AN EMPIRICAL THERMAL HISTORY OF THE EARTHS UPPER-MANTLE

We have compiled petrological and geochemical data from 71 ophiolite s uites and greenstone belts, which range in age from 15 to 3760 Ma. We have selected those rocks whose compositions indicate that they are ei ther normal mid-ocean ridge basalts (MORBs) or hotspot-type MORBs. The n we used the data base to calculate the most primitive liquidus tempe rature for each rock suite. The results show that the liquidus tempera ture of the Phanerozoic ophiolites ranges from a low of 1212-degrees-C to a high of 1417-degrees-C. Using these data and two exponential cur ves bracketing the maximum and minimum temperatures versus time, we in fer that the Phanerozoic suites had a mean liquidus temperature of 127 2+/-7-degrees-C and a mean temperature range of 1218-degrees to 1425-d egrees-C. The liquidus temperatures of Archean MORBlike greenstones ra nge from 1305-degrees to 1576-degrees-C. Using these data and two expo nential curves bracketing the maximum and minimum temperatures versus time, we infer that Archean melts at 2.8 Ga had a mean liquidus temper ature of 1399+/-13-degrees-C and a temperature range from 1301-degrees to 1533-degrees-C. Using two different methods, we show that the chan ge in the mean liquidus temperature since the late Archean is from 96/-13-degrees-C (from temperature ranges) to 127+/-20-degrees-C (from t emperature means). When we convert these liquidus temperatures to pote ntial temperature of the mantle, we find that the change in the mean u pper mantle potential temperature since the late Archean is from 137+/ -8-degrees-C (from temperature ranges) to 187+/-42-degrees-C (from tem perature means). This change is less than that which was previously th ought to have occurred. We compared the liquidus temperatures calculat ed from our data set with an independent data set from the modem day P acific plate. The resulting histograms have the same shape and the sam e temperature range, showing that our method for calculating mantle te mperatures from MORBlike rocks in ophiolite suites is valid. When our calculated liquidus temperatures for all time intervals are plotted in histograms, the resulting distributions are not bimodal, but skewed u nimodal. That is, the distributions show a high-T tail which results f rom the presence of hotspot magmas in the data set. The Archean temper ature distribution is also skewed unimodal, and the high-temperature A rchean rocks, such as komatiites, plot in the hotspot area of the dist ribution. This strongly supports the contention that komatiites do not represent ''normal'' Archean mantle but rather were probably erupted by hotspots. Our data suggest that the relative proportion of hotspot magmas in oceanic lithosphere has remained nearly constant over geolog ic time.