| 3892 |
abstract |
We present a mathematical model for the evolution of the (226Ra / 230Th) activity
ratio during simultaneous fractional crystallization and ageing of magma. The model
is applied to published data for four volcanic suites that are independently known
to have evolved by fractional crystallization. These are tholeiitic basalt from Ardoukoba,
Djibouti, MORB from the East Pacific Rise, alkali basalt to mugearite from Vestmannaeyjar,
Iceland, and basaltic andesites from Miyakejima, Izu–Bonin arc. In all cases (226Ra
/ 230Th) correlates with indices of fractional crystallization, such as Th, and the
data fall close to model curves of constant fractional crystallization rate. The best
fit rates vary from 2 to 6 × 10− 4 yr− 1. Consequently, the time required to generate
moderately evolved magmas (F ≤ 0.7) is of the order of 500 to 1500 yrs and closed
magma chambers will have lifetimes of 1700 to 5000 yrs. These rates and timescales
are argued to depend principally on the specific power output (i.e., power output
per unit volume) of the magma chambers that are the sites of fractional crystallization.
Equating the heat flux at the EPR to the heat flux from the sub-axial magma chamber
that evolves at a rate of ca. 3 × 10− 4 yr− 1 implies that the magma body is a sill
of ca. 100 m thickness, a value which coincides with independent estimates from seismology.
The similarity of the four inferred differentiation rates suggests that the specific
power output of shallow magma chambers in a range of tectonic settings covers a similarly
narrow range of ca. 10 to 50 MW km− 3. Their differentiation rates are some two orders
of magnitude slower than that of the basaltic Makaopuhi lava lake, Hawaii, that cooled
to the atmosphere. This is consistent with the two orders of magnitude difference
in heat flux between Makaopuhi and the East Pacific Rise. (226Ra / 230Th) data for
magma suites related by fractional crystallization allow the magma differentiation
rate to be estimated and, from this, the thermal budget of the magma chamber addressed,
and where an independent measurement of heat flux exists, to place constraints on
the size of the magma chamber. Such results have the potential to constrain the likely
timescale and size of future eruptions of evolved magmas. |