All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe‐Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene‐dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust‐mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low‐density, residual hawaiitic melts. Minor assimilation of MORB‐related wall rocks, reflected by a trend toward lower 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low‐density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent.

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Mauna Kea Volcano has three exposed rock units. Submarine shield‐building tholeiites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70–240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4–66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite‐rich olivine (up to Fo90.5) providing evidence for MgO‐rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (<2%) of melting of a garnet Iherzolite source which had near‐chondritic abundances of heavy rare‐earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe‐Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated phenocryst‐rich, high‐MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene‐dominated fractionation at lower crustal pressures.

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SUMMARY:The aeration of culture media can be measured in terms of q5L (defined below) which is dependent on rate of air flow, agitation, etc. Two methods of measuring q5L are given, based on the polarographic estimation of dissolved oxygen. For penicillin and streptomycin fermentations there is a correlation between the amount of antibiotic produced and q5 L .Recent advances in industrial stirred and aerated fermentations have emphasized the need for a suitable technique to measure aeration in terms of the oxygen available to the organism rather than in terms of the volume of air which is passed through the medium. In the case of submerged cultures, the organism is dependent upon the oxygen dissolved in solution. In order to assess the efficiency of the aeration it is therefore necessary to compare the rate of solution of oxygen into the culture medium with the rate of consumption of oxygen by the organism. Determination of the oxygen demandThe Warburg apparatus is usually used for measuring oxygen demands, but in the present work it was found convenient to use a polarographic method first used by Petering & Daniels (1938). The culture is saturated with air and then the air supply is cut off. The concentration of dissolved oxygen in the culture medium decreases steadily from the saturation concentration to zero as the organism consumes the oxygen. A typical determination is shownin Fig. 1, from which it will be seen that the graph of the concentration of dissolved oxygen against time is linear, i.e. the oxygen demand is of zero order with respect to time and oxygen concentration over the period of measurement. This behaviour was always found with the organisms studied, and in the following theory it is assumed that the oxygen demand at a particular moment is of zero order. The polarographic measurements required only a few minutes so that no significant growth occurred during the measurement. The oxygen demand of the culture varies comparatively slowly with time as growth proceeds, and by measuring the oxygen demand of samples taken a t various times throughout the culture period it is possible to draw the curve connecting oxygen demand with the time after the inoculation of the medium.It has many times been noted (for a review see Tang, 1933) that the oxygen demand of many organisms is of zero order although only above a critical oxygen tension. Below the critical oxygen tension, the oxygen demand becomes first order. The critical oxygen tensions found are very low. For instance, in the present work the critical oxygen tensions were too low to be apparent from curves such as those shown in Fig. 1, so that the assumption that the oxygen demand is of zero order is justified.