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首页> 外文期刊>Bulletin of Volcanology >The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions
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The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions

机译：海底金伯利岩岩浆的挥发性含量：天然岩石成分实验的一些限制

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Kimberlites are volatile rich magmas that ascend from deep in the mantle at high velocities, then as they reach a ‘root zone’ at 1–3 km in depth they either discharge explosively through to the surface or stall to form dykes and sills. Understanding this eruptive behaviour is difficult due to a lack of data on volatile solubility, particularly at conditions where the magmas enter the ‘root zone’ (∼30–80 MPa). In this study, we perform experiments on some putative primary kimberlite magma compositions to assess the amount of CO2 and H2O retained if these compositions represent magma as it enters the root zone. At the conditions investigated (100–200 MPa and 1,275–1,100°C) the results suggest that none of these particular kimberlite compositions reproduce a magma that can retain the observed high volatile content when intruded at these pressures (∼4–8 km). In our experiments, the low volatile retention is due to a combination of factors including a high proportion of solid phases, none of which are volatile-bearing, and inadequate volatile solubilities in the subordinate amounts of melt present. Modelled solubilities also suggest that the dissolved volatile contents remain too low even at super-liquidus temperatures (i.e. 100% melt). For water, the higher values observed in natural rocks can be explained by the addition of H2O associated with ubiquitous post-emplacement serpentinization. The high CO2 contents in hypabyssal rocks are unlikely to be related to alteration. We suggest that most kimberlites originally had lower SiO2 contents and as such may have been ‘transitional’ between silicate and carbonate melts. This results in both higher CO2 solubilities and lower liquidus temperatures. For such compositions, it is possible that both CO2 and water solubility may first decrease and then increase as magmas decompress and crystallize. Such unusual behaviour can help explain why kimberlite magmas can be very explosive or form shallow hypabyssal intrusions.
机译：金伯利岩是易挥发的富集岩浆，它们以较高的速度从地幔深处上升，然后到达深度为1-3 km的“根区”，要么爆炸性地排放到地表，要么失速形成堤坝和窗台。由于缺乏挥发性溶解度的数据，很难理解这种喷发行为，尤其是在岩浆进入“根区”（约30-80 MPa）的条件下。在这项研究中，我们对一些假定的金伯利岩岩浆原岩成分进行了实验，以评估当这些成分代表岩浆进入根部时保留的CO 2 和H 2 O的量区。在所研究的条件下（100–200 MPa和1,275–1,100°C），结果表明，这些特殊的金伯利岩成分均未产生岩浆，该岩浆在这些压力（〜4-8 km）下侵入时仍能保留观察到的高挥发分。在我们的实验中，低的挥发性保留是由于多种因素的组合，包括高比例的固相，这些固相都不具有挥发性，并且在存在的次要熔体中挥发性溶解度不足。建模的溶解度还表明，即使在超液相线温度（即100％熔融）下，溶解的挥发物含量仍然过低。对于水，在天然岩石中观察到的较高值可以通过添加与普遍存在的就位蛇纹石化相关的H 2 O来解释。海马岩石中较高的CO 2 含量不太可能与蚀变有关。我们建议大多数金伯利岩最初的SiO 2 含量较低，因此可能是硅酸盐和碳酸盐熔体之间的“过渡”。这会导致较高的CO 2 溶解度和较低的液相线温度。对于这样的组合物，CO 2 和水溶性都可能随着岩浆减压和结晶而先降低然后增加。这种不寻常的行为可以帮助解释为什么金伯利岩岩浆会爆炸性很大或形成浅层的hybybysal侵入体。

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