Kvassnes
Astri Jæger Sweetman
Kvassnes
Astri Jæger Sweetman
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ArticleThe Atlantis Bank Gabbro Massif, Southwest Indian Ridge(SpringerOpen, 2019-11-14) Dick, Henry J. B. ; Kvassnes, Astri Jæger Sweetman ; Robinson, Paul T. ; MacLeod, Christopher J. ; Kinoshita, HajimuThis paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.
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ThesisEvolution of oceanic gabbros : in-situ and ancient examples(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2004-06) Kvassnes, Astri Jæger SweetmanThis study is a geochemical investigation into the accretion of lower oceanic crust and processes of shallow melt-rock reaction at mid-ocean ridges. Major-, trace-elements, and isotopes from whole-rocks and minerals from the Lyngen Gabbro, a 480-My old dismembered ophiolite from the Scandinavian Caledonides, indicate that this igneous complex was produced from hydrous supra-subduction zone magmas, a remnant of an incipient ocean-arc. Such ophiolites are better models for the structural evolution than the geochemical evolution of the lower oceanic crust at mid-ocean ridges. Minerals in gabbros from Atlantis Bank, Southwest Indian Ridge, a modern, insitu example of lower ocean-crust, were analyzed for major and trace-elements. The MELTS algorithm indicates that these gabbros formed by near-fractional crystallzation at mid-crustal pressures. The gabbroic crust is more evolved than the lavas and represents melts fractionated 50-95% relative to a mantle-derived melt-composition, supported by trace-element models. This argues against the often-cited gabbro-glacier accretion model, where mantle-derived melts are transported to a shallow melt-lens and fractionates there before eruption. There remain > 770-m of additional primitive cumulates below 1500-m deep Hole 735B or within the underlying mantle. Thus, the seismic Moho, beneath Hole 735B, could be the crust-mantle boundary, rather than an alteration front as suggested elsewhere. The Atlantis Bank gabbros have augites that are more primitive than plagioclases and olivines with which they coexist. Melt-rock interaction, where ascending melts dissolve the pre-existing gabbroic rocks and create hybrid magma may have caused this. Dissolution-experiments for plagioclase-olivine and plagioclase-augite mineral pairs were performed at 1180°-1330°C and 20-min - 24hrs. Dissolution occurs rapidly and out of equilibrium, with the dissolution rates dependent on the ΔT above the solidus. Rocks with small grain-boundary areas (coarse grained or nearly mono-mineralic) heat internally when enclosed in hot magma, causing xenoliths or wall-rock to melt and disaggregate. The dissolution-derived magma crystallizes minerals more refractory-looking than the melts that precipitated the original gabbroic rocks. Assimilation of gabbroic rocks increases the Na content and decreases the Fe content of the melt that digests it, thus basaltic glasses formed after this hybridization will falsely reflect a lower degree and pressure of mantle melting.