TY - JOUR
T1 - Heck and Heckle seamounts, northeast Pacific Ocean
T2 - high extrusion rates of primitive and highly depleted mid-ocean ridge basalt on off- ridge seamounts
AU - Leybourne, M. I.
AU - Van Wagoner, N. A.
PY - 1991
Y1 - 1991
N2 - We analyzed, petrographically and chemically, basalt from
eight dredge hauls from the Heck and Heckle seamounts, northeast Pacific Ocean.
Major elements were determined for mineral, glass, and whole rock samples, and
trace and rare earth elements were determined for glass and whole rock samples.
The dredge hauls included hyaloclastites and fragments from sheet flows and
pillows. The clinkery fragments are interpreted to be deformed sheet flow tops,
characteristic of high effusion rates. The hyaloclastites recovered are
reworked deposits, as indicated by the wide compositional range of the glass
shards, abundance of clay and calcite matrix, and bedding. Most rocks are
aphyric, but the analyzed plagioclase and olivine phenocrysts and microcrysts
are equilibrium compositions and show minor compositional zonation (up to 7.5%
An, <1 %Fo). Calculated quenching temperatures range from 1228° to 1216°C,
decrease with decreasing Mg #, and are consistent with phase equilibria. The
seamount lavas are primitive (Mg # 68.7–59.5) and restricted in compositional
range ([La/Ce]n = 0.65–0.77). The basalts are equivalent to highly
depleted mid-ocean ridge basalt ([La/Ce]n = 0.29–0.41) and are at
the depleted end of the range in composition of basalts from East Pacific
seamounts. They are more depleted than basalts from the adjacent West Valley
spreading axis. The lavas can be produced by partial melting of a depleted
source. Mineralogical and geochemical evidence is consistent with rapid ascent
of the Heck and Heckle lavas through oceanic crust with little residence time
in magma chambers. Lavas of the Heck and Heckle Seamount chains have a limited
range of incompatible element ratios, whereas the adjacent West Valley Segment
of the Juan de Fuca Ridge is highly heterogeneous. In contrast, lavas from the
East Pacific near-ridge seamounts exhibit a wider range of incompatible element
ratios than do the adjacent East Pacific Rise basalts. On the West Valley
Segment, magma supply is less robust associated with lower spreading rates
compared to the East Pacific Rise at 10°N. In contrast, at fast spreading
centers robust melting produces a mixed mantle signature in axial lavas, while
suppressed melting at the seamounts reveals the heterogeneities. We suggest
that at some spreading ridges, more fertile portions of the mantle are
preferentially melted such that the outwelled portions of the mantle tapped by
the seamounts are more depleted.
AB - We analyzed, petrographically and chemically, basalt from
eight dredge hauls from the Heck and Heckle seamounts, northeast Pacific Ocean.
Major elements were determined for mineral, glass, and whole rock samples, and
trace and rare earth elements were determined for glass and whole rock samples.
The dredge hauls included hyaloclastites and fragments from sheet flows and
pillows. The clinkery fragments are interpreted to be deformed sheet flow tops,
characteristic of high effusion rates. The hyaloclastites recovered are
reworked deposits, as indicated by the wide compositional range of the glass
shards, abundance of clay and calcite matrix, and bedding. Most rocks are
aphyric, but the analyzed plagioclase and olivine phenocrysts and microcrysts
are equilibrium compositions and show minor compositional zonation (up to 7.5%
An, <1 %Fo). Calculated quenching temperatures range from 1228° to 1216°C,
decrease with decreasing Mg #, and are consistent with phase equilibria. The
seamount lavas are primitive (Mg # 68.7–59.5) and restricted in compositional
range ([La/Ce]n = 0.65–0.77). The basalts are equivalent to highly
depleted mid-ocean ridge basalt ([La/Ce]n = 0.29–0.41) and are at
the depleted end of the range in composition of basalts from East Pacific
seamounts. They are more depleted than basalts from the adjacent West Valley
spreading axis. The lavas can be produced by partial melting of a depleted
source. Mineralogical and geochemical evidence is consistent with rapid ascent
of the Heck and Heckle lavas through oceanic crust with little residence time
in magma chambers. Lavas of the Heck and Heckle Seamount chains have a limited
range of incompatible element ratios, whereas the adjacent West Valley Segment
of the Juan de Fuca Ridge is highly heterogeneous. In contrast, lavas from the
East Pacific near-ridge seamounts exhibit a wider range of incompatible element
ratios than do the adjacent East Pacific Rise basalts. On the West Valley
Segment, magma supply is less robust associated with lower spreading rates
compared to the East Pacific Rise at 10°N. In contrast, at fast spreading
centers robust melting produces a mixed mantle signature in axial lavas, while
suppressed melting at the seamounts reveals the heterogeneities. We suggest
that at some spreading ridges, more fertile portions of the mantle are
preferentially melted such that the outwelled portions of the mantle tapped by
the seamounts are more depleted.
UR - http://www.scopus.com/inward/record.url?scp=0026309956&partnerID=8YFLogxK
U2 - 10.1029/91JB00595
DO - 10.1029/91JB00595
M3 - Article
AN - SCOPUS:0026309956
SN - 0148-0227
VL - 96
SP - 16275
EP - 16293
JO - Journal of Geophysical Research
JF - Journal of Geophysical Research
IS - B10
ER -