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为什么在裂谷碱性玄武岩发现中心和最重要的“羽毛”,而MORBs一般拉斑玄武岩质?

所以我实际上是想知道为什么它是玄武岩之上的“羽毛”在“热点”(有时称为OIBs,大洋岛玄武岩)和大陆裂谷中心丰富的碱性金属(Na和K)拉斑玄武岩质MORBs相比。

另外,我想知道有人会向我解释化学为什么比拉斑玄武岩碱性玄武岩更“成熟”?即是离子更比拉斑玄武岩中发现的钙离子不兼容?这是为什么呢?

回答

* *简短回答* *钠和钾在地幔是不相容的。程度低融化发生深地幔岩浆形式丰富Na和k .岩浆形成由于热点形成地幔深处。* *长回答* *拉斑玄武岩质玄武岩的岩浆形成由于减压地幔的岩石。地幔然后融化在相当大的程度上在低压力,形成一个相对减少岩浆(因此拉斑玄武岩,而不是钙碱性)。相对消耗在不相容元素源于地幔橄榄岩被融化的事实是一个回收的岩石,也失去了它很久以前不相容元素。另一方面,生产碱性玄武岩在较小的体积比MORB(大洋中脊)拉斑玄武岩质玄武岩。一般OIB(大洋岛玄武岩)是碱性,但碱性岩石不限于海洋设置。他们通常发生的热点和裂谷区,无论他们爆发的岩石圈板块的本质(海洋、大陆)。>即是离子* (Na和K) *更比拉斑玄武岩中发现的钙离子不兼容?这是为什么呢? Yes. Sodium and potassium are less compatible than calcium in minerals that comprise mantle rocks. This figure, taken from my answer to //www.hoelymoley.com/questions/2950/what-are-the-high-field-strength-and-large-ion-lithophile-hfs-lil-elements-a, shows why: ![trace elements][1] MRFE stands for Mantle Rock Forming Elements. These elements are compatible in olivine, pyroxene, garnet, spinel and anorthite. Na and K are only monovalent and larger than the MRFE. Thus, they are compatible and in cases of mantle melting, they are partitioned to the liquid (*i.e.* magma) phase. In order to understand why this enrichment in Na and K occurs, we need to understand *where* in the mantle these magmas form. Whereas tholeiitic magmas form in shallow depths, alkaline magmas form much deeper. Experiments show that the silica undersaturated nature of these rocks requires very deep melting (tens of kilometers). Since the melting does not occur by decompression, it has to melt by heating: a hot spot. This hot spot melts mantle rocks that did not experience prior melting events by the "ocean conveyor belt", and thus still retain their incompatible elements, including Na and K. There are two other possible sources of Na and K enrichment in alkali rocks. 1. Mantle metasomatism: Na and especially K are LILs: They are highly mobile in fluids. It is possible that mantle rocks were metasomatised by K and Na bearing fluids. This would cause minerals such phlogopite (a mica) and certain varieties of amphiboles to form in the mantle. These minerals are readily melted, thus contributing Na and K to the alkali magma. 2. Contamination by crustal rocks: This process is especially important in continental rifts. Continental crust is rich in Na and K (micas, feldspars, amphiboles) and these elements can be easily assimilated to the rising alkali magma. > why it is that alkali basalts are more 'mature' than tholeiites? I'm not exactly sure what do you mean by 'mature' but I will address the timing of alkali magmatic activity. Here is a *very* simplified depiction of what happens: ![volcanism and plumes][2] At first (A), a young plume will still be deep. The melts that it forms are low degree and deep. Then, when the plume 'matures' (B), it causes shallow melting producing voluminous tholeiitic basalts. The margins of the plumes still produce low degrees alkali melts. When the plume wanes (or just migrates somewhere else), you get only alkali melts again (sorry, I was too lazy to draw C in the figure, but it should look not too different than A). This is similar to what's happening at the moment in Hawaii. The newest (still underwater) volcano, Loihi, erupts alkali magmas. This corresponds to stage A in the figure. This is also what the oldest rock on Hawaii (the island) are made of: alkali rocks. Most of the rocks by volume on Hawaii (again, the island) are tholeiitic basalts erupted some hundred thousands year ago, corresponding to stage B in the figure. Recent (geologically) eruptions are yet again alkaline, similar to stage C. For some extra reading about alkali rocks and related subjects, see these other questions and answers: - //www.hoelymoley.com/questions/2950/what-are-the-high-field-strength-and-large-ion-lithophile-hfs-lil-elements-a - //www.hoelymoley.com/questions/310/what-are-rare-earths-and-why-do-they-cluster-near-alkaline-magmatism - //www.hoelymoley.com/questions/2293/are-there-significant-amounts-of-rare-earth-elements-on-iceland [1]: https://i.stack.imgur.com/RXCuD.png [2]: https://i.stack.imgur.com/TBVws.png

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