MEXICO CITY--President Carter and Mexican President Jose Lopez Portillo concluded their summit meeting yesterday with a joint communique agreeing to establish talks quickly that will determine a price framework for U.S. purchases of Mexican natural gas.

Carter told members of both houses of the Mexican congress shortly before his departure that his 48-hour visit helped settle differences between the United States and Mexico, although key issues dividing the two nations will take years to settle.

Lopez Portillo accepted Carter's suggestion to meet again in three months, and the two leaders also agreed on scientific exchanges, desert management and housing.

Carter returned to the White House for a meeting with Vice President Mondale and foreign policy advisers on developments in Iran before flying to Camp David, Md., for the weekend.

At a news conference after Carter left, Lopez Portillo announced that Carter reached on petroleum sales to the U.S., but that the talks with Carter "will provide the foundation for future agreements, adding. "I am deeply satisfied with the result of this meeting."

Lopez Portillo announced that Carter had asked Reubin Askew, former governor of Florida, to head a congressionally-mandated commission to report by 1980 on immigration and refugee problems, an issue not resolved during the summit.

In the joint communique, Mexico agreed for the first time that it shares the migration problem with the U.S.

During his speech to the Mexican congress, which Carter delivered in Spanish, the president said Mexico's new found oil and natural gas riches will help lift the Mexican standard of living closer to that of the United States, which in turn will help slow immigration north.

Carter said he would enforce U.S. immigration laws "as fairly and as humanely as I can," and would "protect the basic human rights of all people within the borders of my country."