Observations of seafloor bathymetry and gravity surveys indicate that magma focuses in
the center of slow spreading Mid-Ocean Ridge (MOR) segments, however; it is not well
constrained how magma is generated, stored, and transported to the segment ends. There
are two end-member models for magma transport: 1) a focused magma model wherein the magma
upwells beneath the entire ridge axis, is focused and pools beneath the center of the segment,
and is then transported towards the segment ends via lateral diking in the shallow crust and
2) a distributed magma model wherein magma vertically upwells and is erupted on the seafloor
along the entire segment, but there is enhanced focusing in the segment center. (Figure 1).
Both models are supported by the bathymetric and geophysical observations but have different
implications for the chemistry of lavas erupted along the segment.
To test how lava chemistries vary along a slow-spreading MOR, we systematically sampled a segment
of the Mid-Atlantic Ridge. The segment (~14°N) (Figure 2) is known to host Popping Rocks,
gas-rich basalts which, upon reaching surface pressures, explode. Two expeditions to this region
in 2016 and 2018 collected both ship-based bathymetry (75 m gridded resolution) aboard the R/V
Atlantis and high-resolution bathymetry (1 m) from the Autonomous Underwater Vehicle (AUV)
Sentry. 27 dives from the Human Occupied Vehicle (HOV) Alvin collected 382 lavas
all of which have been analyzed for major element contents, and 162 have been analyzed for trace
element contents. During these expeditions, samples were collected both along and across axis from
the magmatically robust segment center, through a transition region, to a sparsely magmatic region.
Analytical results show that there is significant chemical variability along this segment. For
example, there is less variability at the segment center (K/Ti ratios from 0.24 to 0.46 and La/Sm
from 2.58 to 3.59) compared to the sparsely magmatic region (K/Ti values from 0.06 to 0.42 and
La/Sm). This suggests that magmas erupted at the segment center are more homogeneous compared to
lavas erupting in the sparsely magmatic region. Major element contents in each region vary, but
on average, become more mafic moving southward away from the magmatically robust segment center
towards the sparsely magmatic region. Petrologic modeling of fractional crystallization and trace
element contents show that fractional crystallization dominates the chemical variability in the
sparsely magmatic region, while either extent of melting or differing mantle sources dominates
the variability in the transition regions and the sparsely magmatic region. Reconciling these
data with both physical and geophysical observations of a slow spreading ridge, we present a
model of magma generation, storage, and transport that is a hybrid of the two proposed models.
Trace and major element contents, microbial communities, and enzymatic activities of urban soils of Marrakech city along an anthropization gradient
