TY - JOUR
T1 - Evidence for magma mixing and a heterogeneous mantle on the West Valley segment of the Juan de Fuca Ridge
AU - Van Wagoner, N. A.
AU - Leybourne, M. I.
PY - 1991
Y1 - 1991
N2 - We studied basaltic rocks of the northern end of the Juan de
Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of
spatially related rocks of an active spreading center (West Valley and West
Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour
and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and
major, trace, and rare earth elements (REE). Crystallization temperatures and
equilibrium phase compositions were calculated. The complex zoning and
disequilibrium compositions of plagioclase and olivine megacrysts and
phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123-7),
and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks
range from primitive (up to Mg # = 68) in the West Valley south and West Ridge
areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West
Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley
axis span the range of Mg #. Rocks from West Valley are transitional to
enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge
(e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle
Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE
depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks
are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically
heterogeneous character of the lavas exists both within site and within a
single dredge haul (West Valley north and Endeavour Ridge rocks) and between
sites, indicating that the mantle source is heterogeneous on a variety of
scales. The range in incompatible element ratios can be modelled as binary
mixing between an enriched source and a depleted source. Further, trace element
systematics require variations in the degree of partial melting. Lava chemistry
and ridge morphology are consistent with southward propagation of spreading in
West Valley to form an overlap with the Endeavour Segment. West Valley (West
Valley north, central and south) is dominated by extension over volcanism and
is thus thermally and magmatically depressed. Rocks from the West Valley
Segment span a broader compositional range than ridge systems to the south,
reflecting the heterogeneity of the source and the lack of robust volcanism to
homogenize the chemistry. Zero age rocks from West Valley are similarly
enriched in [La/Ce]n as zero age rocks from the Endeavour and
Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the
older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de
Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment
of the mantle source north of the Cobb Offset has been a recent phenomena.
AB - We studied basaltic rocks of the northern end of the Juan de
Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of
spatially related rocks of an active spreading center (West Valley and West
Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour
and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and
major, trace, and rare earth elements (REE). Crystallization temperatures and
equilibrium phase compositions were calculated. The complex zoning and
disequilibrium compositions of plagioclase and olivine megacrysts and
phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123-7),
and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks
range from primitive (up to Mg # = 68) in the West Valley south and West Ridge
areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West
Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley
axis span the range of Mg #. Rocks from West Valley are transitional to
enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge
(e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle
Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE
depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks
are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically
heterogeneous character of the lavas exists both within site and within a
single dredge haul (West Valley north and Endeavour Ridge rocks) and between
sites, indicating that the mantle source is heterogeneous on a variety of
scales. The range in incompatible element ratios can be modelled as binary
mixing between an enriched source and a depleted source. Further, trace element
systematics require variations in the degree of partial melting. Lava chemistry
and ridge morphology are consistent with southward propagation of spreading in
West Valley to form an overlap with the Endeavour Segment. West Valley (West
Valley north, central and south) is dominated by extension over volcanism and
is thus thermally and magmatically depressed. Rocks from the West Valley
Segment span a broader compositional range than ridge systems to the south,
reflecting the heterogeneity of the source and the lack of robust volcanism to
homogenize the chemistry. Zero age rocks from West Valley are similarly
enriched in [La/Ce]n as zero age rocks from the Endeavour and
Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the
older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de
Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment
of the mantle source north of the Cobb Offset has been a recent phenomena.
UR - http://www.scopus.com/inward/record.url?scp=0026301269&partnerID=8YFLogxK
U2 - 10.1029/91JB00592
DO - 10.1029/91JB00592
M3 - Article
AN - SCOPUS:0026301269
SN - 0148-0227
VL - 96
SP - 169295
EP - 116318
JO - Journal of Geophysical Research
JF - Journal of Geophysical Research
IS - B10
ER -