Geochemistry of boninite and harzburgite of ODP Leg 125 basement samples

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form
the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component.
The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250 infinity C and 1.5 kb for an average boninite) of boninite segregation. We thus envisage that boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 boninites and boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting.

在第125航次（Leg 125）中，于伊豆-小笠原（Izu-Ogasawara）和马里亚纳（Mariana）前弧地体的火山岩及超基性基底中钻进的钻孔，为西太平洋始新世（Eocene）俯冲作用启动后形成的部分最早期岩石圈（lithosphere）提供了研究数据。该火山基底包含三套玻安岩（boninite）系列与一套拉斑玄武岩（tholeiite）系列。（1）在786站位（Site 786）的外弧高地深地壳剖面最底部，以始新世低钙玻安岩及低钙古铜辉石安山岩枕状熔岩与脉岩为主。（2）786站位玻安岩质火山构造的主体部分，由始新世中钙玻安岩及其分异产物（古铜辉石安山岩、安山岩、英安岩及流纹岩）构成。（3）渐新世（Oligocene）早期的中钙至高钙玻安岩岩床或岩脉侵入该火山构造，并可能为786站位顶部的角砾岩单元提供物源。（4）782站位（Site 782）外弧高地的最顶部地层，由始新世或渐新世早期的拉斑质安山岩、英安岩及流纹岩构成。
上述四套岩石组合均可通过洋中脊处部分熔融作用发生亏损后的大洋岩石圈地幔在俯冲带上方发生重熔得到解释。我们估算，平均玻安岩的源区在俯冲作用启动后不久发生二次熔融（熔融程度5%~10%）前，已在洋中脊处损失了10%~15%重量百分比的熔体。方辉橄榄岩（harzburgite，<2%单斜辉石（clinopyroxene），橄榄石Fo值约为92%）的成分表明，其相对于富集型洋中脊玄武岩（MORB）地幔总计经历了约25%的熔融作用。玻安岩中铌（Nb）含量较低，说明俯冲作用发生前的大洋岩石圈并未受到任何软流圈（OIB）组分的富集改造。
俯冲组分具有如下特征：（1）相对于钐（Sm）、钛（Ti）、钇（Y）及中重稀土元素（REE）而言，锆（Zr）与铪（Hf）含量较高；（2）轻稀土元素富集；（3）相对于钍（Th）、铷（Rb）或镧（La）而言，铌与钽（Ta）含量较低；（4）钠（Na）与铝（Al）含量较高；（5）铅（Pb）同位素位于北半球参考线（Northern Hemisphere Reference Line）上。该组分与伊豆-马里亚纳地区或其他区域活动弧火山的俯冲组分均不相同。模拟结果显示，这些特征符合角闪岩相（amphibolite facies）板片熔融形成的奥长花岗岩质熔体（trondhjemitic melt）。经交代作用改造的地幔可能含有约0.15%重量百分比的水。
整体熔融环境受实验数据约束，对应玻安岩分离结晶的浅部深度与高温条件（平均玻安岩的温度为1250℃，压力为1.5千巴）。因此我们认为，玻安岩由经交代作用改造后的残余洋中脊玄武岩地幔底辟体发生减压熔融形成，而方辉橄榄岩则作为该第二阶段熔融作用中最上部、亏损程度最高的残余物留存下来。热学约束条件表明，在玻安岩形成时期，俯冲岩石圈与上覆地幔楔的大洋岩石圈均需非常年轻。该结论与如下模型一致：太平洋地区始新世板块重组后，一条切割两条洋脊轴的活动转换断层处于挤压或压扭环境中。
将第125航次的玻安岩与西太平洋其他区域的玻安岩及相关岩石进行对比，可发现区域间在岩石成因方面存在显著差异，具体体现在地幔矿物学、部分熔融程度、俯冲组分成分以及俯冲前岩石圈性质等方面。从区域尺度来看，俯冲作用的启动可能涉及不同年龄与成分的俯冲地壳及岩石圈地幔楔，这在该类构造环境中实属常见。