Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

Abstract

International Ocean Discovery Program (IODP) Site U1437 recovered an 1,800‐m‐long sediment sequence in a volcano‐bounded basin on the Izu rear arc. Pore fluid studies revealed a pattern of repeated fluid inputs, fluid diffusion, and methane and ethane accumulations, which represent “fluid anomalies” that disturb the fluid profiles. First‐order reversal curve analysis, magnetic hysteresis, saturation isothermal remanent magnetization, and low‐temperature remanence cycling reveal a detrital input dominated by vortex‐state and multidomain magnetite, which passes through an initial stage of partial magnetite dissolution and greigite authigenesis in the upper few tens of meters. Progressive magnetic mineral diagenesis comprises the continued loss of fine‐grained magnetite and gradual pyritization of greigite and produces a background logarithmic decrease in saturation isothermal remanent magnetization normalized by magnetic susceptibility. This process implies a continuing slow supply of S2− to depths >1,500 m. Thermally driven diagenesis, which would cause extensive loss of greigite at these depths, does not appear to be significant here. Multidomain magnetite grains dominate the magnetic mineralogy in the deepest part of the sequence, but some single‐domain magnetite survives as inclusions in silicates. Fluid anomalies representing sulfate influx drive locally renewed greigite authigenesis, as do methane and ethane accumulations. In some cases, where methane is accompanied by H2S (“sour gas”), fine‐grained greigite is converted to pyrite. We term these multiple episodes of enhanced magnetic mineral alteration “punctuated magnetic mineral diagenesis.” Despite both progressive and punctuated magnetic mineral diagenesis, enough depositional remanence survives to allow recognition of the magnetostratigraphy to 1,320 m below seafloor.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

“…The ARM/SIRM ratio, sensitive to the magnetic grain‐size (Q. Liu et al., 2012; Peters & Dekkers, 2003), displays “base‐line” values of 4.9 × 10 −4 ± 1.2 × 10 −4 ( N = 31) and two prominent peaks at 1.7 × 10 −3 and 3.4 × 10 −3 that correspond to Anomalies A and B, respectively (Figure 4g). The SIRM/ χ ratio is sensitive to grain‐size and has also shown to increase with the introduction of greigite in diagenetically altered marine sediments (e.g., Larrasoaña et al., 2007; Musgrave et al., 2019; Snowball, 1991). It mostly mimics the variations in NRM intensity and χ described above (Figure 4h).…”

“…Rapid sedimentation rates and an associated upward shift of the SMTZ at continental margins favor the preservation of greigite over pyrite formation. However, a change in redox conditions has been widely described to drive enhanced or secondary mineral diagenesis (e.g., Housen & Musgrave, 1996; Larrasoaña et al., 2007; Musgrave et al., 2019). Localized enhanced magnetic mineral diagenesis in active deformation zones have previously been linked to fluid flux, methane trapped in high permeability fracture zones (e.g., Larrasoaña et al., 2007; Musgrave et al., 2019), methane accumulating below low porosity intervals and gas hydrate occurrences (Kars et al., 2019; Kars & Kodama, 2015).…”

“…While at its top Anomaly "A" displays a sharp peak from baseline values to its maximum expression, in its lower portion the magnetic properties more gradually return to base-line values with depth, indicating a progressive increase in greigite content. We suggest that the highest degree of diagenesis coincides with the most prolonged accumulation of solutes at the top of the main brittle fault zone, where fluids are trapped beneath less permeable hanging wall sediment and forced to migrate in a fault parallel direction (see also Musgrave et al, 2019). At first sight it appears contradictory that Anomaly "A" is capped at the top of the fault zone, even though the lower hanging wall forms a wider and fractured damage zone (Figure 2c).…”

Authigenic Greigite as an Indicator of Methane Diffusion in Gas Hydrate-Bearing Sediments of the Hikurangi Margin, New Zealand

“…There are two major possible explanations for the absence of greigite: (i) it was never preserved because biogeochemical conditions favored complete pyritization at the sulfatemethane transition zone (SMTZ) during burial (e.g., Schoonen, 2004), or (ii) greigite was formed during early/synsedimentary diagenesis and later reduced to pyrite when environmental conditions changed. Secondary diagenesis to pyrite may for example be caused by advective or diffusive transport of fluids and methane through the sediment (e.g., Musgrave et al, 2019). The high sedimentation rate at this active margin probably resulted in a rapid burial beneath the SMTZ during initial deposition which would have prevented complete pyritization and enabled the preservation of greigite.…”

Magnetic Mineral Diagenesis in a Newly Discovered Active Cold Seep Site in the Bay of Bengal

“…Destabilization of gas hydrates due to increase in pressure owing to sedimentation, burial and tectonic activities can release vast amount of methane; a fraction of which may rise up to seafloor and form cold seep (Haq, B.U, 1998;Henriet et al, 1998;Vogt and Jung, 2002;Sultan et al, 2004;Handwerger et al, 2017;Argentino et al, 2019). Diagenesis of magnetic minerals involves postdepositional changes either by altering the detrital magnetic mineral assemblages or through authigenic growth of secondary minerals (Roberts, 2015;Musgrave et al, 2019). Therefore, the sediment magnetic signals in marine sedimentary system represent the primary depositional and secondary diagenetic processes.…”