Examining the Effect of Olivine Addition on Olivine Liquid Equilibrium Temperatures: Implications for the Temperatures of the North Atlantic Igneous Province

Abstract:

Large igneous provinces (LIPs) and seaward dipping reflectors (SDRs) are massive outpourings of basaltic magma associated with supercontinent breakup. One hypothesized driver for both the breakup and the volcanism is the impingement of a hot mantle plume at the base of the supercontinent. Plume genesis requires an excess temperature of at least 160-200 degrees Celsius (Sleep, 1990). The temperature of the mantle source for LIPs and SDRs can in theory be determined from magmas using relevant thermometers, such as those based on olivine-melt equilibrium. However, due to magmatic differentiation, all magmas must be back-corrected to their primary compositions by reversing the fractional crystallization process until an assumed forsterite content of the residual mantle is attained (Herzberg et al., 2007; Lee et al., 2009; Putirka, 2005).

We compiled a database of North Atlantic Igneous Province (NAIP) and Central Atlantic Magmatic Province (CAMP) magmas and olivines. From this dataset, we observed a linear trend in NAIP lavas in Ni-MgO space for high Mg number (Mg/(Mg+Fe)) samples. This linear trend cannot be replicated by fractional crystallization, which predicts rapid depletion of Ni. This suggests that most of the high Mg number magmas in the NAIP are better explained as mixtures of melts with excess olivine. Plotting FeO versus Ni allows us to identify magmas that have escaped olivine addition. These magmas have lower total FeO then previous magmas assumed to be primitive. All other assumptions remaining the same, a lower FeO will result in 60 degree cooler temperatures.

Finally, our compilations show that olivine forsterite contents exhibit a bimodal distribution. This could be explained if the high forsterite olivines are xenocrystic while the lower forsterite olivines are primary. Using the lower Mg number olivines along with the lower FeO contents, we arrive at primary magma temperatures <1425 Celsius and within 75 degrees of MORB (1350) (Lee et al., 2009). This result suggests that the NAIP and CAMP may not be driven by a thermal plume. Implying that supercontinent breakup is driven by far-field forces and that LIPs and SDRs are due to lithospheric extension. If our conclusions are correct, the creation of large amounts of magmas from extension instead of a thermal anomaly is perplexing.