The roof of an axial magma chamber: A hornfelsic heat exchanger

Gillis, K. M.

doi:10.1130/g24590a.1

Cited by 71 publications

(102 citation statements)

References 2 publications

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“…The granoblastic dikes and underlying dike screens represent the conductive boundary layer between mafic magma and the overlying hydrothermal system, and the rocks from Hole 1256D are similar to those observed in ophiolites and elsewhere in oceanic crust (e.g., Gillis and Roberts, 1999;Gillis, 2008;France et al, 2009). The granoblastic basalt sampled beneath Gabbro 2 during Expedition 335 is part of a dike screen within the transition from sheeted dikes to gabbros, consistent with the presence of a significant underlying gabbro heat source.…”

Section: Alteration and Metamorphismmentioning

confidence: 99%

Expedition 335 summary

Teagle¹,

Ildefonse²,

Blum³

et al. 2012

Proceedings of the IODP

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Section: Alteration and Metamorphismmentioning

confidence: 99%

Expedition 335 summary

Teagle¹,

Ildefonse²,

Blum³

et al. 2012

Proceedings of the IODP

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“…Gillis and Coogan, 2002), and are consistent with melting at the top of the shallow melt lens. The depth of the melt lens, however, may vary due to migration within crust over time and, therefore, the melt may interact with the upper gabbros and/ or lower sheeted dikes (Gillis, 2008). If alteration is caused by circulation of hydrothermal fluids, it is likely that this is largely restricted by permeability to the base of the sheeted dikes and uppermost gabbros of Layer 3.…”

Section: Composition Of Potential Assimilantsmentioning

confidence: 99%

Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

et al. 2011

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a b s t r a c t a r t i c l e i n f oMost geochemical variability in MOR basalts is consistent with low-to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR highsilica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H 2 O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ 18 O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ 18 O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.

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“…values derived from metamorphic data at the dike-gabbro transition in ophiolites and at fast-spread oceanic settings (e.g., Hess and Pito Deep [Gillis, 2008]). Assuming that heat is extracted over a 1 km-wide AMC [Kent et al, 1990] this heat flux would provide about 30 MW per km of ridge, which is within estimates of time-averaged values of heat available per kilometer of ridge axis [Wilcock and Delaney, 1996;Mottl, 2003;Cannat et al, 2004].…”

Section: Heat Fluxesmentioning

confidence: 99%

“…In the light of our model, this could be explained in two ways: 1) that subcritical (in regard to hydrothermal convection) permeabilities (<10 −16 m 2 ) characterize the axial lithosphere above high crystallinity AMC sections, or 2) that crustal permeability above the mushy sections is moderate (∼10 −14 -10 −15 m 2 ) and comparable to values used in our model (Figure 3), but that high-temperature fluids do not make it to the seafloor because of fluid-rock interactions (e.g., mineral precipitation [Fontaine et al, 2001]), mixing and cooling of hydrothermal fluids along the volcanic sections, and/or permeability structure [e.g., Wilcock, 1998]. In this latter case, the hydrothermal circulation will mine heat from the AMC and either replenishment rates commensurate to the AMC cooling rates reproduced in our models are required to ensure steady-state, or the magmato-hydrothermal transition will propagate downward as described by Gillis [2008]. In this respect, it is worth noting that our model considers a uniform permeability hydrothermal layer.…”

Section: Amc Segmentationmentioning

confidence: 99%

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

Fontaine

Olive

Cannat

et al. 2011

Geophys. Res. Lett.

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[1] The heat output and thermal regime of fast and intermediate spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber (AMC), which remain to be fully understood. Here, we model the interactions between a shallow two-dimensional cellular hydrothermal system at temperatures <700°C, and a deeper AMC at temperatures up to 1200°C. We show that hydrothermal cooling can freeze the AMC in years to decades, unless melt injections occur on commensurate timescales. Moreover, the differential cooling between upflow and downflow zones can segment the AMC into mush and melt regions that alternate on sub-kilometric length scales. These predictions are consistent with along-axis variations in AMC roof depth observed in ophiolites and oceanic settings. In this respect, fine-scale geophysical investigations of the structure of AMCs may help constrain hydrothermal recharge locations associated with active hydrothermal sites.

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The roof of an axial magma chamber: A hornfelsic heat exchanger

Cited by 71 publications

References 2 publications

Expedition 335 summary

Expedition 335 summary

Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

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