The formation of peridotitic suite inclusions in diamonds

Olivine, orthopyroxene and garnet grains belonging to the peridotitic suite of mineral inclusions in natural diamonds typically show compositions poorer in Ca and Al and richer in Mg and Cr than the same minerals in peridotite nodules in kimberlite. Other features suggest the crystallisation of diamonds from magmas of kimberlitic affinities, and it is suggested that the genesis of peridotitic suite diamonds is linked with that of a CO₂-bearing magma. It is shown that the generation of kimberlitic magma from common garnet-peridotite (with 5 wt.% clinopyroxene) in the presence of CO₂ may rapidly remove by melting all Ca-rich solid phases (clinopyroxene and/or carbonate). Further melting may form liquids in equilibrium with olivine, orthopyroxene, and garnet with the distinctive compositions of the diamond inclusions. The amount of melting and CO₂ necessary for the loss of clinopyroxene (and/or carbonate) are estimated at approximately 5.0 wt.% and 0.5 wt.% respectively.     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

柱状扰动体引起圆柱谐振腔共振频率偏移的数值模拟

基于微扰理论,研究了内部存在一个同轴圆柱扰动体时圆柱谐振腔共振频率的偏移问题.其中,圆柱谐振腔的边侧面是刚性的,上下底面应力自由.经过推导,得出了圆柱扰动体存在时谐振腔的声势与共振频率的表达式.在此基础上,分析了谐振腔与圆柱扰动体各种参数对谐振腔共振频率的影响.数值模拟结果表明,谐振腔的共振频率受扰动体在谐振腔中的位置影响较大.圆柱谐振腔的共振频率在圆柱扰动体居中时是最大值,并且其共振频率对扰动体的声速敏感;当扰动体在谐振腔两端时,谐振腔共振频率是最小值,并且其对扰动体的密度敏感.最后,通过数值模拟结果和实验测量结果之间的对比,发现两者的基本变化趋势是吻合的.