Boron isotopic constraints on the source of Hawaiian shield lavas
Boron isotopic constraints on the source of Hawaiian shield lavas
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DOI：
10.1016/j.gca.2005.03.009
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发表时间：
2005-07
影响因子：
5
通讯作者：
R. Tanaka;E. Nakamura
中科院分区：
文献类型：
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作者：
R. Tanaka;E. Nakamura
用热电离质谱法分析了夏威夷三个代表性盾状火山（基拉韦厄火山、莫纳罗亚火山和库劳火山）熔岩的硼同位素组成。每个样品的硼同位素组成进行了两次分析，一次进行了酸浸，一次没有进行酸浸，以评估后续硼污染的影响。我们的酸浸程序溶解玻璃、橄榄石、二级沸石和吸附硼；这些溶解的硼被完全从残渣中除去，残渣由斜长石、辉石和新形成的无定形二氧化硅组成。我们证实，适当的酸浸工艺可以在不改变原始11b /10B比的情况下消除所有海底样品中吸附和掺入的硼污染。另一方面，当样品风化时，即橄榄石有一个边缘，耐酸矿物的11B/10B也发生了变化，因此无法从风化样品中得到爆发前的11B/10B值。通过这种事后污染的消除和评价程序，库拉火山（N = 8）、基拉韦厄火山（N = 11）和莫纳罗亚火山（N = 6）的盾状熔岩喷发前δ11B值分别为- 4.5 ~ - 5.4‰、- 3.6 ~ - 4.6‰和- 3.0 ~ - 3.8‰。历史基拉韦厄熔岩在δ11B不变的情况下，B含量和Nb/B与其他放射性成因同位素比值和微量元素比值具有系统的时间变化趋势，表明这些熔岩很少或没有同化地壳物质。库劳熔岩和莫纳罗亚熔岩的B含量和δ11B不相关也可能表明这些熔岩的地壳同化作用很小或没有影响。以基拉韦厄火山为代表的夏威夷源幔的Kea组分（与夏威夷熔岩中所谓的Kea端元相同）的来源应该是俯冲洋壳下部或再循环俯冲板块中的难熔橄榄岩。从基拉韦厄到库劳的系统趋势是δ11B减小，εNd减小，87sr /86Sr和206pb / 204pb增大，这与EMK（富集的玛卡普乌）组分（以库劳熔岩玛卡普乌期为代表）俯冲沉积组分的参与相一致。
Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original11B/10B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim,11B/10B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive11B/10B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ11B values for the shield lavas are −4.5 to −5.4‰ for Koolau (N = 8), −3.6 to −4.6‰ for Kilauea (N = 11), and −3.0 to −3.8‰ for Mauna Loa (N = 6). Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ11B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ11B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ11B coupled with decreasing εNd as well as increasing87Sr/86Sr and206Pb/204Pb—is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.