微量元素地球化学课件（中国地质大学） 4微量元素在不同地质体中的分布与分配.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * * * * * * Continue with: % partial melting increases to ~ 15-40% as diapirs of melting mantle rise toward the surface % melted when reaches top depends on the source depth, temperature, and the rate of rise (and spreading) Melting is terminated by conductive heat loss to the surface near the top of the column, perhaps aided by the consumption of clinopyroxene, which, when gone, will create a discontinuous temperature jump in melting * * More enigmatic processes and less voluminous than activity at plate margins No obvious mechanisms that we can tie to the plate tectonic paradigm As with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB 41 well-established hot spots Estimates range from 16 to 122 * MORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulation * * Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough) * Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough) * Ocean Island Basalt (Hawaiian alkaline basalt) Looks like partial melt of ~ typical mantle Mid Ocean Ridge Basalt (tholeiite) How get (+) slope?? * Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr 0.7035