CO2 and the eruptibility of picrite and komatiite
CO2 and the eruptibility of picrite and komatiite
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CO2 与苦味铁矿和科马提岩的喷发性
DOI：
10.1016/0024-4937(95)90005-5
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发表时间：
1995
期刊：
影响因子：
3.5
通讯作者：
A. T. Anderson
中科院分区：
文献类型：
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作者：
A. T. Anderson
1959年，夏威夷基拉韦厄火山顶区喷发出拉斑玄武岩苦橄岩。Wright、Helz、Schwindinger和安德森的研究表明，1959年的岩浆是一种新的、热的、富含MgO的岩浆的混合物，这些岩浆上升并与较冷的储存岩浆混合，导致大量橄榄石斑晶的结晶。1959年火山喷发的橄榄石斑晶中的熔融包裹体中溶解的CO2表明，大多数橄榄石的生长压力小于1千巴（100 MPa）。因此，新的岩浆可能是浮力相对于存储的岩浆在1千巴。富含MgO的岩浆的特点是熔体相对致密，只有当它含有比储存的岩浆更多的气体时才能浮起来。包裹体成分分析表明，该气富含CO2。由于气体是高度可压缩的，岩浆浮力所需的富CO2气体的质量分数在更大的压力下增加。一个临界压力是指含气的、富含MgO的母岩浆与储存的岩浆具有相同的密度。如果母岩浆以足够大的压力侵入储存的岩浆，它将具有负浮力，其随之而来的下降将抑制其喷发。基拉韦厄的脱气作用表明，基拉韦厄母玄武岩可能含有约0.3重量%的二氧化碳。含气苦橄质岩浆（含15重量% MgO + 0.3体积重量% CO2）相对于脱气拉斑玄武岩（8 wt.% MgO液体）。对于基拉韦厄1959年的苦橄岩，新的富含MgO的岩浆侵入储存的岩浆时，推断的1千巴压力与其浮升到表面和喷发是一致的。虽然基拉韦厄的母岩浆被认为具有苦橄岩成分，但喷发的苦橄岩并不常见，特别是在基拉韦厄火山的山顶喷发中。在基拉韦厄的顶峰之下，一个间歇性岩浆储存库向下延伸约10公里，对应的压力约为2.7千巴。Ryan认为，岩浆储存库的底部是由将孔隙消除与矿物压缩分开的过渡区控制的。基拉韦厄上苦橄岩作为熔岩喷发的罕见性可能反映了两个因素的结合：（1）基拉韦厄的苦橄岩母岩浆的CO2含量可能小于0.3wt.%; (2)大多数母岩苦橄质岩浆在其底部附近进入储层，在那里压力足够高，导致母岩岩浆的体积密度大于储存的岩浆的体积密度。1959年新的苦橄质岩浆通过一条不寻常的路线上升，可能首先在一个异常浅的地方遇到了储存的岩浆，那里的新岩浆相对浮力大。由于孔隙的作用，次火山岩浆储集层的基底仅取决于压力和岩石强度。因此，在古代和现代，活火山下储存的岩浆库的底部可能发生在2至3千巴之间。由于不含气体的科马提质岩浆相对致密，预计它也会被困在储存的岩浆之下。科马提质岩浆，含有超过约0.6wt.%然而，即使在岩浆储存库的底部，CO2的含量相对于储存的、脱气的玄武岩熔体也是有浮力的。科马提质熔岩在太古代的普遍存在可能反映了幔源太古代母岩浆中较高的CO2含量。
A tholeiitic picrite erupted from the summit region of Kilauea volcano, Hawaii in 1959. Work by Wright, Helz and Schwindinger and Anderson shows that the 1959 magma was a mixture of new, hot, MgO-rich magma that rose through and mixed with cooler stored magma causing crystallization of abundant olivine phenocrysts. CO2dissolved in melt inclusions in olivine phenocrysts from the eruption of 1959 reveals that most of the olivines grew at pressures less than 1 kbar (100 MPa). Therefore, the new magma probably was buoyant relative to stored magma at 1 kbar. The MgO-rich magma is characterized by a relatively dense melt and could only be buoyant, if it contained more gas than the stored magma. Compositions of inclusions indicate that the gas was rich in CO2. Because gas is highly compressible, the mass fraction of CO2-rich gas that is required for magma buoyancy increases at greater pressures. A critical pressure is that at which gassy, MgO-rich, parental magma has the same density as stored magma. If the parental magma invades stored magma at a sufficiently great pressure, it will be negatively buoyant and its consequent descent will inhibit its eruption. Kilauea's degassing suggests that Kilauean parental basalt may contain about 0.3 wt.% CO2. Gassy picritic magma (liquid with 15 wt.% MgO + 0.3 bulk wt.% CO2) is buoyant with respect to degassed tholeiitic basalt (8 wt.% MgO liquid) at pressures less than about 2 kbar. For Kilauea's 1959 picrite the inferred 1 kbar pressure at which new MgO-rich magma invaded stored magma is consistent with its buoyant rise to the surface and eruption. Although the parental magma of Kilauea is considered to have a picritic composition, erupted picrite is uncommon, particularly in Kilauean summit eruptions. Beneath Kilauea's summit a reservoir of intermittent magma storage extends downward to about 10 km corresponding to a pressure of about 2.7 kbar. Ryan has argued that the base of the magma storage reservoir is controlled by the transition region that separates porosity elimination from mineral compression. The rarity of erupted picrite as a lava on Kilauea probably reflects a combination of two factors: (1) parental Kilauean picritic magma probably has a CO2content less than about 0.3 wt.%; (2) most parental picritic magma enters the reservoir near its base where the pressure is sufficiently high to result in the bulk density of parental magma being greater than that of stored magma. The new, 1959 picritic magma ascended through an unusual route and probably first encountered stored magma at an unusually shallow level where the new magma was relatively buoyant. Because of the role of porosity, the base of subvolcanic magma storage reservoirs is dependent only on pressure and rock strength. Therefore, the base of reservoirs of stored magma beneath active volcanoes likely occurred between 2 and 3 kbar during ancient as well as modern times. Because gas-free komatiitic magma is relatively dense, it too is expected to be trapped beneath stored magma. Komatiitic magmas containing more than about 0.6 wt.% of CO2, however, would be buoyant relative to stored, degassed basalt melt even at the base of magma storage reservoirs. The common occurrence of komatiitic lavas in Archaean times may reflect a greater CO2content of mantle-derived Archean parental magmas.