Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in saprolites and bauxites
Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in saprolites and bauxites
复制标题
DOI：
10.1016/j.chemgeo.2021.120103
复制
发表时间：
2021-04
期刊：
影响因子：
3.9
通讯作者：
A. Greaney;R. Rudnick;S. Romaniello;Aleisha C. Johnson;A. Anbar;M. Cummings
中科院分区：
文献类型：
--
作者：
A. Greaney;R. Rudnick;S. Romaniello;Aleisha C. Johnson;A. Anbar;M. Cummings
美国南卡罗莱纳变质辉绿岩上形成的腐辉岩和俄勒冈州和华盛顿州哥伦比亚河玄武岩上形成的两种含铁铝土矿3条深风化剖面的钼同位素特征阐明了大陆风化过程中的钼同位素行为。腐岩记录了Mo相对于新鲜基岩的总体损失，这可以用负的τMoTi来表示，它定义了Mo相对于相对不动元素Ti的损失。变质岩的δ98Mo值在- 0.89‰~ - 0.05‰之间（相对于NIST3134），平均δ98Mo = - 0.40‰，而下伏未风化基岩的δ98Mo值为+0.55‰。相比之下，含铁铝土矿相对于新鲜基岩（τMoTi为0 ~正）通常有Mo的添加，δ98Mo值一般高于母岩玄武岩：铝土矿的δ98Mo值在- 0.14‰~ +0.38‰之间，而未风化的母岩玄武岩的δ98Mo值为- 0.33‰和+0.02‰。腐泥岩δ98Mo值低可能反映了风化过程中同位素轻Mo优先吸附在辅助铁氧氢氧化物和粘土上，而铝土矿δ98Mo值高反映了地下水中同位素重Mo的加入。三种剖面的δ98Mo与τ motii值存在正相关关系，表明大陆风化过程中Mo损失时，形成的风化层同位素较轻，而地下水的加入则使风化层同位素较重。由于腐岩是比铝土矿更常见的风化产物，因此我们得出结论，一般来说，大陆风化将Mo同位素分馏，使风化的上地壳保留了同位素轻的Mo。相反，从风化地壳中浸出Mo的地下水同位素重。因此，大陆化学风化作用形成了在全球范围内观测到的重同位素河流特征，并部分促成了重同位素海水特征。最后，这些数据与先前发表的冰川二晶岩数据相结合，可以用来评估过去2.9 Ga的地壳Mo同位素特征的变化。
Molybdenum isotopes in three deep and well-characterized weathering profiles – a saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMoTi, which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰NIST3134for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMoTi) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMoTiand δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.