VOLUMETRIC CALCULATION OF THE PACIFIC MANTLE 0-30 MA

Since the breakup of Pangea, the Pacific Basin has been shrinking and since about 30 mya, the mantle beneath the basic has been confined by the surrounding continental cratons and subducting slabs. We propose that the asthenosphere between 100-200 km depth is horizontally blocked by these features and vertically blocked by the lithosphere and internal pressure. The mantle to 440 km is similarly vertically blocked and depending on craton depth, may be blocked horizontally. Therefore, the upper 440 km of mantle as well as the asthenosphere may be a semi-closed system. To determine if it was a closed system we calculated the output of the mid-ocean ridge (MOR) and oceanic lithosphere underplating in 5 million year increments from 0-30 mya. Volumes were calculated using age vs thickness equations. The theoretical volume of mantle available to supply the MOR in a closed system was calculated using slab rollback rates to calculate both the asthenospheric and mantle volume displaced towards the MOR. The results show a 106-108 difference between outputs and inputs. This is to suggest the amount of mantle added at the MOR is significantly higher than that which is given from subducting slabs at both the 100-200 km depth and to 440 km depth. This is held as a near linear difference as time increases. This is to suggest that the subducting plates, on their own, do not produce enough to explain the amount the ridge requires, but rather that the MOR is supplied by other means. However, the volumetric calculations do not take into account displacement by cooling which could significantly impact the overall volume of the Pacific Mantle.