或结晶的沉淀。复合捕虏体证明可观察到的岩壁反应极为有限（约1厘米）。在CaO-MgO-Al_2 ' ' O_3 ' ' -SiO_2 ‘ （CMAS）和SiO_2 ’ - ' MgO-Al_2 ' ' O_3 ' - ' CaO-Cr_2 ' ' O_3 ' （SMACCR）体系中进行的实验表明，在CHAS体系中，橄榄岩组合中的尖晶石和石榴石是由一个不变的反应联系在一起的，但在SMACCR体系中，在一定的P、T条件下，它们是共存的。该场在P，T空间中的宽度和位置都取决于体成分的Cr/Cr+Al比。用Wells（1977）的方法对含有两种辉石的组合进行温度估计，在所有研究系统（CMS， SMACCR和多组分“自然”系统）中，温度范围为1000- 1400°C和15-40 kb，结果令人满意。对这种温度计的准确性存在怀疑，特别是在低温（<900°C）时，但对于大多数天然石榴石橄榄岩的xenolithic矿床，可以用这种方法相当有信心地估计温度。在SMACCR体系中，Cr-Al在正辉石和斜辉石之间以及辉石和尖晶石之间的分布表现出规律性，不受压力和温度的影响。在1200°C以上的SMACCR体系中，石榴石和尖晶石之间的Cr-Al交换对温度敏感，并以此为基础对简单体系的反应进行了标定。然而，在多组分系统中的实验表明，这种反应的组分依赖性很强，因此在实际的地温测量中没有用处。Al_2 ‘ ‘ O_3 ’和Cr_2 ’ ‘ O_3 ’在与石榴石和斜辉石共存的正辉石中的溶解度取决于共存相的压力、温度和组成。通过热力学推理和经验曲线拟合，在CMAS和SMACCR系统中模拟了这些相关性，得到了这些系统的气压表达式。结果与近期含铁系统的实验研究结果相结合，并对两个独立的经验地压计进行了校准，在多组分系统和天然石榴石辉橄榄岩的实验中得到了令人满意和一致的结果。各套石榴石-辉橄榄岩捕虏体的P、T估计显示出不同省份和寄主岩石的P、T平衡条件的不同分布。来自南非的低温捕虏体（1100°C）来自48-51 kb的狭窄压力区间，覆盖1100-1400°C的温度范围。俄罗斯样品的过渡温度（1050-1200°C）表明与南非捕虏体相似的趋势。来自lanprophy岩（美国西南部四角地区）的石榴石辉橄榄岩在1000-1200°C范围内，在36- 39kb的狭窄压力范围内平衡。来自海洋环境（所罗门群岛）、年轻大陆地区（澳大利亚新西南地区）和比查德岛地区（加拿大）的样本显示，P、T平衡条件与基于热流数据或对流地幔模型估算的海洋上地幔P、T条件一致。南非石榴石辉橄榄岩的P、T估计数据与橄榄岩系统的熔融研究和金伯利岩的流体研究相结合，推断南非金伯利岩的起源条件为160-180公里深度和1400-1500°C。在这些金伯利岩中发现的巨晶和堆积物是岩浆在地幔深处分选和/或结晶的沉淀。金刚石在接近石墨-金刚石边界的条件下（900-1300℃，40-55 kb）从预定年岩浆中结晶。巨晶岩、堆积物和钻石可能与金伯利岩岩浆有关，但与寄主金伯利岩相比则是xenolithic/ xenoccrystal。
Spinel and garnet peridotites are the dominant rock types of the upper mantle. Petrography, mineralogy, bulk rock and mineral chemistry of spinel lherzolites from inclusions in basanites in Victoria, Australia were studied to provide evidence for their petrogenesis and relation to host rocks and to other xenolithic rock types occurring within these host rocks. Experimental studies in ultramafic systems were carried out to infer P,T conditions of the origin of garnet peridotites, particularly those of xenolithic occurrence in kimberlites reported in the literature, to investigate the relation between garnet and spinel peridotites and to give clues to the relation between host rocks, garnet peridotites and simultaneously occurring xenoliths, megacrysts and minerals. Ultramafic inclusions from two neighbouring localities in Victoria, Australia (Lake Bullenmerri and Mt Leura) include spinel lherzolites with and without hydrous phases, wehrlites, pyroxenites and hornblendites. Mineral chemistry provides evidence for equilibrium crystallization for individual nodules at a depth near 45 km but over a range of temperatures. Temperature estimates yield systematic differences between anhydrous assemblages > phlogopite-bearing assemblages > amphibole-bearing assemblages. Bulk rock variation of `MgO, CaO, Al_2``O_3` and compatible element contents in lherzolite and harzburgite has been modelled as an early partial melting event, giving rise to various degrees of depletion. The extracted liquid was of picritic composition. Amphiboles in spinel lherzolites are developed independently of the early partial melting event and postdate it They crystallized as a response to near-isochemical metamorphic reaction, consequent on addition of water. The hydration events predate but are not precursor conditions for production of basanite. Hydration-metasomatism occurs in the uppermost mantle above the LVZ, but this is most probably not the region of formation of the alkaline magams. The emplacement and passage of alkaline magmas through the lithosphere/upper mantle may be the cause of local metasomatism and hydration. Wehrlites, pyroxenites, some lherzolites, and hornblendites are precipitates from magmas fractionating and/or crystallizing at mantle depths. Observable wallrock reaction is extremely restricted (about 1 cm) as evidencedby composite