Metasomatism of oceanic gabbros by late stage melts and hydrothermal fluids: Evidence from the rare earth element composition of amphiboles

Abstract

We report new compositional and textural observations on amphiboles hosted in oceanic gabbros recovered from the slow spreading Mid‐Atlantic (MARK area) and Southwest Indian (Hole 735B) ridges and the fast spreading East Pacific Rise (Hess Deep) in an effort to find a set of reliable criteria that unequivocally distinguishes magmatic from hydrothermal amphibole. We find that there is no simple set of criteria that may be applied to distinguish between these parageneses. Rather, it is necessary to assess both a magmatic and hydrothermal origin for all amphibole grains in order to establish appropriate criteria for a particular sample suite. The rare earth element (REE) content of ∼75% of our amphibole grains may be accounted for by crystallization from residual melts; some of these melts were enriched in the light rare earth elements by the crystallization of apatite and/or amphibole. Of significance, only a small subset of these grains crystallized at magmatic temperatures (>825°C). We conclude that the grains that equilibrated at lower temperatures were initially magmatic but that their major elements were reset during subsolidus, hydrothermal alteration, and deformation. This decoupling of the major and rare earth elements was most prevalent in the Hess Deep and MARK suites, both of which were tectonically exhumed at amphibolite to greenschist facies conditions. The remaining REE data identify hydrothermal amphibole that formed by mineral‐scale reactions involving clinopyroxene, fluids, and ± plagioclase at subsolidus conditions. Evaluation of fluids in equilibrium with amphibole shows that deep hydrothermal fluids are unlike those venting at the seafloor in terms of REE contents and/or patterns.

Long‐Lasting Evolution of Layer 2A in the Western South Atlantic: Evidence for Low‐Temperature Hydrothermal Circulation in Old Oceanic Crust

“…Fluid circulation at a range of temperatures triggers alteration of the host rock close to the ridge axis, resulting in low or medium and sometimes even high-grade metamorphism (Alt et al, 1996;Christensen & Salisbury, 1972;Gillis & Meyer, 2001;Hayman & Karson, 2007;Kempner & Gettrust, 1982;Liou, 1979;Salisbury & Christensen, 1978;Wilson et al, 2006). Also, precipitation of secondary minerals alters the morphology of pores and cracks, causing an increase of seismic velocities (Carlson, 2010(Carlson, , 2014Wilkens et al, 1991).…”

Petrology and geochemistry of the lower ocean crust formed at the East Pacific Rise and exposed at Hess Deep: A synthesis and new results

“…Brown to green calcic amphibole rims clinopyroxene and forms discrete interstitial grains. Major and trace element data for pegmatitic amphibole from ODP Site 894 suggest a magmatic origin with subsequent interaction with hydrothermal fluids [ Gillis , 1996; Gillis and Meyer , 2001]. Hydrogen isotope ratios for amphibole separates from samples recovered near the crest of the intrarift ridge [ Agrinier et al , 1995; Früh‐Green et al , 1996], and fluid inclusion data for the Site 894 gabbros [ Kelley and Malpas , 1996], suggest the presence of magmatic fluids in these patches.…”

“…Here we report the results of new thermometric calculations for hydrothermal amphibole in samples from the southern slope of the intrarift ridge and additional data for the northern rift valley scarp (Figure 9; Table 4 and Table 5). Hydrothermal amphibole was identified on the basis of texture (microscopic and macroscopic vein fill, rims on clinopyroxene and olivine; Figures 3d–3f) and major element composition such that grains with TiO 2 contents ≥1 wt % are likely magmatic and hence were excluded [ Gillis and Meyer , 2001].…”

What underlies the Philippine island arc? Clues from the Calaton Hill, Tablas island, Romblon (Central Philippines)

“…Despite their geochemical differences, LREE-enrichment in the three types of amphiboles suggests that the all the amphiboles may have been affected and have undergone modification by an LREE-enriched melt (Tribuzio et al, 1999;Gillis and Meyer, 2001). Alternatively, they may have been modified by circulating aqueous fluids with the same REE content.…”

“…Amphibole may form as a late stage magmatic phase, crystallizing from chemically evolved hydrous melts or exsolved magmatic fluids as well as a hydrothermal phase replacing pyroxene and olivine or filling fractures in oceanic plutonic sequences (Mevel, 1987;Coogan et al, 2001;Gillis and Meyer, 2001). They therefore record the complex evolution from solidification of primary cumulates and magmatic fluids to alteration by seawater-derived fluids.…”