26 million years of mantle upwelling below a segment of the Mid Atlantic Ridge: The Vema Lithospheric Section revisited
26 million years of mantle upwelling below a segment of the Mid Atlantic Ridge: The Vema Lithospheric Section revisited
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DOI：
10.1016/j.epsl.2009.05.046
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发表时间：
2009-07-30
影响因子：
5.3
通讯作者：
Ligi, Marco
中科院分区：
文献类型：
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作者：
Cipriani, Anna;Bonatti, Enrico;Ligi, Marco
从沿沿着一个>300公里长的海洋岩石圈剖面（Vema岩石圈剖面或VLS）取样的橄榄岩中，重建了20至4 Ma前大西洋中脊80公里长的部分下地幔上涌的温度和成分的时间变化，该剖面暴露于北纬11度的Vema变换以南[Bonatti，E.，Ligi，M.，Brunelli，D.，Cipriani，A.，Fabretti，P.，Ferrante，V.，加斯佩里尼湖，科多利尼湖，2003.中大西洋海岭下的地幔热脉冲和大洋岩石圈形成的时间变化，自然，423，499-505]。我们延长这一时间间隔从26至2马采样地幔超镁铁岩在18个新的网站沿着的VLS。橄榄岩斜方辉石、单斜辉石和尖晶石化学特征表明，26 ~ 18.5Ma前地幔熔融程度减弱，叠加短波长（类似于4 Ma）振荡，随后18.5 ~ 2 Ma前熔融程度稳定增加，叠加3-4 Ma振荡。由重力资料计算的剩余地幔布格异常推断的地壳厚度的时间变化也显示出类似的趋势。老地幔套（26 ~ 18.5Ma）和新地幔套（18.5 ~ 2 Ma）的cpxNa_2 O含量和CaO/Al_2 O_3比值不同，表明这两套地幔不仅热体制不同，而且地幔源区的组成也可能不同。这两个趋势被一个类似于1.4 Ma长的伸展（从18.2到16.8 Ma）分开，变形超镁铁质糜棱岩占主导地位，这可能表明在脊轴处有一个近岩浆岩石圈侵位的间隔，对应于一个热极小期。沿着地壳厚度的VLS的空间偏移相关（即，地幔释放的玄武质熔体量）和地幔橄榄岩熔融程度导致固体地幔上涌平均速度的估计值接近16.1 mm/a，接近VLS覆盖的26 Ma间隔的平均半扩展速率。然而，橄榄岩单斜辉石地质测温显示振荡叠加在计算的平衡温度从26至18.5 Ma前的下降趋势，其次是稳定的增加，从16至4 Ma前，这表明固体地幔上涌速度随时间而变化。这些结果暗示了大西洋中脊北方存在类似10-20 Ma的活动旋回。(C)2009 Elsevier B. V.保留所有权利。
Temporal variations of temperature and composition of the mantle upwelling below a 80-km long segment of the Mid Atlantic Ridge were reconstructed from 20 to 4 Ma ago from peridotites sampled along a >300-km long section of oceanic lithosphere (Vema Lithospheric Section or VLS) exposed south of the Vema transform at 11 degrees N [Bonatti, E., Ligi, M., Brunelli, D., Cipriani, A., Fabretti, P., Ferrante, V., Gasperini, L., Ottolini, L., 2003. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variation in the formation of oceanic lithosphere, Nature, 423, 499-505]. We extended this time interval from 26 to 2 Ma by sampling mantle ultramafics at 18 new sites along the VLS. Peridotite orthopyroxene, clinopyroxene and spinel chemistry suggest a weak trend of decreasing extent of melting of the mantle from 26 to 18.5 Ma ago with superimposed short-wavelength (similar to 4 Ma) oscillations followed by a steady increase of degree of melting from 18.5 to 2 Ma ago, with superimposed 3-4 Ma oscillations. Temporal variations of crustal thickness inferred from the Residual Mantle Bouguer Anomaly calculated from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2 Ma) mantle suites differ in cpx Na2O content and CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might have been different in the two suites. The two trends are separated by a similar to 1.4 Ma-long stretch (from 18.2 to 16.8 Ma) where deformed ultramafic mylonites prevail, indicating probably an interval of nearly a-magmatic lithospheric emplacement at ridge axis, corresponding to a thermal minimum. Spatially offset correlation along the VLS of crustal thickness (i.e., quantity of basaltic melt released by the mantle) and mantle peridotite degree of melting led to an estimate of similar to 16.1 mm/a for the solid mantle average velocity of upwelling, a value close to the average half spreading rate for the 26 Ma interval covered by the VLS. However, peridotite clinopyroxene geothermometry shows oscillations superimposed on a decreasing trend of calculated equilibration temperature from 26 to 18.5 Ma ago, followed by a steady increase from 16 to 4 Ma ago, suggesting that the solid mantle upwelling velocity varied through time. These results hint at the existence of similar to 10-20 Ma cycles in the activity of the northern Mid Atlantic Ridge. (C) 2009 Elsevier B.V. All rights reserved.