Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise
Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise
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DOI：
10.1007/s00410-002-0420-3
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发表时间：
2003-02-01
影响因子：
3.5
通讯作者：
Falloon, TJ
中科院分区：
文献类型：
--
作者：
Danyushevsky, LV;Perfit, MR;Falloon, TJ
东太平洋隆起北部Siqueiros转换断层中玄武岩橄榄石斑晶的熔融包裹体地球化学数据为中洋脊玄武岩（MORB）的岩石成因提供了新的认识。新鲜熔岩中含有约10%的橄榄石斑晶（Fo(89.3-91.2)）和罕见的小斜长石斑晶（< 1 mm），形状为半面状至不规则，成分多样(An（80-90), An(57-63)）。橄榄石斑晶中的熔融包裹体呈玻璃状，形状一般为圆形，大小从几个到200个不等。虽然大多数包裹体的组成与枕边玻璃的母熔体的代表大致一致，但有几个包裹体明显不同。其中一个包裹体含有高铝、低钛尖晶石的自面体晶粒，其组成不同于先前在MORB中从原始斑晶中描述的任何熔融包裹体。它具有非常高的Al2O3（约20 wt%），非常低的TiO2（约0.04 wt%）和Na2O（约1 wt%）含量，以及非常高的CaO/Na2O值（约14 wt%）。玻璃包裹体在所有不相容元素（La =0.052 ppm, Yb =0.34, La/Sm(n)接近0.27）中都被强烈地耗尽，但它具有较大的正Sr和Eu异常（Sr/Sr*接近30，Eu/Eu*接近3）和负Zr异常。它还具有低S （0.015 wt%）和相对较高的Cl （180 ppm）。我们认为，这种不寻常的成分是橄榄石将斜长石困在热的、强斜长石不饱和的岩浆中，以及随后斜长石与宿主橄榄石产生的熔体和残余尖晶石之间的反应的结果。另外两种不同橄榄石斑晶中的熔体包裹体的成分通常介于“正常”包裹体和铝包裹体之间，但含有更高的CaO和Sr含量。它们也有不相容元素的贫化，但贫化程度低于铝包裹体，且Sr和Eu异常较小。在冰岛和大西洋中脊北部的高磷橄榄石斑晶中也发现了类似的包裹体。我们认为这些包裹体的组成代表了辉长岩物质在热原始岩浆中的同化作用。这种同化作用的局域性与它发生在一个晶体糊状带是一致的，在这个区域，当原始岩浆穿过早期形成的辉长岩“堆积”时，孔隙度很高。在这样的环境中，污染物的成分应该是相当多样化的。虽然原始熔体与辉长岩物质的相互作用可能不会大规模地影响喷发的MORB熔体的成分，但这一过程可能在某些MORB套件中很重要，应该在岩石成因模型中加以考虑。另一个重要的含义是，在原始MORB斑晶中观察到的熔体包裹体成分的变化不一定总是反映地幔中发生的过程。特别是，基于超贫熔体包裹体的地球化学对分式熔融过程的推断可能并不总是有效的。
Geochemical data from melt inclusions in olivine phenocrysts in a picritic basalt from the Siqueiros Transform Fault on the northern East Pacific Rise provide insights into the petrogenesis of mid-ocean ridge basalts (MORB). The fresh lava contains similar to10% of olivine phenocrysts (Fo(89.3-91.2)) and rare, small (< 1 mm) plagioclase phenocrysts with subhedral to irregular shapes with a range of compositions (An(80-90), An(57-63))Melt inclusions in olivine phenocrysts are glassy, generally rounded in shape and vary in size from a few to similar to200 mum. Although most of the inclusions have compositions that are generally consistent with being representative of parental melts for the pillow-rim glasses, several inclusions are clearly different. One inclusion, which contains a euhedral grain of high-Al, low-Ti spinel, has a composition unlike any melt inclusions previously described from primitive phenocrysts in MORB. It has a very high Al2O3 (similar to20 wt%), very low TiO2 (similar to0.04 wt%) and Na2O (similar to1 wt%) contents, and a very high CaO/Na2O value (similar to14). The glass inclusion is strongly depleted in all incompatible elements (La =0.052 ppm; Yb =0.34; La/Sm(n) similar to0.27), but it has large positive Sr and Eu anomalies (Sr/Sr* similar to30; Eu/Eu* similar to3) and a negative Zr anomaly. It also has low S (0.015 wt%) and relatively high Cl (180 ppm). We suggest that this unusual composition is a consequence of olivine trapping plagioclase in a hot, strongly plagioclase-undersaturated magma and subsequent reaction between plagioclase and the host olivine producing melt and residual spinel. Two other melt inclusions in a different olivine phenocryst have compositions that are generally intermediate between 'normal' inclusions and the aluminous inclusion, but have even higher CaO and Sr contents. They are also depleted in incompatible elements, but to a lesser degree than the aluminous inclusion, and have smaller Sr and Eu anomalies. Similar inclusions have also been described in high-Fo olivine phenocrysts from Iceland and northern Mid-Atlantic Ridge. We suggest that the compositions of these inclusions represent assimilation of gabbroic material into the hot primitive magma. The localised nature of this assimilation is consistent with it occurring within a crystal mush zone where the porosity is high as primitive magmas pass through earlier formed gabbroic 'cumulates'. In such an environment the contaminants are expected to have quite diverse compositions. Although the interaction of primitive melts with gabbroic material may not affect the compositions of erupted MORB melts on a large scale, this process may be important in some MORB suites and should be accounted for in petrogenetic models. Another important implication is that the observed variability in melt inclusion compositions in primitive MORB phenocrysts need not always to reflect processes occurring in the mantle. In particular, inferences on fractional melting processes based on geochemistry of ultra-depleted melt inclusions may not always be valid.