Petrological interpretation of deep crustal intrusive bodies beneath oceanic hotspot provinces
Petrological interpretation of deep crustal intrusive bodies beneath oceanic hotspot provinces
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DOI：
10.1029/2012gc004448
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发表时间：
2013-03
期刊：
影响因子：
3.7
通讯作者：
M. Richards;E. Contreras‐Reyes;C. Lithgow‐Bertelloni;M. Ghiorso;L. Stixrude
中科院分区：
文献类型：
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作者：
M. Richards;E. Contreras‐Reyes;C. Lithgow‐Bertelloni;M. Ghiorso;L. Stixrude
对某些海洋热点地区下的深地壳和上地幔结构的地震折射研究表明，在莫霍面或以上存在P波速度为Vp ~ 7.4-8.0 km/s的超镁铁质体，例如，夏威夷马克萨斯和留尼汪岛。然而，在其他热点地区，如加拉帕戈斯、纳斯卡海岭和路易斯维尔，下地壳被大量辉长岩（镁铁质）侵入（Vp ~ 6.8-7.5 km/s）。超镁铁质原生熔体形成于成熟大洋岩石圈之下，压力约为2-3 GPa（60-90 km深度），并由于其相对较高的密度而在莫霍面形成积水，可以解释所观察到的超镁铁质深地壳体。相比之下，在薄岩石圈下约15-30公里深处形成的地幔柱熔融结晶组合更辉长岩。速度和密度梯度在0.6-1.5 GPa的压力范围内特别强，这是由于随着熔融压力（和程度）的增加，熔体变得更加富含MgO，斜长石被橄榄石取代。这种异常的密度梯度表明可能存在一种过滤效应，即在非常年轻和薄的海洋岩石圈下相对较浅的深度处平衡的地幔柱熔体可能具有接近辉长岩（镁铁质）的成分（~6-10% MgO），而超镁铁熔体（MgO ~ 12-20%）形成于较老之下，较厚的大洋岩石圈必须下沉，并经历广泛的橄榄石和单斜辉石分馏，然后才能演化出具有足够浮力的玄武岩成分的残余岩浆，以便在表面喷发。一项对侵位时岩石圈年龄变化很大的热点地区的调查表明，深地壳和上地幔的地震折射数据与这一假设一致。这些结果突出了大体积侵入过程在热点岩浆演化中的重要性，在大多数情况下，侵入体积明显大于喷发的熔岩。地幔岩熔融可以解释，一阶，为总地壳柱的岩浆产物，而替代模型，如选择性熔融的辉石岩斑点可能不能。
Seismic refraction studies of deep‐crustal and upper mantle structure beneath some oceanic hotspot provinces reveal the presence of ultramafic bodies with P‐wave velocities of Vp ~ 7.4–8.0 km/s lying at or above the Moho, e.g., Hawaii, the Marquesas, and La Reunion. However, at other hotspot provinces such as the Galapagos, Nazca Ridge, and Louisville the lower crust is intruded by large volumes of gabbroic (mafic) rocks (Vp ~ 6.8–7.5 km/s). Ultramafic primary melts formed beneath mature oceanic lithosphere at pressures of ~2–3 GPa (60–90 km depth), and ponded at the Moho due to their relatively high density, can explain the observed ultramafic deep‐crustal bodies. By contrast, plume melts formed at depths of ~15–30 km beneath thin lithosphere crystallize assemblages that are more gabbroic. The velocity and density gradient is particularly strong in the pressure range 0.6–1.5 GPa due to the replacement of plagioclase by olivine as melts become more MgO‐rich with increasing pressure (and degree) of melting. This anomalous density gradient suggests a possible filtering effect whereby plume melts equilibrated at relatively shallow depths beneath very young and thin oceanic lithosphere may be expected to be of nearly gabbroic (mafic) composition (~6–10% MgO), whereas ultramafic melts (MgO ~ 12–20%) formed beneath older, thicker oceanic lithosphere must pond and undergo extensive olivine and clinopyroxene fractionation before evolving residual magmas of basaltic composition sufficiently buoyant to be erupted at the surface. A survey of well‐studied hotspot provinces of highly‐varying lithospheric age at the time of emplacement shows that deep‐crustal and upper mantle seismic refraction data are consistent with this hypothesis. These results highlight the importance of large‐volume intrusive processes in the evolution of hotspot magmas, with intrusive volumes being significantly larger than those of the erupted lavas in most cases. Pyrolite melting can account, to first order, for the total crustal column of magmatic products, whereas alternative models such as selective melting of pyroxenite blobs probably cannot.