xenoliths. Experiments in the systems `CaO-MgO-Al_2``O_3``-SiO_2` (CMAS) and `SiO_2`-`MgO- Al_2``O_3`-`CaO-Cr_2``O_3` (SMACCR) show that spinel and garnet in peridotitic assemblages are related by an univariant reaction in the CHAS system, but coexist with each other over a range of P,T conditions in the SMACCR system. Both the width and location of this field in P,T space is dependent on the Cr/Cr+Al ratio of the bulk composition. Temperature estimates for assemblages containing two pyroxenes by the method of Wells (1977) give satisfactory results for the range of 1000- 1400°C and 15-40 kb in all systems studied (CMS, SMACCR and multicomponent "natural" systems). Doubts on the accuracy of this thermometer exist particularly at loWtemperatures (<900°C), but for most xenolithic occurrences of natural garnet lherzolites,'temperatures may be estimated by this method with considerable confidence. Cr-Al distributions between ortho- and clinopyroxene as well as between pyroxenes and spinel show a regular behaviour in the SMACCR system, independent of pressure and temperature. The Cr-Al exchange between garnet and spinel is sensitive to temperature in the SMACCR system above 1200°C and a thermometer based on this, reaction has been calibrated for the simple system. . Experiments in multicomponent systems show however strong compo-sitional dependencies of this reaction, rendering it as of no use for practical geothermometry. The solubility of both `Al_2``O_3` and `Cr_2``O_3` in orthopyroxene coexisting with garnet and clinopyroxene is dependent on pressure, temperature and composition of the coexisting phases. By both thermodynamic reasoning and empirical curve-fitting these dependencies have been modelled in the CMAS and SMACCR systems, resulting in barometric expressions for these systems. The results have been combined with recent experimental investi-gations in Fe-bearing systems and two independent empirical geobarometers were calibrated, giving satisfactory and consistent results when applied to experiments in multicomponent systems and natural garnet lherzolites. P,T estimates for various suites of garnet lherzolite xenoliths show differing distributions of P and T comditions of equilibration for different provinces and host rocks. Low temperature xenoliths ( 1100°C) from South Africa are derived from a narrow pressure interval around 48-51 kb, covering a range of temperatures from 1100-1400°C. Russian samples with transitional temperatures (1050-1200°C) indicate a parallel trend to South African'xenoliths. Garnet lherzolites from lanprophyre (Four-Corners area, southwestern U.S.A.) equilibrated over a range of =1000-1200°C at a narrow pressure range around 36-39 kb. Samples from oceanic environments (Solomon Islands), young continental areas (N.S.W., Australia) and the Ile Bizard locality (Canada) show P,T conditions of equilibration in agreement for estimates of P,T conditions for oceanic upper mantle based on heat-flow data or convecting mantle models. The data on P,T estimates for garnet lherzolites from South Africa have been combined with melting studies on peridotitic systems and lquidus studies on kimberlite to infer conditions of 160-180 km depths and 1400-1500°C for the origin of South African kimberlites. Megacrysts and cumulates found in those kimberlites are precipitates from pre-dating magmas fractionating and/or crystallizing at mantle depths. Diamonds crystallized from pre-dating magmas at conditions close to the graphite-diamond boundary (900-1300°C, 40-55 kb). Megacrysts, cumulates and diamonds are probably related to kimberlitic magmas, but are xenolithic/ xenocrystic to the host kimberlite.