Full page fax print - International Seabed Authority

eing derived from seawater 78. The extraterrestrial component (meteorite debris) in bulk
crusts is small. Platinum is a redox-sensitive element and its varying concentration
probably reflects changing seawater redox conditions, and diagenesis (maturation of
oxide phases and phosphatisation) within the older generation of crusts where it is
concentrated 79.
2.5. Phosphatisation of Fe-Mn Crusts
Most thick hydrogenetic Fe-Mn crusts that formed in the open Pacific consist of
two growth generations: A phosphatised older generation and a younger nonphosphatised
generation. Phosphatisation of older crust layers was widespread and is
found in crusts from many locations in the central and south Pacific 80. Crusts from
relatively deep water (>2800 m), such as from the Teahitia-Mehetia region 81 and from the
Tasman Rise 82, show only minor or no phosphatisation, which is also demonstrated by a
decrease in CFA-associated elements with increasing water depth of crust occurrence
(see next section). Phosphatised crusts are also not found in most continental-margin
environments, such as in the EEZ off western North America.
Precipitation of the old crust generation began for most crusts between about 55
and 25 Ma ago (e.g., 83) and was interrupted by several Cenozoic phosphogenic events 84.
Phosphogenesis entailed CFA impregnation of the older crust and formation of
phosphorite partings within and at the top of the older generation of some crusts.
Phosphatisation took place by CFA replacement of calcite infiltered into crust pore
spaces, by direct precipitation of CFA in pore space, and by replacement of other crust
phases, most commonly iron oxyhydroxide. The thickness of the phosphatised layer can
be as large as 12 cm, and calcium and phosphorus concentrations can increase up to 15
wt.% and 5 wt.%, respectively. Growth of the younger crust generation started during
the Miocene and continued to the present without interruption by further phosphogenic
events.
According to Hein et al. 85, two major and possibly several minor Cenozoic
episodes of phosphogenesis were responsible for formation of phosphorite on equatorial
Pacific seamounts. Those same phosphogenic events also phosphatised the older
generation of Fe-Mn crusts. Crust ages would allow for the major phosphogenic events
centred on the Eocene-Oligocene (˜34 Ma) and Oligocene-Miocene boundaries (˜24 Ma),
as well as the minor middle Miocene event at about 15 Ma, to have affected growth of
Pacific Fe-Mn crusts. Extraction and dating of the CFA from older crust generations
support the coeval phosphatisation of substrate rocks and crusts (Chan, Hein,
Koschinsky unpublished data).
64 International Seabed Authority