Occurrence and source of quartz in six basaltic soils from Northland, New Zealand

Abstract

Quartz abundance, distribution and oxygen isotope composition was determined for six basaltic soils from Northland, New Zealand. Quartz > 125 µm in size occurred in two soils, the Kiripaka and Ruatangata. ä18O values of 8.2% for the >250 µm sized quartz in both soils and 8.1-8.4¿ for that in the 250-125 µm size fraction of the Ruatangata indicate a high temperature origin for this quartz. A probable origin is from air-fall rhyolitic tephra (particularly Kaharoa Ash) erupted from the central North Island. Another potential source is quartzose beach sand from the western (windward) coast of Northland. Quartz from these beach sands has a ä18O value of 8.7¿, also indicating a high temperature origin. Quartz in the 63-20 µm size fractions of the basaltic soils has a mean 6180 value of 12.0¿, consistent with derivation from local sedimentary rocks as loess. Quartz in the 5-2 µm fraction of the soils has a mean ä18O value of 13.9¿, similar to that of aerosolic dusts previously reported from this latitude. The Kiripaka soil had anomalously high ä18O values of 17.3-19.6¿ for 5-2 µm sized quartz. This quartz can be shown to originate from marine shales in the vicinity of the soil profile sampled and indicates that local sources may contribute to the 'aerosolic' size fraction of soils in addition to tropospheric aerosolic dust.