Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

[1] We present an estimate for the composition of the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent-daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell-Masuda diagram (spider diagram). The remaining elements of the Coryell-Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low-degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2003GC000597

Composition of the depleted mantle

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot-spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow-spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow-spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

Oceanic crustal thickness from seismic measurements and rare earth element inversions

Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium

Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14'N, MAR)

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

MORB alteration : rare-earth element/non-rare-earth hygromagmaphile element fractionation

Different parts of the world ridge system have quite different morphologies, which reflect different constructional processes. It appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust. This hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes. The TAG (26°N) and Snake Pit (23°N) hydrothermal sites on the Mid-Atlantic Ridge (MAR) display Mn/CH4 ratios in overlying seawater similar to those of nonsedimented East Pacific Rise sites, i.e., about 0.2 mol/L. In contrast, large methane anomalies with very low Mn/CH4 ratios of 0.005 mol/L are associated with ultramafic topographic highs near 15°N, close to the axis and on ultramafic walls of the rift valley. The association of ultramafic bodies and CH4 anomalies in seawater indicates active serpentinization processes. CH4 is produced during serpentinization according to the Fisher Tropsch reaction (Charlou et al., 1991). Many ultramafic rock assemblages have been sampled on the MAR away from fracture zones, in particular by the Deep Sea Drilling Project. Changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges.

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Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH4 output

Mantle heterogeneity from trace elements: MAR triple junction near 14°N

Hydrothermal effluents enriched in gases such as helium, hydrogen and methane have been found outgassing from mid-ocean ridge zones, crystal rifts, volcanoes and other areas of tectonic activity. Although methane is not conservative in seawater, its analysis on board ship can be used qualitatively or semi-quantitatively to locate hydrothermal fields as a crusie progresses. Between 12° and 15°N over the Mid-Atlantic Ridge, CH 4 anomalies (up to 44 nl −1 in the water column reveal the presence of plumes originating from hydrothermal discharged. The large amplitude of the CH 4 anomaly at one station (HY-36) integrated over more than 1000 m, reflects a large CH 4 input and thus extensive hydrothermal activity in this slow spreading section similar to inputs from fast spreading sited like the East Pacific Rise.

Leonid Dmitriev

Papers

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

MORB alteration : rare-earth element/non-rare-earth hygromagmaphile element fractionation

Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH4 output

Mantle heterogeneity from trace elements: MAR triple junction near 14°N

Hydrothermal CH4 between 12°N and 15°N over the Mid-Atlantic Ridge