Geochemistry of basalts from ODP Hole 113-690C

Basalts from Maud Rise, Weddell Sea, are vesicular and olivine-phyric. Major, trace, and rare earth element concentrations are similar to those of alkali basalts from ocean islands and seamounts. The rocks are low in MgO, Cr, Ni, and Sc, and high in TiO2, K2O, P2O5, Zr, and LREE contents. The abundance of "primary" biotite and apatite in the matrix indicates the melting of a hydrous mantle. Prevalence of olivine and absence of plagioclase in the rocks suggests that the volatile in the melt was an H2O-CO2 mixture, where H2O was <0.5.
Mantle derived xenocrysts in the basalt include corroded orthopyroxene, chromite, apatite, and olivine. Olivine (Fo90) is too magnesian to be in equilibrium with the basalts, as they contain only 5-6 wt% MgO.
Based on the presence of mantle xenocrysts, the high concentration of incompatible elements, the spatial and chemical affinity with other ocean island basalts from the area, and the relative age of the basalt (overlain by late Campanian sediments), it is suggested that Maud Rise was probably generated by hot-spot activity, possible during a ridge crest jump prior to 84 Ma (anomaly 34 time).
Iddingsite, a complex intergrowth of montmorillonite and goethite, is the major alteration product of second generation olivine. It is suggested that iddingsite crystallized at low temperatures (<200°C) from an oxidized fluid during deuteric alteration.
Vesicles are commonly filled by zeolites which have been replaced by K-feldspars.

威德尔海（Weddell Sea）莫德海岭（Maud Rise）产出的玄武岩为气孔状（vesicular）且含橄榄石斑晶（olivine-phyric）。其主量元素、微量元素及稀土元素（rare earth element, REE）含量特征与洋岛及海山的碱性玄武岩（alkali basalt）相似。该类岩石MgO、Cr、Ni及Sc含量偏低，而TiO₂、K₂O、P₂O₅、Zr及轻稀土元素（LREE）含量偏高。基质中发育大量“原生”黑云母（biotite）与磷灰石（apatite），指示源区含水地幔发生熔融。岩石中橄榄石普遍发育且不含斜长石（plagioclase），表明熔浆中的挥发分以H₂O-CO₂混合体系为主，且H₂O含量小于0.5。
玄武岩中产出的地幔来源捕虏晶（xenocryst）包括熔蚀状斜方辉石（orthopyroxene）、铬铁矿（chromite）、磷灰石及橄榄石。其中橄榄石（Fo₉₀）的镁含量过高，无法与该玄武岩达到平衡，因为后者仅含5~6 wt%的MgO。
基于地幔捕虏晶的产出特征、不相容元素（incompatible element）的高富集度、与区域内其他洋岛玄武岩的空间及地球化学亲缘性，以及该玄武岩的相对年代（被晚坎潘期沉积物覆盖），研究认为莫德海岭可能形成于热点活动，极有可能发生在84 Ma（磁异常34期）之前的洋脊跃迁过程中。
伊丁石（iddingsite）是蒙脱石（montmorillonite）与针铁矿（goethite）的复杂交生体，为次生橄榄石的主要蚀变产物。研究表明，伊丁石是在低温（<200℃）条件下，由氧化流体在岩浆自蚀变（deuteric alteration）过程中结晶形成。
气孔通常被沸石（zeolite）充填，随后沸石被钾长石（K-feldspar）交代。